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HUMAI^  EMBRYOLOGY  AND  MOEPHOLOGY 


Digitized  by  the  Internet  Archive 

in  2010  with  funding  from 

Open  Knowledge  Commons  (for  the  Medical  Heritage  Library  project) 


http://www.archive.org/details/humanembryologymOOkeit 


HUMAN  EMBRYOLOGY 
AND  MORPHOLOGY 


BY 

ARTHUR   KEITH 

M.D.,  F.R.S.,  LL.D.  (Aberdeen),  F.R.C.S.  (Eng.) 

CONSERVATOR   OF   THE   MUSEUM   AND   HUNTERIAN   PROFESSOR 

ROYAL   COLLEGE    OF   SURGEONS,    ENGLAND 

FDLLERIAN   PROFESSOR    IN   COMPARATIVK   ANATOMY,    ROYAL   INSTITUTION,    LONDON 


FOURTH  EDITION,  REVISED  AND  ENLARGED 
WITE  NEARLY   500  ILLUSTRATIONS 


NEW  YORK 

LONGMANS,   GREEN  &   CO. 

LONDON:    EDWARD   ARNOLD 

1921 

All  rights  reserved 


PREFACE  TO  FOURTH  EDITION 

The  issue  of  a  new  edition  has  given  tlie  author  an  opportunity  not  only 
of  incorporating  recent  additions  to  our  knowledge  of  the  development 
and  morphology  of  the  human  body,  but  also  of-  recasting  many  of  the 
chapters.  Over  eighty  new  illustrations  have  been  added.  The  chief 
alterations  relate  to  sections  dealing  with  the  origin  of  the  foetal  mem- 
branes, the  growth  of  the  embryo  and  foetus,  and  the  nature  of  the  basal 
ganglia  of  the  brain.  The  chapters  dealing  with  the  pharynx,  the  ear, 
the  heart,  and  the  lymphatic  system  have  been  rearranged  and  to  a  large 
extent  rewritten.  The  enquiries  of  the  late  Professor  Franklin  P.  Mall 
have  shown  that  human  embryos,  in  their  earlier  stages,  are  a  week  older 
than  was  formerly  believed.  The  estimated  ages  of  embryos  now  given  in 
this  work  are  based  on  Professor  Mall's  calculations. 

Experience  has  confirmed  the  author  in  his  earlier  opinion  that  the 
facts  of  embryology  are  barren  and  meaningless  until  they  are  interpreted 
in  the  light  of  our  knowledge  of  the  evolution  of  the  human  body — 
a  knowledge  which  must  be  founded  on  a  comx^rehensive  survey  of 
comparative  anatomy  and  physiology.  Hence  in  this  new  edition  the 
author  has  sought  to  give,  not  only  a  descriptive  history  of  the  develop- 
ment of  the  various  systems  of  the  body,  but  to  make  the  facts 
intelligible  by  bringing  a  knowledge  of  comparative  anatomy  and 
evolution  to  bear  on  them. 

Human  Embryology  and  Comparative  Anatomy  have  become  vast 
fields  of  knowledge.  Here  they  are  dealt  with  only  in  so  far  as  they  bear 
directly  on  the  nature  of  the  human  body,  and  reflect  what  the  author 
has  found  to  be  useful  in  the  course  of  his  daily  work  and  teaching.  Every 
effort  has  been  made  to  make  the  book  representative  of  the  latest  British 
Research. 

In  the  preparation  of  the  present  edition  the  author  has  become  indebted 
to  a  very  wide  circle  of  friends  too  numerous  to  be  mentioned  individually. 
He  cannot,  however,  allow  the  occasion  to  pass  without  a  warm  acknow- 
ledgment of  his  indebtedness  to  Dr.  Alexander  Low-,  Lecturer  on  Em- 
bryology in  the  University  of  Aberdeen,  for  the  help  he  has  given. 

ARTHUR  KEITH. 

Royal  College  oe  Surgeons  of  England, 

Lincoln's-Inn-Fields,  W.C.  2,  3Iay,  1921. 


CONTENTS 


CHAPTER  PACK 

I.  Eably  Changes  in  the  Development  of  the  Ovum  and 

Embryo    -        -        -        - 1 

II.  The  Manner  in  which  a  Connection  is  Established  between 

THE  Foetus  and  Uterus 22 

III.  The  Primitive  Streak,  Xotochord  and  Somites         -         -  35 

IV.  The  Age  Changes  in  the  Embryo  and  Eoetus  -         -         -  45 
V.  The  Spinal  Column  and  Back      -         -         -         -         -         -  52 

VI.  The  Segmentation  of  the  Body  ------  66 

VII.  Central  Nervous  System — Differentiation  of  the  Spinal 

Cord         -        -        .        . 74. 

VIII.  The  Mid-  and  Hind-Brains  -------  85 

IX.  The  Fore-Brain  or  Prosencephalon  -----  101 

X.  The  Fore-Bbain  OR  Prosencephalon  (co«<("«??erZ).     Cerebral 

Vesicles  ----------  m 

XI.  The  Cranium  ----------  135 

XII.  Development  of  the  Face    -------  155 

XIII.  The  Teeth  and  Apparatus  of  Mastication         -         -         -  182 

XIV.  The  Nasal  Cavities  and  Olfactory  Structures         -         -  192 

XV.  Development  of  the  Structures  concerned  in  the  Sense 

OF  Sight 203 

XVI.  The  Organ  of  Hearing         -.---..  223 

XVII.  Pharynx  and  Neck        ---.--..  240 

XVIII.  Tongue,   Thyroid   and   Structures  developed   from  the 

Walls  of  the  Primitive  Pharynx         -         -         .         .  255 

XIX.  Organs  of  Digestion     --------  267 

XX.  Circulatory  System 301 

vii 


viii  CONTENTS  . 

CHAPTER  PAGE 

XXI.  Circulatory  System  (continued)     -        -        -         -         -         -  329 

XXII.  Respiratory  System      - 340 

XXIII.  Urogenital  System        .        .        - 358 

XXIV.  Urogenital  System  {continued)      ------  379 

XXV.  Body  Wall  and  Pelvic  Floor     ------  404 

XXVI.  Development  and  Differentiation  of  the  Limb  Buds        -  425 

XXVII.  Morphology  of  the  Limbs 441 

XXVIII.  Skin  and  its  Appendages      -------  462 

Index      -----------  475 


HUMAI^  EMBRYOLOGY  AKD 
MORPHOLOGY. 

CHAPTER  I. 

EARLY  CHANGES  IN  THE  DEVELOPMENT  OF 
THE  OVUM  AND  EMBRYO. 

The  First  Five  Weeks  of  Development.— In  the  first  five  weeks  of 
human  development   changes  take  place  very  rapidly.     In  that   shoit 


CORONA     RAOIATA 


CYTOPLASM 


GERMINAL  VESICLE 
/"nucleus) 


YOLK    GRANULES 
ZONA    RADlATA 


+      100    TIMES 


UMBILICAL    CORD 


MNION 
LEG    BUD 


+   3   TIMES 

Fig.  1. — The  parts  of  a  Mature  Human  Ovum.     (After  Van  der  Stricht.) 
Fig.  2. — Human  Embryo  and  its  Membranes  at  the  end  of  the  fifth  week  of  develop- 
ment.    (After  Kollmann.) 

time  the  fertilized  ovum  passes  from  the  condition  of  a  single  cell,  with  a 
diameter  of  -^ i^^  of  an  inch  ^  to  a  fully  formed  human  embryo  about  V  of  an 
inch  in  length  (.5  mm.),  and  contained  within  a  spherical  envelope  of 
embryonal  membranes  which  measures  nearly  an  inch  in  diameter  (see 
Figs.  1  and  2).     By  the  end  of  the  fifth  week  the  beginnings  of  all  the 

^  Measurements  are  given  at  first  according  to  our  English  standard,  but  throughout 
this  book  the  more  convenient  metric  system  vnW  be  employed.  One  inch=25-'i  mm. 
One  millimetre  =  1000^,  or  micromillimetres,  or  mikrons. 


2       HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

parts  of  tlie  adult  body  are  recognizable — the  head,  the  trunk,  the  limb- 
buds,  the  primitive  segments,  the  eyes,  the  nose  and  mouth.  A  section 
across  the  abdominal  cavity  of  an  embryo  at  this  stage  (see  Fig.  23)  reveals 
the  fact  that  the  foundations  of  the  genital  glands  are  already  laid,  and  that 
certain  cells  have  been  set  aside  for  the  reproduction  of  another  generation. 
Thus  by  a  cycle  of  developmental  changes  a  new  generation  of  reproductive 
cells  has  been  produced  from  the  fertilized  ovum  of  a  former  generation 
all  taking  place  within  the  short  space  of  five  weeks.  In  this  chapter  we 
are  to  follow  the  changes  which  lead  from  the  fertilized  egg  or  oocyte  of 


V^ OVP>R  .  AflTERY 

KIDNEY 


PLICA    VASCULARIS 


Fig.  3. — The  position  of  the  Ovary  and  Fallopian  Tube  in  the  5th  month. 

the  mother  to  the  establishment  of  a  new  brood  of  genital  cells  in  the 
embryo.     In  this  way  life  is  handed  on  from  one  generation  to  another. 

Descent  o£  the  Ovary. — In  tracing  the  cycle  of  changes  which  lead 
from  the  fertilized  ovum  of  one  generation  to  the  production  of  a  repro- 
ductive gland  in  the  next  generation,  we  may  begin  our  study  at  any 
point,  but  for  many  reasons  it  is  convenient  to  commence  with  the  con- 
dition of  the  ovary  in  a  fifth-month  foetus.  The  ovary  is  descending  or 
migrating  from  the  region  of  the  kidneys  where  it  was  formed,  and  has 
reached  the  iliac  fossa.  In  all  primitive  vertebrate  animals  the  genital 
glands  are  stationed  above  the  kidneys,  but  in  mammals,  for  reasons 
to  be  explained  afterwards,  they  descend  to  the  lower  part  of  the  trunk 
— a  change  which  is  especially  well  seen  in  the  human  subject.  In  the 
fifth  month  the  ovary  is  long  and  narrow,  with  an  upper  or  cranial  and 
lower  or  caudal  pole  ;  it  is  three-sided  in  section — the  surfaces  being 
medial,  lateral  and  inferior  or  ventral  (Fig.  3).  The  Fallopian  tube  or 
oviduct  lies  along  the  outer  side  of  the  ovary  in  the  iliac  fossa  ;   its  upper 


DEVELOPMENT  OF  THE  OVUM  3 

fimbriated  end  teriiiinati'S  at,  and  is  attached  to,  tlie  upper  or  cranial 
l^ole  of  the  ovary  (Fig.  3).  As  the  parts  lie  on  the  iliac  fossa,  the  tube 
and  the  ovary  are  supported  each  by  its  own  mesentery,  the  mesosalpinx 
and  mesovarium.  The  two  mesenteries  have,  however,  a  common  origin 
or  attachment  to  the  jiosterior  abdominal  wall,  and  to  the  common  attach- 
ment the  name  of  common  genital  mesentery  may  be  given  (Fig.  4).  The 
upper  end  of  the  common  mesentery — the  plica  vascularis  (Fig.  3),  as  it  is 
reflected  from  the  cranial  pole  of  the  ovary  and  fimbriated  extremity  of 
the  tube,  is  continued  up  towards  the  diaphragm  and  in  it  the  ovarian 
vessels  and  nerves  pass  to  the  ovary  and  tube.  The  caudal  pole  of  the 
ovary  is  joined  to  the  uterus  by  its  round  ligament.     The  round  ligament 


COM  .MESENT. 
AORTA 
MESOVAft . 


WOLFFIAN 
BODY 


WOLFF/ AN 
DUCT 


MES0-5ALP. 


Fig.  4. — Diagrammatic  Section  of  a  Foetus  at  the  beginning  of  the  3rd  month  (30  mm.  long), 
showing  the  attachments  of  the  Ovary  and  Miillerian  duct. 

of  the  uterus,  corresponding  to  the  gubernaculum  testis  of  the  male, 
passes  from  the  brim  of  the  pelvis,  where  it  is  attached  to  the  horn  of  the 
uterus,  almost  straight  to  the  internal  inguinal  opening  and  assists  in  the 
descent  of  the  ovary  and  tube. 

By  full  term  the  ovary  lies  at  the  brim  of  the  pelvis  or  partly  within  it ; 
after  birth  the  ovary,  uterus  and  rectum  come  gradually  to  occupy  their 
adult  positions  within  the  pelvis.  This  is  due  to  a  relatively  great  growth 
in  the  pelvis,  which  becomes  marked  as  the  child  learns  to  walk,  and 
especially  in  the  female  at  the  time  of  puberty.  The  ovary,  as  is  more 
frequently  the  case  with  the  testicle,  may  be  arrested  in  its  descent. 

In  Fig.  4  an  earlier  stage  is  shown  ;  it  represents  the  condition  about 
the  beginning  of  the  third  month.  The  ovary  and  tube  with  the  remnants 
of  the  Wolffian  body — a  primitive  form  of  kidney — occupy  the  position 
in  which  they  are  developed.  Both  are  suspended  by  mesenteries  from 
the  dorsal  wall  of  the  peritoneal  cavity,  at  the  side  of  the  mesentery 
of  the  gut. 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


Normal  Position  of  the  adult  Ovary. — When  the  ovary  descends 
within  the  pelvis  it  usually  occupies  a  definite  triangle — the  ovarian 
triangle — on  the  lateral  wall  of  the  pelvic  cavity  (Fig.  5).  The  ovarian 
triangle  is  bounded  above  by  the  upper  half  of  the  external  iliac  artery, 
below  and  behind  by  the  internal  iliac  artery,  with  the  ureter  lying  on  the 
artery  ;  in  front  by  the  reflection  of  the  posterior  layer  of  the  broad  liga- 
ment on  the  side  of  the  pelvis.  The  peritoneum  covering  the  triangle 
forms  a  depression,  or  occasionally  a  pouch,  for  the  ovary.  The  fimbriated 
end  of  the  Fallopian  tube  is  applied  to  the  ovary,  ready  to  receive  the  ripe 
ova  and  transfer  them  to  the  uterus.  One  of  the  fimbriae — the  ovarian 
fimbria — tethers  the  tube  to  the  ovary.  It  will  be  seen  that,  with  the 
descent  of  the  ovary,  the  mesosalpinx,  the  mesovarium,  and  the  common 


ureter 


ouario-peluic  lig.. 
Fallopian  tube^ 
meso-salp.^ 

rd.  lig.  ut 


common  iliac  art 

'.  iliac 
reter 
-fimbriated  end 
ovary 


uterus 


mesouarium 
'cl.  lig.  ou. 


Fig.  5.- 


-Showing  the  position  of  the  Ovary  on  the  lateral  wall  of  the  Pelvis  and 
its  relation  to  the  Fallopian  Tube. 


genital  mesentery  have  come  to  form  the  major  part  of  the  broad  ligament. 
The  upper  end  of  the  common  genital  mesentery  now  forms  the  ovario- 
pelvic  ligament  (Figs.  3  and  5).  The  ovary  brings  down  with  it,  too,  the 
ovarian  arteries,  veins,  lymphatics  and  plexus  of  nerves.  The  nerves  come 
through  the  aortic  plexus  from  the  10th  and  11th  dorsal  segments  of  the 
spinal  cord,  and  the  lymphatic  vessels  carry  the  ovarian  lymph  to  a  group 
of  glands  situated  high  up  in  the  lumbar  region. 

An  Ovum.^ — As  the  infantile  ovaries  descend,  each  is  laden  with  thou- 
sands of  ova  (over  10,000,  T.  G.  Stevens  ;  100,000,  F.  H.  A.  Marshall). 
It  is  estimated  that  not  more  than  200  in  all  become  ripe  and  are  shed. 
The  ova  are  embedded  in  the  stroma  of  the  ovary,  each  being  surrounded 
by  a  special  company  or  cluster  of  epithelial  cells,  which  provide  both 

^  For  fuller  details  and  literature  see  Francis  H.  A.  Marshall,  The  Physiology  of 
Reproduction,  London,  1910.  For  more  recent  investigations  on  the  maturation  of 
Graafian  follicles  in  Man  see  Prof.  Arthur  Thomson,  Journ.  of  Anat.  1919,  vol.  53, 
p.  172,  vol.  54,  p.  1.  See  also  Prof.  Arthur  Robinson,  Trans.  Roy.  Soc.  Edin.  1918, 
vol.  52,  p.  303. 


DEVELOPMENT  OF  THE  OVUM 


a  nest  and  nourishment  for  the  ovum  or  oocyte  (Figs.  6,  7).  The  cells 
which  surround  an  ovum,  with  a  condensed  layer  of  the  stroma  cells  outside 
them,  form  a  Graafian  follicle.  As  the  ovary  descends  it  is  covered  by  a 
cubical  epithelium,  derived  from  the  germinal  epithelium  which  formed 
a  stratum  on  the  free  surface  of  the  ovary  at  its  first  appearance  in  the 
roof  of  the  abdominal  cavity.  The  ova  and  their  accompanying  follicular 
cells  are  derived  from  the  surface  stratum.  Amongst  the  columnar  cells 
of  the  germinal  epithelium  and  also  in  the  stratum  immediately  beneath 
them  are  large  peculiar  cells.  These  are  the  primordial  ova  from  which 
brood  ova  arise.  The  ova  are  thus  carried  within  the  ovary  by  ingrowths 
of  the  germinal  epithelium.  These  tubular  invasions  into  the  ovary 
become  broken  up,  the  isolated  masses  of  the  germinal  epithelium  remaining 
to  form  the  linings  of  the  Graafian  follicles.  In  the  outer  or  cortical  zone 
follicles  continue  to  form  in  early  foetal  life,  but  after  birth  and  even  to  the 


GELRMINAL     EPITHELIUM 


ISOLATED    NEST 


PENING    OVUr 


TOmCA  ALBUGlNtA 
STROMA  CAPSULE 


ZONA    RADIATA 


lAL 


VESICLE 
GERMINAL   SPOT 
OVUM 
DISCUS    PROLIGERUS 


Pig.  6. — Diagrammatic  Section  of  the  Ovary  of  a  fiftli  month  Foetus,  showing 

Nests  of  Germinal  Epithelium  and  unripe  Graafian  Follicle. 
Fig.  7. — Ripe  Graafian  Follicle  at  Puberty. 

end  of  the  fifth  decade  of  a  woman's  life,  follicles  are  being  continually 
formed.  With  this  new  formation  there  is  an  equally  constant  process  of 
degeneration  or  atresia  of  follicles.  We  shall  see  that  another  important 
constituent  of  the  ovary, also  arises  from  the  tubular  incursions  of  the 
germinal  epithelium — namely  the  interstitial  cells,  which  are  glandular 
in  nature,  and  supply  an  internal  secretion  which  has  much  to  do  with  the 
growth  and  regulation  of  the  sexual  structures  of  the  body. 

Discharge  of  the  Ova. — At  puberty  especially,  also  before  it,  and  for 
30  years  after  it,  one  egg  after  another  ripens  ;  the  ovum  enlarges  ;  so 
does  its  Graafian  follicle  (Fig.  7).  The  cells  of  the  epithelial  lining  pro- 
liferate and  a  cavity  appears  within  the  follicle,  due  to  a  collection  of 
fluid — the  liquor  folliculi — amongst  the  cells.  The  ovum  remains  attached 
to  the  wall  of  the  follicle  by  a  group  of  epithelial  cells,  the  discus  pro- 
ligerus  or  cumulus  (Fig.  7).  As  the  fluid  collects,  the  follicle  works  its 
way  to  the  surface  of  the  ovary  ;  the  tunica  albuginea,  which  forms  a 
capsule  for  the  ovary,  and  the  covering  epithelium,  gradually  atrophy 
over  it,  and  at  last  it  bursts  and  discharges  the  ovum.^  Ova  may  be  shed 
^  As  to  the  mechanism  of  rupture,  see  references  given  on  p.  4. 


6       HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

at  any  point  of  a  woman's  menstrual  cycle  but  the  most  usual  time  is  during 
or  just  after  the  menstrual  period.  All  the  circumstantial  evidence  at 
our  disposal  points  to  the  10th  or  11th  day  of  a  menstrual  cycle,  counting 
from  the  first  day  of  the  menstrual  flow,  as  being  the  most  common  for 
conception.  Whether  ova  are  discharged  from  both  ovaries  at  once,  or 
from  only  one,  and  whether  one  or  more  than  one  in  a  month,  are  points 
not  yet  settled  ;  but  the  usual  opinion  is  that  one  ovum  is  shed  each  month, 
and  only  from  one  ovary.  An  ovum  shed  from  one  ovary  may  occasionally 
pass  down  by  the  opposite  Fallopian  tube. 

The  Graafian  follicle,  after  rupture,  fills  up  with  blood  ;  a  cellular 
tissue  is  soon  developed  within  its  cavity  from  the  lining  cells  of  the  follicle 
but  particularly  from  cells  of  the  inner  sheath  of  the  follicle.  The  inner 
sheath  cells  break  into  the  follicle  and  carry  blood  vessels  with  them. 
These  cells  come  to  contain  lutein,  which  gives  them  a  yellowish  colour 
when  seen  in  the  mass.  If  pregnancy  does  not  occur,  a  false  corpus  luteum 
is  formed,  a  formation  which  begins  to  disappear  before  the  next  menstrual 
period.  If  pregnancy  occurs,  however,  the  cellular  mass  continues  to 
increase  in  size  until  it  forms  a  glandular  body  as  large  as  a  pigeon's  egg 
and  is  known  as  a  true  corpus  luteum.  It  reaches  its  maximum  size  about 
the  fourth  or  fifth  month  of  pregnancy  ;  it  is  much  reduced  in  size  by 
the  end  of  that  period.  Experiments  have  been  made  by  Marshall  and 
Jolly  and  by  Blair  Bell  which  show  that  the  secretion  of  the  corpus  luteum 
acts  on  the  decidual  or  lining  membrane  of  the  uterus,  sensitizing  it  so  that 
it  responds  by  growth  when  the  fertilized  ovum  comes  in  contact  with  the 
decidua.  If  the  corpus  luteum  is  excised  pregnancy  is  prevented,  or  if 
begun,  is  arrested.  Both  forms  of  corpus  luteum  lead  to  the  formation  of 
cicatrices  which  are  to  be  seen  on  the  surface  of  the  ovary.  The  ovary 
of  an  old  person  is  commonly  covered  with  such  scars.  The  Graafian 
follicles  may  become  cystic  and  give  rise  to  enormous  ovarian  tumours. 

The  Fallopian  Tube. — When  the  ovum  or  oocyte  drops  from  the  ovary 
it  cannot  easily  escape  the  ciliated  fimbriae  of  the  Fallopian  tube  which 
surround  and  clutch  the  ovary.  In  Fig.  5  the  relationship  of  the  Fallopian 
tube  to  the  ovary  is  shown.  The  tube  may  be  demarcated  into  three 
parts  :  (a)  the  isthmus  or  arm  directed  outwards  to  the  wall  of  the  pelvis 
(I  to  1  inch)  ;  (6)  the  forearm  or  ampullary  part,  directed  backwards  on 
the  lateral  pelvic  wall  above  the  ovary  ;  (c)  the  hand,  infundibular,  or 
fimbriated  part,  folded  backwards  and  grasping  the  free  border  and 
exposed  surface  of  the  ovary.  The  tube  is  fastened  by  one  of  its  fimbriae 
to  the  cranial  pole  of  the  ovary. 

Course  of  the  Ovum  in  the  Tube. — The  cilia  on  the  fimbriae  work 
towards  the  ostium  abdominale,  the  abdominal  mouth  of  the  Fallopian 
tube,  which  is  situated  at  the  bases  of  the  fimbriae,  and  carry  the  dis- 
charged ovum  through  the  ostium  within  the  tube.  The  ostium  abdominale 
is  shut  when  the  tube  is  examined  after  excision  ;  the  closure  is  probably 
due  to  reflex  contraction  of  the  tube  muscle,  caused  by  handling  and 
cutting.  In  the  infundibular  and  ampullary  segments  of  the  tube,  the 
mucous  membrane  is  thrown  into  long  plicated  folds  shown  in  section  in 
Fig.  8.     They  are  covered  with  ciliated  epithelium,  which  urge  the  ovum 


DEVELOPMENT  OF  THE  OVUM  7 

towards  the  uterus.  Between  these  folds,  in  the  upper  reach  of  the  tube, 
the  ovum,  if  it  is  to  be  fertilized,  usually  meets  the  male  cell  or  spermatozoon, 
for  we  know  that  spermatozoa  can  remain  alive  in  the  tube  for  at  least 
seven  days  after  connection.  The  passage  of  the  fertilized  ovum  along 
the  tube  takes  place  slowly  for  it  undergoes  its  first  developmental  changes 
during  this  journey  which  is  supposed  to  extend  over  a  period  of  four  or 
five  days.  If  the  passage  of  the  fertilized  ovum  is  obstructed,  which  may 
result  from  an  inflammation  or  cicatrization  of  the  epithelial  lining  of  the 
tube,  development  may  proceed  at  the  point  of  obstruction.^  When 
tubular  pregnancy  occurs,  the  growing  ovum  expands  and  ultimately 
perforates  the  tube — usually  in  the  second  month — an  accident  which  is 
always  attended  by  a  grave  haemorrhage. 


Ff^LLOPIIKN    TUBE 


yVOLFFIAN  DUCT 


MESOSALPINX 


VARY 
ME.SOVARIUM 


Fig.  8. — Diagrammatic  Section  of  tlie  Broad  Ligament  and  Fallopian  Tube. 

Fig.  9. — Mature  Ovum  of  Bat,  showing  the  separated  Polar  Bodies,  the  Female 

Pronucleus  and  a   Spermatozoon  about  to  form  a  Male  Pronucleus.    (After 

Van  der  Stricht.) 

The  History  of  the  Ovum  within  the  Fallopian  Tube. — When  the 
ovum  enters  the  Fallopian  tube,  it  is  a  cell  of  very  considerable  size 
(100/ji)  with  a  cell  wall — the  zona  radiata  (Fig.  1),  a  nucleus — the  germinal 
vesicle,  and  a  nucleolus — the  germinal  spot.  Then,  or  before  then,^  the 
ovum  prepares  for  fertilization  by  the  extrusion  from  its  nucleus  of  first 
one,  then  a  second  polar  body,  and,  with  the  extrusion,  the  germinal 
vesicle  becomes  the  female  pronucleus  (Fig.  9).  The  polar  bodies  or 
polocytes,  for  they  really  represent  cells,  lie  outside  the  protoplasm  of  the 
ovum,  but  within  the  zona  radiata  ;  they  are  parts  of  the  germinal  vesicle 
which  are  extruded  with  all  the  display  of  karyokinesis — the  peculiar 

1  See  F.  P.  Mall,  Surg.  Gynaec.  and  Obstet.  1915,  vol.  21,  p.  289. 

^  Prof.  Arthur  Thomson  has  sho-\vn  that  ova  are  to  be  seen  in  the  human  ovary  \^^th 
both  polar  bodies  already  extruded  and  that  maturation  changes  can  be  seen  to  take 
place  before  the  ova  are  shed,  Joiirn.  Anat.  1919,  vol.  53,  p.  172. 


8       HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

changes  manifested  by  the  nucleus  when  a  cell  is  about  to  divide.  We  shall 
see  that  the  three  polar  bodies  really  represent  three  aborted  ova — which 
have  left  their  cell  bodies  to  enrich  the  principal  ovum. 

Karyokinesis.^ — The  preparatory  or  maturation  changes  which  take 
place  in  the  nucleus  of  the  ovum  and  also  in  the  nucleus  of  the  male  germinal 
cell  are  of  the  greatest  interest  to  us,  for  we  have  good  grounds  for  sus- 
pecting that  the  mechanism  which  regulates  the  shaping  of  the  adult  body 
is  represented  in  the  substance  of  the  nucleus  of  the  germ  cell.  The 
nucleus  appears  to  be  the  chief  vehicle  of  heredity — the  medium  by  which 
the  features  of  the  parent  are  handed  on  to  the  child.  Hence  the  im- 
portance attached  by  embryologists  to  the  elaborate  changes  undergone 
by  the  nucleus  of  a  maturing  male  or  female  germ-cell.  When  an 
ordinary  cell  of  the  body  is  about  to  divide,  the  nucleus  undergoes  certain 

attract  sphere 

line  of  division 

mtract  sphere 

A  S 

Fig.  10,  A. — Diagram  of  Karyokinesis  in  a  somatic  cell  (homotypical  division). 

B. — Diagram  of  Karyokinesis  during  the  production  of  matured  ova  and 
spermatozoa  (heterotypical  division).     (After  J.  B.  Farmer.) 

changes  before  cleavage  takes  place.  The  nuclear  division  precedes  that 
of  the  whole  cell.  This  mode  of  cell  division  or  cell  propagation  is  known 
as  Karyokinesis  or  Mitosis.  Two  elements  within  the  nucleus  play  a  part 
in  the  process — the  chromatin,  which  readily  combines  with  certain  staining 
reagents,  and  the  acuromatin,  which  does  not  absorb  dyes.  In  the  resting 
phase,  the  chromatin  is  scattered  as  minute  particles  in  the  substance  of  the 
nucleus,  but  when  mitosis  is  to  take  place  the  particles  unite  into  filaments  ; 
the  filaments  break  up  into  segments  or  rods,  each  rod  being  known  as  a 
curomosome  (Fig.  10).  The  number  of  chromosomes  appearing  in  each 
somatic  cell  is  approximately  constant  for  each  species  of  animal ;  in 
man  twenty-four  is  the  usual  number  (Broman) .  As  the  chromosomes  form, 
an  achromatin  formation  appears  in  the  substance  of  the  cell  body  just 
outside  the  nucleus — the  centrosome,  which  appears  to  yield  a  commanding 
influence  on  the  division  of  the  nucleus.  The  centrosome  divides  ;  the  two 
halves  move  apart  until  they  lie  at  opposite  poles  of  the  nucleus  where 
each  forms  an  attraction  sphere  (Fig.  10).  The  attraction  spheres  become 
joined  by  a  spindle  of  achromatin  threads,  the  chromosomes  of  the  nucleus 

1  For  literature  and  significance  of  Mitosis  see  C.  E.  Walker,  Essentials  of  Cytology, 
London,  1907  ;  Bashford  and  Murray,  "  Significance  of  Mitosis,"  Proc.  Roy.  Soc.  1904, 
vol.  73,  p.  66  ;  R.  Fick,  Ergebnisse  der  Anat.  1906,  vol.  16,  p.  1  ;  S.  Tschassownikow, 
Anat.  Hefte,  1911,  vol.  45,  p.  197  ;  Prof.  W.  E.  Agar,  Cytology,  1920  ;  Prof.  F.  R.  Lillie, 
Fertilization  of  the  Ovum,  1919. 


DEVELOPMENT  OF  THE  OVUM  9 

then  appearing  as  if  they  were  supported  by  the  spindle  between  the 
attraction  or  centrospheres.  The  chromosomes  move  towards  the  equa- 
torial plane  of  the  nucleus — midway  between  the  attraction  spheres  ; 
during  the  movement  each  chromosome  divides  longitudinally,  so  that 
each  is  split  into  two,  the  two  halves  lying  side  by  side,  often  bent  into 
V-shaped  forms  (Fig.  10,  ^) .  As  the  nucleus  divides  in  the  equatorial  plane 
24  chromosomes  pass  into  one  half  and  24  into  the  other.  The  attraction 
spheres  fade  away  ;  the  division  of  the  cell  body  is  completed,  each  half 
having  now  its  own  nucleus  ;  the  chromosomes  break  up  in  the  network 
of  the  daughter  nuclei  and  the  two  cells  enter  a  resting  phase.  By  this 
means  an  equitable  distribution  of  the  chromatin  material  of  the  parent 
nucleus  is  made  to  the  two  daughter  cells. 

The  two  karyokinetic  divisions  undergone  by  the  ovum  before  fertil- 
ization differ  in  three  particulars  from  the  process  as  seen  in  a  somatic 
cell  :  (1)  Only  12  chromosomes  are  formed — each  being  really  double  ; 
(2)  the  chromosomes  are  peculiar  in  shape  and  in  manner  of  division 
(Fig.  10,  B) ;  (3)  the  cell  body  divides  very  unequally — only  a  very  minor 
part  accompanying  that  half  of  the  nucleus  which  is  separated  at  the  first 
and  second  divisions  of  the  ovum  and  which  form,  when  thus  separated, 
the  first  and  second  polar  bodies  or  polocytes.  A  division  of  the  first 
polar  body  accompanies  the  separation  of  the  second  polar  body  from  the 
ovum,  there  being  thus  three  polocytes  formed  during  the  maturation  of  the 
ovum.  Thus  the  two  divisions  undergone  by  the  ovum  result  in  the  forma- 
tion of  one  matured  ovum  and  three  polocytes.  Three-fourths  of  the 
chromatin  in  the  nucleus  of  the  original  ovum  are  extruded  in  the  polar 
bodies.  The  number  of  chromosomes  in  the  ripe  ovum  has  been  reduced 
from  24  to  12. 

The  cells  of  a  malignant  tumour  frequently  show  in  their  divisions  the 
peculiar  mitotic  changes  which  are  seen  in  the  preparation  of  the  female 
pronucleus.  Prof.  J.  B.  Farmer  regards  such  cells  as  essentially  germinal 
in  character.  It  has  also  been  found  that  the  heterotypical  or  reducing 
form  of  mitosis  may  occur  in  leucocytes  and  in  inflamed  tissues. 

Formation  of  Spermatozoa. — Having  thus  described  the  maturation 
of  the  ovum,  and  followed  it  within  the  Fallopian  tube,  it  is  necessary 
to  trace  the  history  of  its  counterpart  in  the  male — the  spermatozoon. 
The  manner  in  which  a  spermatozoon  is  produced  by  a  primary  and  second- 
ary division  from  a  spermatocyte  is  very  similar  to  the  production  of  a 
mature  from  an  immature  ovum.  The  form  of  mitosis  is  the  same  (hetero- 
typical), the  chromosomes  being  reduced  to  12  in  number  and  to  a  peculiar 
shape.  The  two  divisions  take  place  within  the  seminiferous  tubules 
of  the  testis,  and  result  in  the  production  of  four  spermatozoa — correspond- 
ing to  the  matured  ovum  and  three  polar  bodies  (Fig.  11,  B).  The  semin- 
iferous tubules  correspond  to  the  ingrowths  of  germinal  epithelium  which 
carry  the  primordial  ova  within  the  ovary.  Lining  the  tubules  are  two 
kinds  of  cells — those  of  Sertoli  (Fig.  11,  B),  large  cells  for  nourishing  the 
spermatozoa — representing  those  of  the  stratum  granulosum  in  the 
Graafian  follicles — and  other  cells  known  as  spermatogonia,  corresponding 
to    primordial  ova.      Spermatogonia   divide   and    give    rise    to    primary 


10 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


spermatocytes  which  correspond  to  immature  ova.  A  primary  sperma- 
tocyte divides  into  two  secondary  spermatocytes,  each  of  which 
again  divides  and  thus  four  cells  are  produced,  which  become  modified 
into  spermatozoa  (Fig.  11,  jB).  The  nucleus  forms  the  head,  the  junctional 
part  represents  the  centrosome,^  while  the  tail  is  all  that  remains  of  the 
substance  of  the  cell  body  (Fig.  11,  A).  While  the  ripe  ovum  has  a  dia- 
meter of  100^  (^iijth  in.)  the  total  length  of  a  spermatozoon  is  only  50//. 
While  the  ovum  represents  a  large  passive  cell,  laden  with  nourishment 
or  yolk,  its  male  counterpart  becomes  highly  modified  within  the  Sertoli 
or  nurse  cell,  has  its  cell  substance  reduced  to  a  minimum  and  is  given  a 
power  of  active  motion.  While  ova  are  ripened  singly,  spermatozoa 
ripen  by  the  million.     Gemmill  has  shown  that  spermatozoa  can  be  kept 


tail- 


middle  piece- 
head- — ^    "• 


spermatozoon 

spermatocyte  II 
spermatocyte  I 
spermatogone 

wall  of  tubule 


B. 


Fig.  11,  A. — Diagram  of  a  Spermatozoon. 

B. — Diagram  showing  the  origin  of  Spermatozoa  from  tlie  lining  cells 
(spermatogonia)  of  the  tubules  of  the  testicle. 

alive  for  many  days  in  nutritive  media  ;  probably  the  secretions  of  the 
epididymis,  vesiculae  seminales,  prostate  and  Cowper's  glands  are  for  this 
purpose. 

Fertilization. — In  the  course  of  fecundation  thousands  of  sperma- 
tozoa are  lodged  in  the  genital  passage  ;  many  stem  the  adverse  current 
of  the  uterine  cilia,  reach  and  live  for  days  within  the  interlaminar  grooves 
in  the  wider  parts  of  the  tube.^  In  the  course  of  its  descent  within  one 
of  the  grooves  the  egg  may  be  fertilized.  The  spermatozoon,  attracted 
to  the  ovum  by  a  force  we  do  not  yet  understand,  bursts  through  the  zona 
radiata,  loses  its  tail,  its  head  enlarges,  and  forms  the  male  pronucleus. 
The  male  and  female  pronuclei  unite,  and  from  their  union  springs  the 
nucleus  of  the  fertilized  ovum.  This  is  the  centre  from  which  all  future 
developmental  changes  start.  In  the  pronuclei,  it  will  be  remembered 
that  the  chromosomes  were  reduced  to  half  the  usual  number  ;  by  their 
union  the  full  complement  of  twenty-four  is  again  restored  in  the  fertilized 

^  In  the  sheath  of  the  middle  piece  is  also  included  an  element  scattered  through 
the  substance  of  the  cell  body  of  the  parent  germinal  cell — the  element  known  as 
mitochondria  or  chondriosomes  (see  J.  Duesberg,  Biol.  Bulletin,  1919,  vol.  36,  p.  71  ; 
E.  V.  Cowdry,  Contributions  to  Embryology,  1918,  vol.  8,  p.  41. 

2  For  literature  on  fate  of  spermatozoa  in  the  uterus  see  J.  H.  F.  Kohlbrugge,  Boux's 
Archives,  1912,  vol.  35,  p.  1. 


DEVELOPMENT  OF  THE  OVUM 


11 


ovum.  By  the  process  of  fertilization  the  characters  of  two  human  indi- 
viduals are  mingled.  The  mixed  chromosomes  of  the  nucleus  of  a  fertilized 
ovum  are  laden  with  an  assortment  of  the  virtues  and  vices  of  both  father 
and  mother  in  a  latent  form.  They  transmit  the  characters  of  the  race 
from  one  generation  to  another.  The  ovum  may  be,  but  rarely  is,  fertilized 
in  the  ovary,  or  between  the  ovary  and  ostium  abdominale,  the  result 
being  a  pelvic  gestation.  The  length  of  time  the  fertilized  ovum  takes  to 
reach  the  uterus  is  not  known  exactly,  but  probably  it  spends  from  four 
to  five  days  within  the  Fallopian  tube.  The  musculature  of  the  tube, 
as  well  as  the  action  of  the  cilia,  assist  the  fertilized  and  developing  ovum 
in  its  progress  to  the  cavity  of  the  uterus. 

Formation  of  the  Embryo.^ — We  are  now  to  follow,  step  upon  step, 
the  changes  which  are  to  transform  the  fertilized  ovum  into  a  human 
embryo.  With  the  fusion  of  the  male  with  the  female  pronucleus  the  ovum 
begins  to  divide,  thus  giving  rise  to  the  first  brood  of  cells,  two  in  number  ; 


A. 


B. 


Fig.  12. — Showing  the  production  of  the  Blastula  or  Morula  from  the  Ovum.     The  oolemma 

(zona  radiata  or  egg-membrane)  persists  up  to  the  morula  stage,  even  later. 

A.  The  Ovum  after  the  first  division.     B.  After  the  second.     C.  The  Blastula  stage. 

these  in  turn  give  rise  to  a  second  brood,  four  in  number,  and  so  on  through 
successive  stages,  until  a  minute  mass  of  cells  replaces  the  ovum  (Figs.  12 
and  13)  and  thus  a  blastula  or  morula  is  formed — the  first  stage  in  the 
production  of  an  embryo. 

The  production  of  the  blastula  takes  place  as  the  egg  passes  towards 
the  cavity  of  the  uterus,  but  before  it  has  come  into  actual  contact  with  the 
prepared  lining  membrane  or  decidua,  it  has  entered  a  second  and  very 
important  stage.  A  space  or  cavity  aj)pears  within  the  blastula  (Fig.  14) 
so  that  its  cells  become  arranged  in  a  definite  manner.  The  cells  which 
are  going  to  give  rise  to  the  structures  by  which  the  embryo  is  to  be 
nourished  become  arranged  around  the  central  cavity  as  a  covering  layer, 
while  the  cells  which  are  to  build  up  the  embryo  are  enclosed  within  the 
covering  layer  (Fig.  14).  In  this  manner  the  blastocyst  is  produced.  At 
this  stage,  when  the  developing  ovum  is  probably  only  half  a  millimetre 
in  diameter  (-jL  inch),  it  reaches  the  uterus.  Its  enveloping  layer  or 
trophoblast  comes   in   contact  with  the  decidua.     In  the  blastocyst  we 

1  For  literature  on  early  stages  in  the  formation  of  the  mammalian  blastula  see 
J.  P.  Hill,  Quart.  Joum.  Mic.  8c.  1911,  vol.  56,  p.  1  ;   1918,  vol.  63,  p.  91. 


12 


HUMAN  EMBRYOLOaY  AND  MORPHOLOGY 


recognize  an  embryogenic  and  a  vegetative  or  yolk  pole  (Figs.  13  and  14). 
In  Vertebrates  with  huge  stores  of  yolk  in  their  ova,  such  as  birds  have, 
the  vesicle  is  filled  by  yolk-bearing  cells,  continuous  with  the  enveloping 
layer  at  the  vegetative  pole,  opposite  to  the  inner  cell  mass. 

We  now  pass  on  to  a  further  or  third  stage,  concerning  which  our  know- 
ledge is  as  yet  imperfect.  By  virtue  of  the  phagocytic  power  oi  its  outer 
or  trophoblastic  layer,  the  blastocyst  embeds  itself  in  the  decidual  mem- 
brane of  the  uterus  towards  the  end  of  the  first  week  of  development.  In 
its  earlier  stages  all  developmental  efforts  are  concentrated  on  the  growth 
of  the  outer  or  trophoblastic  layer  which  is  to  provide  the  embryo  with 
nourishment ;   hence  the  rapid  expansion  of  the  blastocyst  and  the  multi- 

embryogenic  pole 
embryogenic  pole  inner  cell  mass  1     .■enveloping  layer     . 


uegetatiue  po'e  enveloping  layer 

Fig.  13.— Stage  I.  The  Blastula. 
Fig.  14. — Stage  II.  The  Blastocyst.     (After  Van  Beneden.) 

plication  and  spread  of  the  trophoblastic  cells.  Early  in  the  second  week 
a  vesicular  structure,  measuring  little  more  than  a  millimetre  in  diameter, 
has  been  produced  (Fig.  15).  The  inner  cell-mass  shown  in  Fig.  14  has 
now  become  differentiated  into  three  sets  or  systems  (Fig.  15)  :  (1)  a 
hypoblastic  or  entodermal  set,  grouped  so  as  to  form  the  wall  of  a  minute 
vesicle — the  fore  shadow  of  the  alimentary  or  archenteric  system  of  the 
embryo  ;  (2)  an  epiblastic  or  ectodermal  set,  enclosing  another  minute 
fluid  space — the  cavity  of  the  Amnion.  We  shall  see  that  the  ectodermal 
cells  in  the  floor  of  this  cavity,  the  side  abutting  on  the  archenteric  vesicle, 
will  go  to  the  formation  of  the  embryo,  while  the  cells  of  the  side  and  roof 
will  form  merely  the  lining  of  the  amniotic  cavity,  within  which  the  embryo 
will  become  developed  ;  (3)  a  third  system — the  mesoblastic  or  meso- 
dermal— of  cells  has  made  a  precocious  appearance,  surrounding  the  arch- 
enteric and  mesodermic  vesicles,  lining  the  inner  surface  of  the  trophoblast 
and  filling  the  space  between  the  vesicles — the  trophoblastic  wall — with 
exceedingly  fine  fibrils  (Fig.  15).  These  mesodermal  cells  are  heralds  of 
the  great  system  out  of  which  are  to  arise  the  blood  and  the  vessels,  muscle, 


DEVELOPMENT  OF  THE  OVUM 


13 


bone,  ligaments  and  all  the  connective  tissue  structures  of  the  body. 
Thus  in  the  second  week,  embedded  within  the  decidua,  the  developing 
human  blastocyst  reaches  a  third  stage — one  in  which  the  embryo  is 
represented  by  two  vesicular  structures — the  bivesicular  blastocyst.  The 
youngest  human  embryo  of  which  we  have  accurate  knowledge  represents 
the  terminal  phase  of  this  stage  of  development.  This  embryo  was  in- 
vestigated and  described  by  Teacher  and  Bryce  in  1908.^  The  inner 
vesicles  are  still  excessively  small,  the  amniotic  measuring  only  -15  mm.  in 
diameter,  while  the  archenteric  is  still  less.  On  the  other  hand,  the 
containing  or  trophoblastic  vesicle  is  relatively  large,  measuring  almost 
2  mm.  in  its  longest  diameter  and  the  trophoblastic  cells  are  pressing 
outwards  into  the  decidua  by  a  process  of  most  active  growth.  It  is 
estimated  that  the  Teacher  and  Bryce  blastocyst  is  at  the  end  of  the 


^^^.^^r.  .^^^^r.^r,^  MESODERMAL    SPACE. 

MESODERM 

Fig.  15. — The  Blasto-dermic  Stage. 

second  week  of  development.  Towards  the  end  of  the  second  week  or 
commencement  of  the  third  a  very  important  change,  reproduced  in 
Fig.  16,  carries  the  blastocyst  on  to  a  further  ov  fourth  stage  of  development. 
The  coelomic  cavity  or  space,  the  primitive  representative  of  the  peritoneal, 
pleural  and  pericardial  cavities  is  produced  by  the  cleavage  or  separation 
of  the  mesoderm  into  two  layers.  One  layer — the  inner — covers  the 
archenteron,  its  wall  being  now  made  up  of  two  strata — an  inner  of  ento- 
derm and  an  outer  of  mesoblast  or  mesoderm  ;  this  double-layered  wall  is 
known  as  the  splanchnopleure.  The  other  layer  of  mesoderm — the  outer, 
covers  the  outer  surface  of  the  amniotic  cavity  (Fig.  16)  and  the  inner 
aspect  of  the  trophoblastic  wall.  We  shall  see  that  the  double-layered 
amniotic  wall  really  represents  the  wall  which  encloses  the  abdominal 
and  thoracic  cavities  ;  the  double  stratum  made  up  of  epiblast  or  ectoderm 

^  For  literature  on  very  early  human  embryos  see  T.  H.  Bryce  and  J.  H.  Teacher, 
Contributions  to  the  Study  of  the  Early  Development  and  Embedding  of  the  Human  Ovum, 
Glasgow,  1908  ;  the  more  recent  literature  and  data  are  given  by  Dr.  Geo.  L.  Streeter, 
Contributions  to  Embryology,  1920,  vol.  9,  p.  389. 


14 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


and  mesoderm  or  mesoblast  is  known  as  the  somatopleure.  The  addition 
of  mesoderm  to  all  of  these  epithelial  walls  endows  them  with  the  power  of 
forming  blood  vessels  and  blood.  So  far  the  growing  blastocyst  has 
depended  for  its  sustenance  on  what  the  trophoblastic  layer  could  absorb 
from  the  decidua  but  with  the  addition  of  mesoderm  to  the  trophoblastic 
layer  we  have  the  outer  or  enveloping  layer  endowed  with  new  and  im- 
portant properties.  We  shall  see  that  the  mesoderm  on  the  archenteron 
and  also  that  which  lines  the  trophoblastic  wall  are  the  first  to  produce 
blood  vessels  and  blood.  With  the  addition  of  mesoderm  to  the  tropho- 
blastic wall  we  apply  a  new  name  to  the  outer  or  enveloping  wall — namely 
the  chorion.     Through  the  chorion  the  embryo  is  to  draw  its  oxygen  and 


ROPHOBLAST 


MUCOUS     ^ 
MEMBRANE. 
of 


Fig.  16. — Showing  the  Origin  of  the  Primitive  Coelom,  the  Mesoblast  and  Cavity 
of  the  Amnion  during  the  Development  of  the  Human  Ovum.  (After  T.  H. 
Bryce.) 

nourishment  from  the  mother  and  get  rid  of  its  carbon  dioxide  and  waste 
products. 

In  1899  Dr.  Peters  gave  a  full  and  clear  description  of  an  embryo  at 
this  stage  of  development.  The  whole  blastocyst  was  as  yet  of  small  size 
— only  1-6  mm.  (y^-  inch)  on  its  longest  diameter  (Fig.  17).  The  envelop- 
ing epiblast  and  its  lining  of  mesoblast  now  form  a  distinct  but  non- vascular 
chorion.  The  archenteric  vesicle  is  still  of  minute  dimensions  (Fig.  17). 
The  amniotic  cavity,  formed  within  the  enclosed  ectoderm  is  larger,  and 
the  cells  lining  it  have  become  differentiated  into  two  kinds  (Fig.  17). 
An  area  of  columnar  cells,  forming  the  floor  plate  of  the  cavity,  produces 
ultimately  the  epithelial  covering  of  the  body,  and  all  the  cells  and  fibres 
of  the  nervous  system.  The  flatter  cells  which  line  the  dome  of  the  cavity 
will  form  the  epithelial  lining  of  the  Amnion  ;  the  outer  layer  of  mesoderm 
afiords  a  covering  to  the  amniotic  ectoderm  (Fig.  17).  Fluid  collects  within 
the  cavity  of  the  amnion  ;  floating  in  the  fluid,  the  human  embryo  will 
develop.      Thus  the  delicate  embryonic   tissues,  being  equally  supported 


DEVELOPMENT  OF  THE  OVUM 


15 


on  all  sides  by  the  amniotic  fluid,  may  pursue  their  developmental  courses, 
unhindered  by  the  influence  of  gravity,  and  uninjured  by  the  pressure,  to 
■  which  the  uterus  within  the  abdomen  is  subjected  by  the  movements  of 
respiration  or  bending  of  the  trunk.  If  the  fluid  is  deficient  or  absent 
then  many  forms  of  malformation  may  result. 

It  is  in  this  stage  (Stage  IV.)  that  it  becomes  possible  to  detect  the  founda- 
tion or  Anlage  of  the  embryo.  It  is  represented  by  the  plate  or  lamina  of 
tissues  which  separates  the  cavity  of  the  archenteron  from  the  cavity  of 
the  amnion  (Fig.  17).     The  growth  of  the  embryo  remains  in  abeyance  ; 


cauity  of  uterus 

.decidual  eel fs 
syncytium 
basal,  layer  of  chorion 

mesoblast  of  chorion 

decidua  reflexa 


decidua  serotina- 
cauity  of  amnion 

decidual  cells. 

syncytium 
basal  layer  of  chorion 

uterine 


embryonic  epiblast 
archenteron 
rimitiue  coelom 

cauity  of  uterus 
esoblast  of  chorion 


Fig.  17. — Stage  IV.  Section  through  the  bivesicular  blastocyst  embedded  in  the  wall  of 
the  Uterus.    (Modified  by  F.  W.  Jones  from  figures  given  by  Peters  and  Selenka.) 

all  the  developmental  energy  is  thrown  into  the  upbuilding  and  expansion 
of  the  enveloping  epiblast  or  Trophoblast  as  Hubrecht  named  it  in  1889, 
for  he  recognized  that  its  chief  function  was  to  provide  the  embryo  with 
the  means  of  nourishment  (rpoc^os,  a  feeder).  Thus  in  the  earlier  stages  of 
development  the  actual  embryo  remains  in  abeyance,  while  the  tissues 
which  protect  it  and  nourish  it  grow  and  develop  with  exceeding  rapidity. 
Already,  in  Stage  IV.,  it  is  seen  that  the  epithelium  forming  the  tropho- 
blast has  become  differentiated  into  (a)  a  Basal  Layer  (Langhan's  cells), 
(b)  masses  of  cells,  which  have  undergone  multiplication  without  separation  ; 
this  formation  is  known  as  Syncytium  (Fig.  17).  The  syncytium  is  chiefly 
developed  on  that  aspect  of  the  developing  ovum  which  is  directly  in  contact 


16      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

with  the  wall  of  the  uterus.  It  is  remarkable  not  only  for  the  imperfect 
-separation  of  its  cells,  due  probably  to  the  rapidity  of  its  growth,  but  also 
for  the  extraordinary  phagocytic  power  it  exercises  on  the  mucous  mem- 
brane of  the  uterus.  Processes  of  the  syncytium  burrow  within  the  thickened 
and  vascular  mucous  membrane  in  a  systematic  and  regulated  manner  ; 
they  absorb  the  tissue  with  which  they  come  in  contact,  and  lay  open  blood 
vessels  of  the  mucous  membrane.  The  maternal  blood  escapes  into  spaces 
enclosed  by  the  syncytial  processes  or  into  lacunae  formed  by  the  vacuola- 
tion  of  processes.  In  certain  circumstances  syncytial  cells  escape  into  the 
general  circulation  and  form  malignant  growths.  If  the  developing  ovum 
be  arrested  in  the  Fallopian  tube  the  syncytium,  owing  to  the  extreme 


NEURENTERIC  CANAL 


r-^T-^n<ro>o.       y^:;;'  \  _,,— 5*^^       -kii-T^:^  •  \        CAVITY  Of 

ECTODERM    ^^  \„<;S?2='''^       FT^I^T^V^     AMNION 

BODY  Stalk 


CAVITY  of 
ALLANTOIS 


ECTODERM 


CHORIONIC 
VILLUS 


CHORION  ^jg       •    ■  ■    ■    -        Jli^      \         ENTODERM 

YOLK    SAC 

Fig.  18. — Stage  V.  Diagrammatic  Section  of  a  human  pregnancy  towards  the  end  of 
the  3rd  week  of  development,  showing  its  demarcation  into  embryo  and  mem- 
branes.   (After  Graf  Spee.) 

thinness  of  the  lining  membrane,  quickly  eats  its  way  into  and  through  the 
wall  of  the  tube. 

In  Fig.  18  there  is  given,  in  a  diagrammatic  form,  the  stage  of  develop- 
ment reached  about  the  end  of  the  third  week.  A  very  rapid  growth  sets 
in  during  this  week  ;  the  chorionic  vesicle  which  at  Stage  IV.  measured 
only  2  mm.  in  its  longest  diameter,  has  become  five  times  that  length — 
an  object  easily  visible  to  the  naked  eye.  Villi  grow  out  from  it,  at  first 
simple  and  then  branched  ;  blood  spaces  filled  by  maternal  blood  are 
formed  between  the  villi.  In  the  villi,  blood  vessels  and  blood  are  being 
formed  but  a  circulation  is  not  yet  established.  The  embryo  is  now 
definitely  represented  by  a  plate  or  shield — the  embryonic  plate,  composed 
of  three  layers  of  tissues — an  upper  or  ectodermal  derived  from  the  floor 
of  the  amniotic  vesicle  ;  a  lower  or  endodermal  formed  by  the  roof  of  the 
archenteric  vesicle  and  an  intermediate  formed  by  mesoderm  or  mesoblast. 
On  the  upper  surface  of  the  flat  embryonic  plate,  which  has  a  total  length 
of  about  1-5  mm.,  appears  on  its  hinder  or  caudal  half,  the  primitive  streak  ; 
at  the  anterior  end  of  the  streak  a  perforation  is  formed — the  neurenteric 


DEVELOPMENT  OF  THE  OVUM 


17 


canal  which  jjlaces  the  amniotic  cavity  in  communication  with  the  arch- 
enteric  vesicle  (Fig.  18).  The  nature  of  the  primitive  streak  and  of  the  neur- 
enteric  canal  we  shall  discuss  later  (see  p.  38).  The  archenteric  vesicle 
has  also  undergone  a  rapid  growth,  now  measuring  2  mm.  in  diameter 
and  we  can  recognize  in  it  (see  Fig.  18)  the  beginning  of  a  division  into 
two  parts,  the  yolk  sac — which  contains  a  stock  of  nourishment  and  will 
come  to  lie  outside  the  embryo  and  a  part  which  remains  applied  to  the 
embryonic  plate  and  will  form  the  alimentary  canal  system.  The  part 
which  will  come  to  lie  within  the  embryo  already  shows  a  division  into  three 
parts — a  forward  diverticulum — the  rudiment  of  the  foregut,  a  posterior 
diverticulum — the  rudiment  of  the  hind  gut  and  an  outgrowth  from  the 


HEADFOLO 


MEDULLARY 
PLATE. 


Fig.  19. — The  formation  of  the  medullary  folds  and  somites  on  the  embryonic  plate. 
From  Prof.  Pfannenstiel's  model  of  an  embryo  measuring  1"95  mm.  in  length. 

hind  gut — which  represents  the  structure  known  as  the  allantois.  The 
embryonic  plate,  with  the  amniotic  and  archenteric  vesicles,  is  bound 
to  the  chorion  by  the  body-stalk  (Fig.  18) — the  rudiment  of  the  umbilical 
cord.  Thus  towards  the  close  of  the  third  week  human  pregnancy  is 
represented  by  (1)  an  embryonic  plate,  (2)  a  yolk  sac,  (3)  amnion,  (4) 
body-stalk,  (5)  chorion. ^ 

We  shall  now  concentrate  our  attention  on  the  growth  of  the  embryo 
which  from  the  end  of  the  third  week  to  the  end  of  the  fifth  undergoes  a 
very  rapid  transformation.  The  changes  to  be  described  follow  very 
rapidly  and  constitute  a  sixth  stage.  About  the  end  of  the  third  week 
two  folds — the  medullary  folds — begin  to  rise  up  along  the  head  end  of  the 

^  For  recent  literature  on  embryos  at  Stasje  V.  see  Geo.  L.  Streeter,  Contributions 
to  Embryology,  1920,  vol.  9,  p.  389;  N.  W.  Ingalls,  ditto,  1918,  vol.  7,  p.  Ill  ;  H. 
Triepel,  Anat.  Hefte,  1916,  vol.  54,  p.  149. 


18      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

embryonic  plate  (Fig.  19),  thus  enclosing  the  neural  plates  from  which  the 
brain  and  spinal  cord  are  to  be  developed.  The  rising  up  of  the  medullary 
folds  is  accompanied  by  the  appearance  of  another  very  important  de- 
velopmental process — the  cleavage  or  segmentation  of  the  mesoderm  on 
each  side  of  the  medullary  folds  into  segments  or  somites.  Segmentation 
commences  in  the  posterior  region  of  the  head  and  spreads  backwards. 
Thus  the  head  region  of  the  embryo  is  the  first  to  be  differentiated  and  we 
have  every  reason  to  suppose  that  the  segments  at  the  cranial  end  are  the 
oldest  in  an  evolutionary  sense.  With  the  appearance  of  somites  the 
human  embryo  manifests  its  vertebrate  character. 

The  human  embryo  reconstructed  from  serial  sections  by  Professor 
Thompson  in  1907  shows  the  great  advance  made  during  the  fourth  week  ^ 


FORE    BRAIN  -- 


ATTACHMENT 

of 
AMNION 


Fig.  20. — Human  Embryo  2' 5  mm.  long,  towards  the  end  of  the  fourth  week  of  develop- 
ment.    (Professor  Peter  Thompson.) 

(Fig.  20).  The  medullary  folds  have  fused  along  their  crests  and  enclosed 
the  neutral  plates  to  form  a  canal  or  tube,  the  process  of  segmentation  is 
spreading  rapidly  backwards,  the  head  and  gill  arches  can  now  be  recog- 
nized and  although  the  embryo  measures  as  yet  less  than  3  mm.  in  length, 
the  main  parts  of  the  adult  body,  saving  the  limbs,  are  clearly  foreshadowed. 
We  have  reached  what  may  be  termed  a  seventh  stage  in  the  develop- 
ment of  the  human  embryo.  We  must  now  turn  to  some  of  the  chief 
internal  changes  which  have  been  taking  place,  and  this  can  best  be  done  by 

1  For  description  of  embryos  in  the  4th  week  of  development,  and  for  references  to 
other  descriptions  see  P.  Thompson,  Journ.  Anat.  and  Physiol.  1907,  vol.  41,  p.  159  ; 
F.  Wood  Jones,  Proc.  Anat.  Soc.  Grt.  Brit.  June,  1903  ;  A.  Low,  Journ.  Anat.  and 
Physiol.  1908,  vol.  42,  p.  237  ;  W.  E.  Dandy,  Amer.  Journ.  Anat.  1910,  vol.  10,  p.  85  ; 
Sussana  P.  Gage,  Amer.  Journ.  Anat.  1905,  vol.  4,  p.  409.  For  the  most  recent  de- 
scriptions :  see  Prof.  D.  Waterston,  Journ.  of  Anat.  1915,  vol.  49,  p.  92  ;  J.  Crawford 
Watt,  Contributions  to  Embryology,  1915,  vol.  2,  p.  5. 


DEVELOPMENT  OF  THE  OVUM 


19 


comparing  sections  across  the  flat  embryonic  plate  of  a  pregnancy  in  the 
third  week  of  development  (Stage  V.)  with  a  section  of  one  such  as  made 
by  Professor  Thompson  which  has  reached  Stage  VII.  (see  Fig.  21,  A,  B). 
When  such  sections  are  compared  the  following  changes  will  be  noted  : 
(1)  A  narrow  plate  of  modified  ectoderm  or  epiblast,  stretching  along  what 
will  be  the  median  dorsal  line  of  the  body,  becomes  depressed,  thus  forming 
the  floor  of  a  groove  ;  the  lateral  margins  of  the  groove  rise  up,  meet 
together  and  fuse  along  the  middle  line.  Out  of  the  neural  tube  thus 
enclosed  are  developed  the  brain  and  spinal  cord.  (2)  In  a  somewhat 
similar  manner  a  strip  of  cells  along  the  median  dorsal  wall  of  the  arch- 
enteron  is  separated  as  a  tube  to  form  the  notochord  (Fig.  21,  A,  B).  Round 
the  notochord  are  developed  the  spinal  column  and  the  greater  part  of  the 
base  of  the  skull.     (3)  Indications  are  to  be  seen  of  a  sejDaration  of  the 


Fig.  21. — Schematic  Transverse  Sections  of  two  Human  Embryos. 
A,  In  the  3rd.  week  of  development.    B,  In  the  4th  week  of  development. 
The  numbers  are  placed    on  corresponding   points  :    Epiblast,  shaded ;    hypoblast, 
black ;  mesoblast,  stippled. 

1.  Neural  groove  and  canal. 

2.  Epiblast  of  embryo. 

3.  Epiblast    lining    amnion. 


Only  the 
attachment  of  the  amnion  is  repre- 
sented in  B. 

4.  Paraxial  mesoblast. 

5.  Intermediate  cell  mass. 


6.  Coelom,  bounded  by  the  somato- 
pleure  externally  and  splanch- 
nopleure  internally. 

8.  Mesoblast  on  amnion. 

9,  10.  Chorion. 

11.  Notochord. 

12.  Archenteron. 


archenteron  into  an  intra-embryonic  part,  which  will  form  the  alimentary 
tract,  and  an  extra-embryonic  part,  which  becomes  the  yolk  sac.  (4) 
The  mesoderm  on  each  side  of  the  embryo  shows  a  division  into  four  parts  : 
(a)  paraxial  mesoderm  (4,  Fig.  21)  from  which  the  voluntary  musculature, 
as  well  as  other  parts  of  the  body  system  arise  ;  (6)  intermediate  cell  mass 
(5,  Fig.  21),  in  which  the  renal  and  genital  organs  are  developed  ;  (c) 
somatic  mesoderm,  this  layer  with  the  ectoderm  over  it  forms  the  som- 
atopleure,  the  outer  wall  of  the  coelom  ;  from  the  coelom  are  developed  the 
pericardium,  pleura  and  peritoneum  ;  [d)  the  splanchnic  mesoderm,  which 
covers  the  intestine  and  yolk  sac  ;  the  splanchnic  mesoderm  and  entoderm 
together  form  the  splanchnopleure.  (5)  Indications  can  be  seen  of  the 
division  of  the  coelom  into  intra-  and  extra-embryonic  parts  (6,  Fig.  21). 


20 


HUMAN  EMBRYOLOaY  AND  MOEPHOLOGY 


Gill  arches 


UMBILICAL  CQRO 


LE& 


When  the  umbilicus  contracts  and  closes,  these  two  parts  of  the  coelom  are 
finally  separated.  It  is  also  during  the  fourth  week  that  the  paraxial 
mesoderm  becomes  separated  into  primitive  segments,  or  somites,  formerly 
known  as  protovertebrae.  In  the  embryo  shown  in  Fig.  20  twenty-three 
pairs  are  already  separated. 

In  Fig.  22  is  represented  the  fully  differentiated  human  embryo — a 
stage  of  development — which  we  may  call  the  eighth  stage,  reached  about 

the  end  of  the  fifth  week. 
From  crown  to  rump  the 
embryo  about  this  time 
measures  5  mm. — one-fifth 
of  an  inch.  The  buds  of  the 
upper  and  lower  extremi- 
ties have  now  appeared ; 
segmentation  has  reached 
almost  to  the  tip  of  the  tail, 
there  being  three  occipital 
and  35  body  somites,  the 
last  representing  the  sixth 
coccygeal  or  caudal.  The 
mouth  is  becoming  ap- 
parent ;  so  are  the  eye  and 
nose  ;  the  gill  arches,  four 
of  which  are  apparent  in 
the  neck,  have  reached 
their  highest  development  ;  a  blood  circulation  is  now  fully  established, 
the  body  stalk  having  been  transformed  into  the  umbilical  cord.  The 
yolk  sac  is  now  joined  to  the  embryonic  gut  by  a  long  narrow  duct — the 
vitello-intestinal  duct  (Fig.  22). 

Origin  of  Ova  and  Spermatozoa. — Towards  the  end  of  the  stage 
just  described,  the  genital  ridges  arise  from  the  intermediate  cell  mass  and 
project  into  the  coelom,  one  at  each  side  of  the  root  of  the  mesentery 
(Fig.  23).  The  mesothelial  cells  which  line  the  coelom  assume  a  columnar 
form  at  the  root  of  the  mesentery  and  over  the  genital  ridges  ;  between 
these  cells  appear  primitive  germ-cells  (primordial  ova)  characterized  by 
their  large  size  and  reaction  to  certain  stains.  Hitherto  it  has  been  as- 
sumed that  the  germ-cells  arose  from  the  mesothelial  columnar  cells 
which  cover  the  ridge.  Beard,  during  a  prolonged  and  accurate  investiga- 
tion of  the  development  of  fishes,  especially  of  the  skate,  discovered  that 
the  germinal  cells  were  not  formed  in  the  genital  ridges  but  appeared  at  a 
very  early  stage  corresponding  to  that  described  here  as  Stage  I.  When 
the  coelom  is  formed  they  migrate  towards  the  genital  ridges.  There  is 
nothing  strange  in  such  a  migration  for  it  is  a  daily  occurrence  in  the  adult 
body  for  leucocytes  to  be  drawn  in  crowds  to  a  site  of  infection  by  an 
obscure  force  which  is  named  chemotactic.  It  is,  then,  far  from  unlikely 
that  the  primitive  germ-cells  are  separated  at  an  early  stage  in  the  division 
of  the  ovum  and  then  subsequently  seek  a  nidus  in  the  genital  ridge. 
We  shall  see  that  nerve  cells  migrate  under  similar  influences.     We  may 


Fig.  22. — Showing  a  human  embryo,  5  mm.  in  length, 
at  the  end  of  the  5th  week  of  development.  (After  Keibel 
and  Mall.) 


DEVELOPMENT  OF  THE  OVUM 


21 


suspect  tiaat  the  germ-cells  whicli  fail  to  reach  the  suitable  nidus,  which 
the  genital  ridges  afford,  are  absorbed,  or,  as  Beard  has  suggested,  they 
may  give  rise  to  those  curious  tumours  known  as  teratomata.  The  manner 
in  which  the  primitive  germ-cells  are  carried  within  the  genital  ridge  by 
tubular  incursions  of  the  mesothelium  covering  the  ridge  has  been  already 
described,  but  we  are  ignorant  of  the  circumstances  which  determine  the 
production  of  spermatozoa  and  a  testicle  in  one  individual,  and  the  forma- 
tion of  ova  and  an  ovary  in  another.  It  is  not  until  the  embryo  has  attained 
a  length  of  15  mm.  in  the  seventh  week  of  development  that  it  is  possible 
to  distinguish  testicle  from  ovary. 


POST.  CARD.VETIN 


POST .CARD    VEIN 


Fig.  23. — Diagrammatic  Section  of  the  roof  of  tlie  Coelomic  cavity  of  a  human 
embryo  in  the  fifth  week  of  development,  showing  the  position  of  the  Genital 
Ridges  in  which  the  Ovary  or  Testicle  is  formed. 

Thus  in  the  space  of  five  weeks  the  cycle  which  produces  new  human 
seed  from  the  old  is  accomplished  and  all  the  parts  of  a  new  human  body 
are  laid  down  in  outline.  In  these  five  weeks  the  fertilized  ovum  has  given 
rise  to  (1)  germ-cells  which  are  endowed  with  the  combined  properties 
of  the  ovum  and  spermatozoon  from  which  they  were  produced  ;  (2)  an 
embryo  in  which  these  cells  are  nourished  and  protected  ;  (3)  membranes 
by  which  the  embryo  is  protected  and  nourished  during  intrauterine  life. 

Having  thus  followed  the  chief  developmental  changes  of  the  ovum,  and 
seen  how  the  embryo,  the  membranes  and  the  reproductive  cells  are 
differentiated,  we  shall  review  in  the  next  chapter  the  manner  in  which 
the  ovum  establishes  itself  in  the  cavity  of  the  uterus  and,  for  the  space 
of  nine  months,  passes  a  parasitic  life  there. 


CHAPTER  II. 

THE   MANNER   IN   WHICH  A  CONNECTION  IS   ESTABLISHED 
BETWEEN  THE   FOETUS   AND   UTERUS. 

The  Decidua.^ — Every  menstrual  period,  the  mucous  membrane  wliicli 
lines  the  cavity  of  the  uterus  becomes  hypertrophied  and  its  vessels  con- 
gested. If  the  ovum  be  not  fertilized,  then  the  surface  layer  of  the  mucous 
membrane  dies  and  is  cast  ofi,  but  if  fertilization  occur  then  the  process 
of  hypertrophy  proceeds  and  the  mucous  membrane  now  receives  the  name 
of  decidua.  The  formation  of  the  decidua  is  characterized  by  (1)  the 
production  of  decidual  cells — cells  with  a  more  or  less  rounded  outline, 
large  cell-body  and  relatively  small  nucleus— from  the-  connective  tissue 
cells  which  lie  beneath  the  epithelial  lining  of  the  mucous  membrane  and 
between  the  tubular  glands  embedded  in  the  mucous  membrane  (Fig. 
17,  p.  15)  ;  (2)  the  epithelial  lining  proliferates,  the  surface  of  the  mucous 
membrane  becoming  rugose  with  pits  and  depressions  ;  (3)  the  uterine 
glands  become  elongated  and  branched  ;  ^  their  mouths  are  closed  by  the 
growth  of  the  decidual  cells  ;  their  fundi,  abutting  against  the  muscular 
coat,  undergo  no  change  ;  the  elongated  bodies  of  the  tubes,  between  their 
mouths  and  fundi  form  cavernous  spaces  ;  (4)  the  vessels  of  the  uterus 
increase  in  size  and  the  capillaries  of  its  mucous  membrane  are  dilated. 
In  this  manner  the  uterus  is  prepared  to  receive  the  fertilized  ovum.  It  is 
highly  probable  that  these  changes  are  influenced  by  an  ovarian  secretion, 
for,  when  the  ovaries  are  removed,  these  changes  soon  cease  to  occur. 
The  internal  secretion  of  the  corpus  luteum  exercises  a  stimulant  action 
on  the  uterine  tissues. 

Implantation  of  the  Ovum.^ — When  the  fertilized  ovum  reaches 
the  cavity  of  the  uterus  it  has  already  attained  the  blastocyst  form  (Fig. 
14,  p.  12).  The  inner  cell  mass,  from  which  the  embryo  will  arise,  projects 
within  the  cavity  and  is  protected  by  the  enveloping  layer  or  trophoblast 
of  the  blastocyst,  the  whole  ovum  measuring  about  '5  mm.  in  diameter. 
Implantation  occurs  in  one  of  the  pits  of  the  mucous  membrane  usually 
on  the  posterior  wall  of  the  cavity  near  the  fundus  of  the  uterus,  but  it 
may  occur  anywhere,  that  form  being  especially  dangerous  in  which 
implantation  occurs  in  the  neighbourhood  of  the  internal  mouth  of  the 
uterus.     The  area  of  the  trophoblast  in  contact  with  the  uterine  pit  grows 

1  See  Baumgartner,  Amer.  Journ.  Anat.  1920,  vol.  27,  p.  203. 

^  See  references,  p.  13. 

22 


THE  FOETUS  AND  UTERUS 


23 


rapidly  and  throws  off  proliferating  masses  of  syncytium  (Fig.  17,  p.  15) 
which  burrow  into  the  decidua,  thus  embedding  and  anchoring  the  blasto- 
cyst and  by  the  absorption  of  the  decidual  tissue,  providing  nourishment 
for  it.  The  blastocyst  is  peculiar  in  man  and  the  anthropoids  in  that  it 
becomes  completely  buried  in  the  decidua.  The  parts  of  the  decidua 
are  thus  distinguished  :  (1)  the  decidua  serotina  or  basalis,  the  part  to  which 
the  ovum  became  attached  and  into  which  the  processes  of  syncytium 
grow  (Figs.  17  and  24) ;  (2)  the  decidua  capsularis  or  refiexa,  the  part 
which  covers  the  ovum  and  is  stretched  as  the  ovum  grows  ;  (3)  the  decidua 
vera,  which  lines  the  rest  of  the  uterus.  The  decidua  vera  ends  at  the 
internal  os,  the  canal  of  the  cervix  producing  no  true  decidual  layer. 


0£ClDUA     VERA 
'5*V,tOEClDUA    RErLECTA 

,DE.CIDL/A    BASALIS 


BLASTODERMIC 
VESICLE. 


DECIDUA   VERA 


DECIDUA    VERA 


Fig.  24. — Section  of  the  Uterus  showing  in  a  diagrammatic  manner  the  Embedded 
Ovum  and  the  differentiation  of  the  Decidua  into  Three  Parts. 


With  the  growth  of  the  embryo  the  decidua  refiexa  is  brought  in  contact 
with  the  decidua  vera.  By  the  fifth  month  they  have  fused  together, 
become  flattened  and  partially  atrophied.  The  decidua  serotina,  on  the 
other  hand,  forms  the  basis  in  which  the  placenta  is  developed. 

Nourishment  of  the  Early  Ovum.^ — The  ova  of  birds  and  reptiles 
are  laden  with  yolk  and  on  this  the  developing  embryo  lives — the  yolk 
being  absorbed  by  the  entodermal  cells  lining  the  archenteron.  Primitive 
forms  of  mammals,  such  as  the  Duckbill  and  Echidna,  have  also  large 
supplies  of  yolk  in  their  ova,  but  in  all  other  mammals  the  ova  contain 
only  a  small  supply  of  yolk  ;  hence  the  developing  ovum  has  to  draw  its 
nourishment  from  the  uterus.  The  secretion  of  the  uterine  glands  contains 
a  proteid  (Emrys  Roberts)  which  probably  affords  nourishment  to  the 

1  E.  Emrys  Roberts,  Journ.  Anat.  and  Physiol.  1910,  vol.  44,  p.  192  (Embedding  of 
Ovum  and  Nutrition — Guinea-pig) ;  Emrys  Roberts,  Proc.  Roy.  Soc.  May  21,  1908. 


24      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

ovum.  The  decidual  cells  contain  vacuoles  of  fat  and  glycogen  (J.  W. 
Jenkinson)  and  these  cells  and  their  contents  are  absorbed  by  the  tropho- 
blast  and  passed  on  to  the  growing  tissue. 

Evolution  of  the  Foetal  Membranes. — In  the  first  chapter  we  have 
seen  that,  almost  from  the  beginning  of  development  the  embryogenic 
mass  of  cells  is  enclosed  within  a  vesicle  formed  of  trophoblastic  cells. 
With  the  addition  of  a  mesodermal  tissue  the  trophoblastic  vesicle  becomes 
the  chorion,  the  most  important  of  the  membranes  which  envelop  the 
foetus  ;  it  comes  soon  to  serve  the  foetus  as  lungs,  stomach  and  kidneys. 
We  have  also  seen  the  origin  of  the  amniotic  cavity  and  its  surrounding 
membrane,  the  amnion  ;  we  saw,  too,  how  the  yolk  sac  arose  from  the 
primitive  gut  cavity  or  archenteron,  and  also  how  a  diverticulum,  known 
as  the  AUantois,  arose  from  the  hinder  end  of  the  archenteron.  They  all 
appear  so  simply  and  in  so  regular  a  sequence  that  we  are  apt  to  forget, 
from  an  evolutionary  point  of  view,  that  they  are  relatively  of  recent 
origin.  We  can  only  understand  their  true  nature  by  an  appeal  to  com- 
parative anatomy.  The  structures  just  named  are  not  found  in  the  lower 
vertebrates — amphibians  and  fishes,  only  in  the  higher — reptiles,  birds 
and  mammals  ;  yet  we  are  certain  that  the  higher  were  evolved  from  the 
lower  and  that  therefore  these  structures  were  evolved  during  the  early 
history  of  the  higher  vertebrates.  We  can  see  that  such  a  highly  evolved 
structure  as  the  human  body  is  at  birth  could  not  have  come  into  existence 
unless  provision  had  been  made  for  maintaining  the  individual  during  the 
months  of  embryonic  and  foetal  life.  Nature  evidently  accomplished 
this  miracle  without  calling  into  being  any  new  kind  of  structure  ;  the 
chorion,  amnion,  allantois  and  yolk  sac  of  higher  vertebrates  were  pro- 
duced from  structures  already  in  existence  in  their  lower  vertebrate 
ancestors.  The  yolk  sac,  we  shall  see,  is  part  of  the  bowel  which  has  under- 
gone an  exaggerated  and  precocious  development  and  is  cut  ofi  from  the 
rest  of  the  bowel  and  cast  away  by  a  species  of  natural  surgery,  when  it 
has  served  its  purpose  in  the  upbuilding  of  the  embryo.  The  allantois 
has  been  evolved  by  a  precocious  development  and  overgrowth  of  the 
apical  portion  of  the  primitive  bladder  ;  when  this  apical  part  has  served 
its  foetal  purpose  it  too  is  sacrificed  and  the  site  of  its  separation  closed. 
More  marvellous  still  is  the  origin  of  amnion  and  chorion  ;  they  represent 
parts  of  the  ventral  body  wall  which  have  been  so  hurried  forwards  in 
point  of  time  of  development  that  they  are  actually  produced  in  the  human 
blastocyst  before  the  main  part  of  the  embryonic  body  has  commenced 
to  form.  The  membranes  which  envelop  the  foetus — the  amnion  and 
chorion — are  precocious  overgrowths  of  a  part  of  the  body  wall  which  is 
removed  at  birth,  the  umbilicus  representing  the  scar  which  marks  the 
site  of  amputation. 

The  Yolk  Sac, — The  most  ancient  method  of  providing  for  the  growth 
of  the  embryo  is  by  loading  the  ovum  with  yolk  or  vitellus.  Everyone  is 
familiar  with  the  embryonic  provision  stored  within  a  fowl's  egg  ;  in 
birds  and  reptiles  the  vitelline  system  reaches  its  highest  development. 
The  meal  or  yolk  is  already  in  the  egg  before  there  is  an  embryonic  stomach 
to  digest,  absorb  and  serve  it  up  as  nourishment  to  the  growing  embryonic 


THE  FOETUS  AND  UTERUS 


25 


structures.  Hence  one  of  the  earliest  efforts  in  the  developing  chick  is 
to  throw  a  containing  wall — the  archenteron — round  the  yolk.  Even  in 
the  human  embryo  the  yolk  sac  plays  a  very  important  part  in  the  up- 
building of  the  body.  In  Fig.  25  is  shown,  in  a  somewhat  diagrammatic 
form,  the  yolk  sac  of  a  human  embryo  at  the  end  of  the  third  week  of 


UMBILICAL    VEIN 
ALLANT0I3 

UMBILICAL  ARrepy 

HEART 
VITELLIHE    VEIN 


Fig.  25. — The  Yolk  Sac  and  early  vessels  of  the  human  embryo  about  the  end  of  the 
3rd  week  of  development.     (Modified  from  Eternod.) 

development  when  the  yolk  sac  measures  2  mm.  in  its  longest  diameter 
and  is  at  its  point  of  maximum  importance.  A  circulation  has  not  yet 
been  established,  but  blood  vessels  and  blood  islands  are  being  rapidly 
formed  in  the  mesodermal  or  mesenchymal  tissues  covering  its  entodermal 
lining  (Fig.  26).  The  aortae — right  and  left — are  being  laid  down  and 
numerous  communications  are  being  opened  up  between  the  aortae  and 


nieso blast 
uasoform.  cell 
hypoblast 


erythrocyte 


blood  ues. 


Fig.  26. — Section  across  the  wall  of  the  Yolk  Sac,  showing  blood  vessels  and  nucleated 
redblood  corpuscles  forming  in  its  mesoblastic  layer.     (After  Selenka.) 

the  vascular-plexus  system  of  the  yolk-sac  and  also  between  the  yolk-sac 
system  and  the  venous  end  of  the  cardiac  tube  where  the  vitelline  veins 
are  forming.  We  see  all  the  parts  being  prepared  for  the  establishment 
of  a  vitelline  circulatory  system.  By  the  end  of  the  fifth  week  of  develop- 
ment the  yolk-sac  lies  outside  the  body  of  the  embryo  (Fig.  22,  p.  20)  and 
is  now  joined  to  the  bowel  by  a  narrow  canal — the  vitello-intestinal  duct. 
Very  soon  after  this,  the  duct  closes  and  atrophies,  but  the  sac  itself  continues 
to  grow  until  it  reaches  a  diameter  of  4  or  5  mm.  Its  further  history  we 
shall  examine  later  (p.  88),  bub  here  we  may  state  that  when  the  umbilical 


26      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

cord  is  fashioned  the  remains  of  the  vitello-intestinal  duct  are  enclosed 
within  it,  while  the  sac  itself  will  be  found  at  or  near  the  placental  end  of 
the  cord.  In  Fig.  26  is  shown  a  section  across  a  small  part  of  the  wall  of 
the  yolk  sac  to  illustrate  the  manner  in  which  embryonic  blood  corpuscles 
(erythrocytes)  and  blood  vessels  are  formed  in  the  mesodermal  or  mesoblastic 
stratum  of  its  wall.  The  lining  entoderm  or  hypoblast  also  gives  rise  to 
glandular  structures. 

The  Allantois. — The  allantois  appears  during  the  third  week  of  de- 
velopment of  the  human  embryo  as  an  outgrowth  from  the  hinder  end  of 
the  archenteron  or  primitive  gut  cavity.  To  understand  its  true  nature 
we  must  examine  the  structures  in  the  lower  vertebrates  from  which  the 
allantois  has  been  evolved.  These  parts  are  represented  in  Fig.  27 — 
depicting  a  condition  found  in  amphibia.  The  rectum  and  also  the  ducts 
of  the  testes — ^the  two  Wolffian  ducts — end  in  a  terminal  passage — the 
cloaca.  An  expansion  or  diverticulum  of  the  cloaca  has  been  established 
as  a  receptaculum  for  urine — the  bladder.     The  blood  supply  is  peculiar. 


POftrAL  VEIN 


CLOACA 


MeN.MESeNTEKY 


FEMORAL  VEINS 
BLAOOCR 
ABDOMIHAU   VEIN 
VEMTHAL  WAlC  or  ASOOMCN 


Fig.  27. — The  cloaca,  bladder  and  abdominal  vein  of  a  frog. 

A  large  vein  passes  along  the  inner  aspect  of  the  ventral  wall  of  the  belly, 
draining  the  blood  from  the  bladder  and  from  the  ventral  wall  of  the  belly, 
as  well  as  from  the  hind-limbs  and  ending  with  the  vein  from  the  bowels 
and  stomach  in  the  portal  system  of  the  liver.  Originally  this  ventral 
abdominal  vein  is  double,  there  being  a  right  and  left  vein  which  convey 
the  blood  of  the  bladder  and  of  the  ventral  wall — but  not  that  of  the  limbs 
— the  connection  of  the  femoral  veins  is  secondary — direct  to  the  heart. 
The  arteries  which  supply  the  bladder  and  ventral  wall  spring  from  the 
common  iliac  arteries — these  latter  vessels  representing  direct  continuation 
of  the  right  and  left  primitive  aortae.  If,  then,  the  allantois  represents  a 
precocious  outgrowth  from  the  apical  region  of  the  bladder  and  the  chorion 
and  amnion  enormous  and  premature  expansions  from  the  ventral  belly 
wall,  we  expect  that  their  arteries  would  arise  from  the  hinder  ends  of  the 
embryonic  aortae  and  their  veins  pass  forwards  on  the  body  wall  to 
terminate  at  first  in  the  heart  and  afterwards  in  the  liver.  That  is  exactly 
what  we  do  find,  as  may  be  seen  from  a  reference  to  Fig-.  25. 

To  see  the  allantois  in  its  complete  form  one  must  examine  the  developing 
chick  embryo  (Fig.  28).  The  young  of  animals  which  are  developed  within 
a  shell,  need  a  recej^taculum  for  the  secretion  from  their  kidneys  ;  for  this 
reason  alone  one  can  understand  the  expansion  of  the  embryonic  bladder. 


THE  FOETUS  AND  UTERUS 


27 


But  even  in  tlie  cliick  its  use  as  a  store  place  for  urinary  excretions  has 
become  of  minor  importance  ;  the  mesodermal  tissue  which  clothes  the 
bladder  has  become  the  most  important  element  ;  it  has  grown  exceedingly 
rich  in  vascular  tissue.  As  the  allantois  expands  in  the  developing  chick 
its  vascular  surface  becomes  applied  to  the  inner  aspect  of  the  chorion 
through  which  it  can  absorb  oxygen  and  discharge  carbon  dioxide.  The 
apical  part  of  the  bladder  has  thus  become  converted  into  a  "  foetal  lung," 
but  its  vessels  are  those  we  have  just  noted  in  the  ventral  area  of  the  frog  ; 
its  arteries — the  umbilical  arteries  appear  to  be  direct  continuations  of  the 
two  aortae,  and  its  veins — the  umbilical  veins,  pass  to  the  heart  and  after- 
wards to  the  liver,  just  as  in  the  frog. 

In  the  human  embryo,  as  is  the  case  in  all  developing  primates,  the 
cavity  of  the  allantois  is  never  represented  by  more  than  a  tubular  out- 
groAvth  into  the  body  stalk  (see  Figs.  18  and  25)  and  even  this  degenerates 
very  soon.  The  human  embryo  has  no  need  for  a  bladder,  as  it  can  dis- 
charge its  urinary  excretion  into  the  maternal  circulation  as  soon  as  the 

chorion 
amnion 


allantois 
embryo 

coelom 


■yolk  sac. 


Fig.  28.— The  primitive  form  of  the  Allantois.     (After  Turner.) 

chorionic  circulation  is  established.  It  is  otherwise  with  the  mesodermal 
covering  of  the  allantois  ;  we  shall  see  that  this  element  takes  the  chief 
part  in  the  vascularization  of  the  chorion.  In  Fig.  25  it  will  be  seen  that 
the  umbilical  vein  is  connected  with  the  vascular  system  of  the  yolk  sac 
at  the  root  of  the  allantoic  diverticulum.  We  may  regard  the  allantoic 
circulation  as  an  enormous  expansion  from  the  more  primitive  circulation 
of  the  archenteron. 

The  Evolution  of  the  Amnion  and  Chorion. — If  our  knowledge 
were  confined  to  the  highly  sj^ecialized  processes  which  give  rise  to  the 
amnion  and  chorion,  the  enveloping  membranes  of  the  human  embryo, 
it  would  be  almost  impossible  for  us  to  guess  that  these  structures  represent, 
in  reality,  folds  of  the  embryo's  own  belly  wall.  They  come  into  existence 
before  even  the  embryo  itself  is  apparent.  Their  very  humble  but  marvel- 
lous origin  is  illuminated  when  we  examine  the  manner  in  which  they  arise 
in  reptiles,  birds  and  the  lowest  mammals.  In  Fig.  29,  which  represents 
diagrammatic  sections  across  chick  embryos,  the  origin  of  the  enveloping 
membranes  is  set  out  in  a  pictorial  form.  The  somatopleure  or  body 
wall  is  seen  to  arise  as  a  fold  at  each  side  of  the  embryo  and  moimting 
upwards  ultimately  meet  and  fuse  along  the  median  dorsal  line.     The  inner 


28 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


fold  separates  from  the  outer  and  forms  the  amnion  ;  the  outer  remains 
as  a  membrane  enveloping  the  embryo,  amnion,  yolk  sac  and  allantois 
and  is  the  basis  of  the  chorion — the  prechorion  it  is  named  in  the  chick. 
From  the  diagrams  one  would  infer  that  the  greater  part  of  the  chick's 
body  wall  was  folded  oif  to  form  the  enveloping  membranes  but  when  we 
remember  that  the  yolk  sac  represents  a  premature  but  enormous  develop- 
ment of  a  localized  part  of  the  bowel,  we  may  justly  conclude  that  the 
enveloping  part  of  the  soniatopleure  represents  a  limited  area  of  the 
ventral  part  of  the  abdominal  wall — the  part  drained  by  the  ventral 
abdominal  vein,  which  has  become  greatly  expanded.  The  assignation  of 
part  of  the  somatopleure  to  form  the  enveloping  membranes  involves  no 


EhlBRYO 


Gut 


LINE  OF  UNION 

EMBRYO 


SPLf\NCHNOPLEUR£. 


Fig.  29. — Illustrating  the  manner  in  wliich  tlie  chorion  and  amnion  arise  in  the 
chick  embryo  from  folds  of  the  somatopleure — the  body  wall  of  the  embryo. 
In  A,  the  folds  are  seen  in  the  act  of  growing  upwards  to  cover  the  embryo ; 
on  B,  they  have  met  over  the  embryo. 

sacrifice  of  muscle  or  nerve  in  the  belly  wall  of  the  embryo  ;  we  shall  see 
that  these  elements  invade  the  somatopleure  long  after  the  membranes 
have  separated.  Only  two  elements  of  the  belly  wall  have  been  utilized 
in  the  formation  of  the  amnion  and  chorion  :  (].)  the  epithelial  or  ecto- 
dermal covering  of  the  skin  which  takes  on  a  phagocytic  action  ;  (2) 
the  mesodermal  element  which  gives  rise  to  connective  tissue,  blood 
vessels  and  blood  cells.  Prof.  J.  P.  Hill  ^  has  demonstrated  that  in  the 
developing  marsupial  ovum,  when  only  three  cell-divisions  have  occurred 
and  only  16  or  fewer  cells  are  formed,  those  which  are  to  give  rise  to  the 
epithelial  covering  of  the  chorion  and  amnion  can  be  distinguished  from  the 
smaller  number  which  is  to  form  the  embryo.  In  the  human  ovum  it  is 
also  so  ;  we  have  seen  that  the  epithelial  covering  of  the  chorion — the 
trophoblast — is  the  first  structure  to  be  differentiated  in  the  development 
of  the  blastocyst.  In  early  days  primitive  man  required  no  scaffolding  or 
machinery  to  build  his  rude  hut ;    in  great  modern  building  extensive 

1  Quart.  Journ.  Mic.  Sc.  1918,  vol,  63,  p.^91. 


THE  FOETUS  AND  UTERUS 


29 


scaffolding  and  c4aborate  macliiues  have  to  be  erected  before  ever  building 
has  begun.  The  chorion  and  amnion  are  the  scaffolding  thrown  up  for  the 
development  of  the  higher  vertebrates  and  they  were  evolved  out  of 
simple  parts  of  the  belly  wall. 

The  amnion  which  contains  a  fluid  in  which  the  embryo  floats  and  has  its 
very  delicate  growing  tissues  equally  supported  on  all  sides,  is  not  required 
in  the  development  of  fishes  or  amphibians  ;  their  eggs  are  hatched  in 
water  and  the  larvae  live  in  water  and  have  therefore  no  need  of  an  amnion. 
This  structure  became  necessary  when  the  ancestry  of  the  higher  verte- 
brates took  to  a  life  on  land.  To  allow  their  young  to  develop  in  the 
ancestral  medium  the  amnion  was  evolved  from  a  duplicature  of  the 
embryo's  body  wall.  Having  thus  given  a  clue  to  the  evolutionary  history 
of  these  marvels  of  adaptation — the  amnion  and  chorion — we  return  to  note 
stages  by  which  the  placenta  is  produced  from  the  chorion  and  a  foetal 
circulation  established. 

Chorionic  Villi.^ — The  origin  of  the  chorion  from  a  combination  of  two 
elements — the  trophoblast  (enveloping  layer  of  ectoderm)  and  an  extension 
from  the  somatic  mesoderm — has  been  already  traced  (p.  14).  The  division 
of  the  trophoblast  into  a  basal  layer  and  syncytium  was  also  mentioned. 
As  soon  as  the  ovum  is  embedded  in  the  decidua,  processes  of  syncytium 
invade  not  only  the  serotinal  but  also  the  reflected  or  capsular  part  (Fig. 

uterine  vessel- 

submuc.  layer^ 
of  uterus 


decidua 
syncytium 


syncytium. 

basal layen 

mesoblast 
of  chorion 


blood  space 


Fig.  30.- — Diagrammatic  Section  of  the  Decidna  Serotina  (formed  frcm  the  mucous 
membrane  of  uterus)  and  Chorion,  to  show  the  manner  in  vhich  the  placental 
blood  spaces  are  formed. 

17,  p.  15).  Villi,  containing  a  core  of  mesoblast  and  a  covering  of  the 
basal  layer  of  chorionic  epithelium,  grow  out  into  the  syncytial  masses 
(Fig.  30).  The  villi  continue  to  divide  and  redivide  thus  becoming  arbor- 
escent.    During  the  third  week  the  mesodermal  tissue  of  the  chorion, 


1  For  details  and  literature  relating  to  the  formation  of  placental  structures  see 
A.  C.  F.  Eternod,  L'oiuf  humain,  Geneva,  1909  ;  A.  Etemod,  Compt.  Reyid.  Congres 
internal.  d'Anat.  1905,  p.  197  ;  Compt.  Rend.  Assoc,  des  Anatomistes,  1909,  p.  1  ; 
A.  W.  Hubrecht,  Anat.  Anz.  1905,  vol.  31,  No.  13  (Nature  of  trophoblast) ;  J.  W. 
Jenkinson,  Vertebrate  Embryology,  comprising  the  early  history  of  the  embryo  and  its 
foetal  membranes,  1913. 


30      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

particularly  of  its  villi,  becomes  the  site  of  active  formation  of  blood  vessels, 
blood  cells  being  developed  within  the  vascular  lumina.  Similar  formations 
are  taking  place  in  the  body  stalk,  in  the  wall  of  the  yolk  sac  and  also  in 
the  embryo  itself,  so  that  by  the  end  of  the  third  week  a  tubular  heart, 
dorsal  aortae,  vitelline  and  umbilical  veins  communicating  with  a  great 
capillary  network  have  been  laid  down  (Fig.  25,  p.  25).  About  the  end 
of  the  third  week  or  beginning  of  the  fourth  a  circulation  of  blood  has  been 
established  in  the  chorion.  Direct  prolongations  of  the  two  dorsal  aortae 
now  extend  through  the  body-stalk  to  the  chorion — these  extensions  form- 
ing the  umbilical  arteries  (Young  and  Robinson).  The  umbilical  veins 
carry  the  blood  from  the  chorion  through  the  body  stalk  to  the  embryonic 
heart.  The  chorionic  circulation  replaces  functionally  that  of  the  yolk  sac. 
Through  the  chorionic  circulation  the  embryo  is  nourished. 

Formation  of  Placental  Blood  Spaces. — The  decidual  nutriment 
only  affords  a  temporary  supply.  In  the  last  few  years  the  researches  of 
a  number  of  German  investigators,  but  especially  of  Peters  and  Selenka, 
have  shown  that  the  maternal  circulation  is  placed  at  the  disposal  of  the 
chorionic  villi  in  a  simple  manner.  The  syncytium,  as  it  burrows  into  and 
replaces  the  serotinal  part  of  the  decidua  (Figs.  24,  30),  invades  the  maternal 
blood  vessels,  and  replaces  their  walls  by  its  own  tissue.  The  masses  of 
syncytium  between  the  main  villi  break  down  and  thus  form  large  spaces 
into  which  the  decidual  vessels,  which  were  enclosed  by  the  syncytium, 
freely  open  (Fig.  30).  Through  these  spaces  the  maternal  blood  circulates, 
supplied  by  the  uterine  arteries  and  carried  away  by  the  uterine  veins. 
The  trophoblast  contains  a  ferment  which  prevents  coagulation  of  the  blood 
in  the  intervillous  spaces  thus  formed  (Young).  The  extension  of  the  syn- 
cytium, the  formation  of  villi  and  of  blood  spaces,  go  on  until  the  5th 
month.  By  that  time  the  basal  and  syncytial  layers  of  epithelium  on  the 
villi  are  replaced  by  a  single  flattened  layer  of  cells.  The  vascular  villi 
of  the  chorion  hang  within  the  decidual  blood  spaces,  and  draw  from  the 
maternal  blood  oxygen  and  nutriment  for  the  supply  of  the  embryo.  Pro- 
cesses and  partitions  derived  from  the  syncytium  remain  to  bind  the  chorion 
to  the  uterine  wall. 

Formation  of  the  Umbilical  Cord. — At  the  end  of  the  third  week 
of  development  (see  Figs.  18,  p.  16,  25,  p.  25),  when  the  embryo  forms  a 
cap  on  the  yolk  sac  and  a  plate  in  the  floor  of  the  amniotic  cavity,  neither 
umbilicus  nor  umbilical  cord  are  differentiated.  The  body-stalk  unites 
the  caudal  end  of  the  embryo  to  the  inner  wall  of  the  chorion,  and  appears 
to  represent  a  posterior  extension  of  the  embryonic  body,  but  in  reality 
it  is  formed  out  of  a  reflection  of  part  of  its  ventral  wall.  It  serves  the 
purposes  of  an  umbilical  cord  to  the  early  embryo.  A  section  across  the 
body-stalk  (Fig.  31)  shows  that  two  umbilical  arteries,  two  umbilical  veins, 
and  the  canal  of  the  allantois  lie  in  its  mesoblastic  basis,  and  while  its 
upper  epiblastic  surface  projects,  like  the  rest  of  the  embryo,  within  the 
cavity  of  the  amnion,  its  lower  surface  lies  in  the  wall  of  the  extra-embry- 
onic coelom,  in  contact  with  the  yolk  sac.  The  structures  in  the 
body-stalk  are  those  which  we  find  in  the  ventral  belly-wall  of  the 
frog  (Fig.  27). 


THE  FOETUS  AND  UTERUS  31 

To  understand  the  origin  of  tlic  umbilical  cord  one  must  observe  closely 
the  attachment  of  the  amnion  at  this  early  stage.  It  is  attached  to  the 
circumference  of  the  embryo  and  body-stalk  (Figs.  20  and  21)  ;  to  the  zone 
of  somatopleure  which  unites  the  embryo  and  the  amnion,  the  name  of 
junctional  ring  may  be  given,  with  the  clear  understanding  that  the  body- 
stalk  enters  into  the  formation  of  the  posterior  part  of  the  ring.  From  the 
junctional  ring  the  umbilical  cord  is  developed.  While  the  embryo  grows 
rapidly  and  expands  within  the  amnion  the  junctional  ring  retains  its 
embryonic  size  (see  Fig.  2).  The  parts  of  the  yolk  sac  and  coelom  which 
are  surrounded  by  the  ring  now  appear  to  be  constricted  (Fig.  21).  In 
the  second  month  the  junctional  ring  begins  to  elongate  and  form  a  cord- 
like structure,  in  which  an  umbilical  and  a  placental  extremity  can  be 
recognized  (Fig.  32).     The  amnion  is  attached  at  its  j)lacental  extremity. 

mnion 

cauity  of  amnion 

continuation  of 
biedullary  groove 

limb,  veins 

somatopleure 

umb.  artery 

coelom  ♦ 

cauity  of  allantois 
vitelline  duct. 

Fig.  31. — Section  across  the  Body-Stalk.     (His.) 

The  mesoderm  of  the  junctional  ring  forms  the  jelly-like  tissue  (Wharton's 
jelly)  of  the  umbilical  cord  in  which  are  embedded  the  umbilical  arteries 
and  one  umbilical  vein,  formed  by  the  fusion  of  the  right  and  left  vein. 
By  the  third  month  the  cord  measures  12  cm.  ;  and  40  cm.  by  the  ninth 
month.  The  elongation  of  the  junctional  ring  to  form  the  cord  necessarily 
aiiects  all  those  structures  which  lie  within  the  ring — the  neck  of  the  yolk 
sac  (vitello-intestinal  duct),  the  coelomic  space  or  primitive  peritoneal  space, 
the  cavity  of  the  allantois.  All  of  these  are  included  within  the  cord,  and 
are  obliterated  during  its  elongation.  The  coelomic  or  peritoneal  space 
at  the  umbilical  end  of  the  cord  closes  in  the  third  month,  but  it  may  remain 
open  to  birth  and  form  the  seat  of  a  congenital  umbilical  hernia.  As  an 
exceptional  occurrence,  the  intra-embryonic  parts  of  the  allantois  or  of  the 
vitello-intestinal  canal  may  remain  patent  as  far  as  the  umbilicus,  and  with 
the  removal  of  the  cord  at  birth  give  rise  to  a  urinary  or  a  faecal  fistula. 

Formation  of  the  Placenta. — The  condition  of  the  membranes  in  the 
third  month  (Fig.  32)  differs  from  that  of  the  first  month  (Fig.  18)  by  the 
formation  of  the  placenta.  In  the  first  month  the  chorion  is  uniformly 
covered  by  shaggy  villi,  this  being  the  permanent  condition  in  low  primates 
(Lemurs).     In  man  the  chorionic  villi  which  project  within  the  decidua 


32 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


serotina  hypertrophy,  while  those  within  the  decidua  reflexa  atrophy, 
and  in  this  way  the  discoidal  placenta  of  man  is  formed  (Fig.  32).  In 
lower  primates  (Monkeys)  there  are  two  discs  (bi-discoidal),  and  this  form 
occasionally  occurs  in  man. 

The  elements  entering  into  the  formation  of  the  placenta  are  diagram- 
matically  shown  in  Fig.  33.     They  are  : 

1st.  The  decidua  serotina,  formed  from  the  mucous  membrane  of  the 
uterus.     It  is  almost  completely  replaced  by  the  syncytium  and  chorionic 


vein 


artery 


artery 

muscular  coat 
blood  spaces 
vein 

chorionic  villus 
decidua  serotina 

— chorion 
amnion. 

decidua  vera 


decidua  reflexa 


Fig.  32. — Showing  the  arrangement  of  the  Amnion,  Chorion,  and  Decidua  in  the 
3rd  month  and  the  Formation  of  the  Placenta, 


villi.  Only  the  basal  layer  remains  to  furnish  a  new  lining  to  the 
uterus  when  the  membranes  and  placenta  are  shed  after  the  birth 
of  the  child. 

2nd.  The  chorion,  or,  strictly,  prechorion. 

3rd.  An  allantoic  element  which  is  fused  with  the  mesoderm  of  the 
chorion  in  the  hiiman  ovum.  In  the  human  placenta  it  is  impossible  to 
distinguish  the  2nd  from  the  3rd  element ;  both  are  fused  in  the  mesoderm  of 
the  chorion  from  the  beginning. 


THE  FOETUS  AND  UTERUS 


33 


4tli.  The  amnion,  which  becomes  applied  to  the  inner  surface  of  the 
chorion,  thereby  obliterating  the  extra-embryonic  coelom  (Figs.  32,  33). 
Thus  it  will  be  seen  that  almost  the  entire  placenta  is  produced  from  the 
ovum  and  is  truly  a  part  of  the  foetal  structures.  The  decidua,  the  only 
maternal  element,  merely  affords  a  nidus  or  suitable  bed  for  the  develop- 
ment of  the  foetal  structures. 

From  the  inner  surface  of  the  fully-formed  placenta,  the  amnion,  a  thin 
transparent  membrane,  is  easily  stripped  off.  The  outer  or  uterine  surface 
of  the  placenta  is  rough  and  shaggy,  being  mainly  composed  of  the  greatly 
hypertrophied  villi  developed  from  the  serotinal  or  attached  area  of  the 
chorion.  The  villi  are  grouped  in  clumps  or  cotyledons,  between  which 
are  fibrous  strands  and  partitions,  which  pass  through  the  whole  thickness 
of  the  placenta  and  thus  maintain  its  fixation  to  the  uterus.  The  manner 
in  which  the  trophoblast  covering  the  villi  becomes  changed  until  it  forms 


muscular  coat 

decidua  serotina 

prechorion 
(somatopleure) 

a/fantois 
(splanchnopleure) 

amnion 

decidua  vera      decidua  reflexa 

Fig.  33. — Diagrammatic  Section  to  sliow  the  Elements  which  enter  into  the  formation 
of  the  Placenta.  The  trophoblast  on  the  outer  side  of  the  prechorion  has  been 
omitted  for  the  sake  of  simplicity. 

merely  a  thin  epithelial  covering  has  been  already  described  (p.  29).  Into 
the  villi  pass  branches  of  the  umbilical  arteries,  ultimately  forming  a  fine 
capillary  network,  from  which  the  arterialized  blood  is  returned  to  the 
foetus  by  the  umbilical  veins.  Everywhere  the  blood  of  the  foetus  is 
separated  from  that  of  the  mother  by  a  thin  capillary  wall  and  a  layer  of 
flat  epithelial  cells  ;  through  this  wall  exchanges  between  the  foetal  and 
maternal  circulation  take  place.  The  villi  project  within  great  blood 
spaces  formed  in  the  decidua  serotina  (Fig.  32).  The  ovarian  and  uterine 
arteries  end  in  these  blood  sinuses,  and  the  ovarian  and  uterine  veins 
begin  in  them. 

At  full  term  all  the  membranes  of  embryonic  origin  come  away  in  the 
after-birth  ;  also  the  decidua,  except  a  thin,  deep  layer  next  the  uterine 
muscle,  Avhich  contains  the  deepest  parts  of  the  uterine  glands.  From 
this  layer  the  mucous  membrane  of  the  uterus  is  regenerated. 

The  establishment  of  the  developing  ovum  within  the  uterus  of  the 
mother  constitutes  one  of  the  most  marvellous  chapters  of  Embryology. 

c 


M      HUMAN  EMBKYOLOGY  AND  MORPHOLOaY 

It  is  apparent  that  in  the  evolution  of  the  higher  mammals  the  young 
have  become  modified  to  pass  the  first  stage  of  life  as  uterine  parasites. 
In  this  chapter  we  have  seen  that  the  ovum  has  already  reached  a  consider- 
able degree  of  development  when  it  enters  the  uterus  from  the  Fallopian 
tube.  All  the  earlier  steps  in  development  are  directed  towards  the 
formation  of  the  structure  necessary  for  the  protection  of  the  embryo — 
the  chorion,  amnion,  yolk  sac,  allantois  and  placenta. 


CHAPTER  III. 
THE   PKIMITIVE    STREAK,   NOTOCHORD   AND   SOMITES. 

Law  of  Recapitulation.- — The  pioneers  of  Embryology  began  in  the 
hope  of  discovering  the  stages  in  the  evolution  of  the  human  body  by  an 
accurate  study  of  its  development.  It  was  expected  that  the  ovum,  as 
it  became  transformed  into  the  embryo,  and  the  embryo  as  it  changed 
into  the  foetus,  would  recapitulate  man's  evolutionary  history.  From 
what  has  been  related  in  the  two  jDrevious  chapters  it  is  plain  that  we  see 
no  resemblance  between  the  successive  stages  of  the  human  embryo  and 
the  succession  of  types  which  compose  the  scale  of  the  Animal  Kingdom. 
Those  who  expected  the  law  of  recapitulation  to  hold  true  in  all  its  details 
forgot  that  the  human  embryo  is  radically  modified  in  order  that  the  first 
nine  months  of  development  may  be  spent  parasitically  in  the  womb  of 
the  mother.  The  storage  of  yolk  in  the  ovum,  the  precocious  development 
of  trophoblast,  chorion,  amnion  and  allantois,  have  transformed  the 
orderly  manifestation  of  evolutionary  stages.  Yet  to  a  certain  degree  the 
law  remains  true  ;  the  human  body  begins  as  a  single  ceU,  similar  in 
constitution  to  the  simplest  form  of  animal  life — a  protozoon  ;  it  becomes 
a  globular  cluster  of  cells  in  its  morula  stage,  similar  to  the  simple  forms 
of  multicellular  organisms.  Further,  there  are  numerous  features  seen 
during  the  development  of  the  embryo  which  can  only  be  explained  by 
supposing  that  the  human  body,  in  the  course  of  its  evolution,  has  passed 
through  those  stages  which  we  see  represented  in  simpler  Invertebrate 
forms — such  as  the  Hydra  and  the  worm.  The  first  of  these  obscure 
embryonic  manifestations  is  the  primitive  streak  and  groove. 

The  Primitive  Streak. — In  the  third  week  when  the  embryonic  plate 
lies  on  the  upper  surface  of  the  yolk  sac  and  measures  only  about  1  mm. 
(-o\  in.)  in  length,  there  appears  along  the  median  line  of  its  hinder  half  a 
linear  demarcation  known  as  the  primitive  streak.  This  line  becomes  the 
site  of  developmental  processes  of  the  highest  significance.  No  sooner  has 
the  primitive  streak  appeared  than  there  is  formed  a  perforation  or  canal 
at  its  crania]  or  anterior  end — the  neurenteric  canal  (Fig.  35,  ^).  If  a  section 
is  made  across  the  embryonic  plate  at  the  site  of  the  canal,  the  entoderm 
lining  the  archenteron  is  seen  to  be  continuous  with  the  ectoderm  on  the 
dorsal  surface  of  the  plate,  the  cavity  of  the  primitive  gut  thus  opening  or 
having  a  mouth,  on  the  dorsal  surface  of  the  embryo  (Fig.  31,  B).  If  a 
section  be  made  further  back,  across  the  region  of  the  primitive  streak 
(Fig.  34,  C)  it  is  seen  that  the  entoderm  fuses  with  the  ectoderm  and  that, 

35 


36 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


at  the  line  of  fusion  the  mesoderniic  plates  are  continuous  with  both 
entoderm  and  ectoderm.  Along  the  line  of  fusion  there  is  a  vigorous 
production  of  mesoderm.  A  section  in  front  of  the  neurenteric  canal 
(Fig.  34,  A)  shows  still  other  appearances  ;  the  ectoderm,  now  being  differ- 


ECTODERM 


ECTODERM 

PRIM.  STREAK 

MESODERM 


NOTOCHORD 


ENTODERM 


ENTODERM 


(A) 


(B) 


(C) 


Pig.  34. — Section  across  a  human  embryonic  plate  measuring  1'5  mm.  in  length. 
(Graf  Spee.) 

A.  In  front  of  the  neurenteric  canal. 

B.  At  the  neurenteric  canal. 

C.  Across  the  primitive  streak,  behind  the  neurenteric  canal. 

entiated  into  the  neural  plate,  is  moulded  to  form  the  rudiment  of  the 
medullary  furrow  ;  beneath  the  furrow  there  is  a  plate  of  cells — the 
notochordal  plate  (Fig.  34,  A)  which  although  apparently  continuous  with 
the  entoderm  is  yet  of  different  origin.     The  notochordal  j)late  will  form  the 


CLOACAL    MEMB. 


BODY  STALK 
ALLAHTOIS 


Fig.  35.— Sections  along  the  median  line  of  two  embryonic  plates,  figured  by  Graf 
Spee,  to  show  the  shifting  backwards  of  the  neurenteric  canal  and  primitive 
streak  as  growth  takes  place. 

notochord,  the  supporting  or  skeletal  rod  of  the  medullary  plate.  At  the 
neurenteric  canal  and  in  front  of  it  the  mesoderm  is  no  longer  continuous 
with  the  ectoderm  or  entoderm  ;  it  has  grown  forwards  from  the  site  of 
23roduction  at  the  primitive  streak. 


PKIMITIVE  STREAK,  NOTOCHORD  AND  SOMITES         37 


If  sections  are  made  along  the  embryonic  plate  (Fig.  35,  A  and  B)  furtlier 
light  is  thrown  on  the  relationship  of  the  neurenteric  canal  and  primitive 
streak  to  the  growth  of  the  embryo.  In  Fig.  35,  A  the  neurenteric  canal  is 
seen  to  be  placed  near  the  middle  point  of  the  plate — which  has  a  total 
length  of  a  little  over  1  mm.  while  in  the  older  embryo  which  measures 
r7  mm.  in  length,  it  is  pushed  backwards  by  the  rapid  growth  and  ex- 
tension of  the  j^recanalicular  part  of  the  embryonic  plate.  The  region  of 
the  primitive  streak — the  postcanalicular  part  of  the  embryonic  plate — ■ 
although  the  site  of  mesodermal  production,  has  undergone  a  lesser  degree 
of  growth  and  is  being  pushed  to  the  hinder  end  of  the  embryonic  plate. 
The  exact  manner  in  which  the  precanalicular  part  expands  we  are  not 
certain  of,  but  it  will  be  noted  that  at  the  anterior  lip  of  the  neurenteric 


MEDULLARY 
PLATE  AND 
FOLDS 


NEUR:  ENTER  ■. 
CAN: 


ANTERIOR  END 


AMNION 
FOLD 
EMBRYO 
NEUR    ENTER:CAN: 
^V^-PRIM    STREAK 

BODY    STALK 


Fig.  36. — Diagram  of  the  Embryogenic  area  of  an  Embryonic  plate  viewed  from  above. 

Fig.  37. — The    Medullary  Plate  and  Primitive  Strealc  on  an  Embryo  towards  the 

end  of  the  3rd  weelv.     (After  Graf  Spee.) 

canal  the  neural  plate  becomes  continuous  with  the  notochordal  plate  and 
we  suspect  that  this  lip  represents  a  growing  edge.  The  first  formed  part 
of  the  precanalicular  plate  represents  the  hinder  cranial  region  ;  as  the 
plate  grows  the  neurenteric  canal  moves  backwards  through  the  cervical 
and  dorsal  regions  until  it  reaches  the  lumbar  region  early  in  the  fourth 
week,  the  embryo  then  being  less  than  3  mm.  in  length.  If,  as  sometimes 
occurs,  the  neurenteric  canal  remains  unclosed,  a  fistula  from  the  bowel 
opens  on  the  lumbar  region  of  the  back.  In  Fig.  34  it  will  be  seen  that 
while  the  neurenteric  canal  lies  at  the  anterior  end  of  the  primitive  streak 
a  very  important  structure — the  cloacal  membrane — marks  its  posterior 
end.  The  cloacal  membrane,  lying  at  the  foot  of  the  body  stalk,  marks 
the  site  of  the  anus  and  vulval  cleft.  Thus  the  whole  of  the  hinder  end  of 
the  human  body  is  developed  on  each  side  of  the  primitive  streak,  a  relation- 
ship which  must  be  understood  if  certain  malformations  of  the  human 
body  are  to  be  adequately  explained. 

In  the  third  week  of  development,  when  the  primitive  streak  is  being 
pushed  backward  on  the  embryonic   plate,  the  medullary  folds   appear 


38      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

on  its  anterior  part,  the  hinder  ends  of  the  folds,  as  they  spread  backwards, 
coming  to  enclose  the  neurenteric  canal  and  anterior  end  of  the  primitive 
streak  (Fig.  37).  The  early  relationship  of  the  medullary  folds  to  the 
primitive  streak  is  shown  diagrammatically  in  Fig.  36.  It  will  be  seen  that 
as  the  medullary  folds  invade  the  postcanalicular  part  of  the  plate  the 
neurenteric  canal  and  anterior  end  of  the  streak  will  be  included  within 
them  and  eventually  lie  in  the  hinder  part  of  the  spinal  cord.  The  hinder 
end  of  the  streak  is  carried  away  from  the  cloacal  membrane  by  the  forma- 
tion of  the  tail. 

The  Blastopore. — The  primitive  streak  with  the  neurenteric  canal  at 
its  front  end  and  the  cloacal  membrane  at  its  hind,  can  be  best  explained 
by  supposing  that  they  represent  the  primitive  mouth  or  blastopore  of 
lower  invertebrate  animal  types.     Its  formation  in  the  vertebrate  body  is 


primitive  mouth. 


coelom       .    , 

subintes.  vessel. 

Fig.  38. — Diagram  showing  three  stages  in  the  early  development'of]Amphioxus. 

A.  Invagination  of  the  entoderm  (shaded)  within  the  ectoderm  (stippled). 

B.  Formation  of  archenteron  and  primitive  mouth  (blastopore). 

C.  Origin  of  mesoderm  (black)  and  coelom  from  margin  of  primitive  mouth,  with 

formation    of    a    ventral    mesentery    round    the    subintestinal    vein.     (After 
Robinson.) 

best  studied  in  amphioxus  (Fig.  38).  At  an  early  stage  of  its  segmentation 
the  ovum  of  this  animal  forms  a  hollow  sphere  (Fig.  38,  A)  ;  one  part 
of  the  sphere  becomes  invaginated  to  form  the  entoderm,  the  uninvaginated 
or  outer  layer  becoming  the  ectoderm.  The  brim  of  the  bilaminar  flask 
(gastrula  or  cup)  thus  formed  served  as  a  mouth  or  blastopore  to  the  cavity 
of  the  entoderm  (archenteron)  (Fig.  38,  B).  The  primitive  streak  and 
groove  seen  in  the  embryos  of  all  vertebrates  are  believed  to  arise  from  a 
linear  fusion  of  the  lips  of  the  blastopore.  The  neurenteric  canal  is  a 
part  of  the  blastopore  which  retains  its  patency  for  a  few  days  only  in  the 
human  embryo.  The  process  of  invagination  or  gastrulation,  which  is 
seen  to  occur  in  the  development  of  amphioxus — by  far  the  most  primitive 
of  vertebrate  forms — has  become  masked  and  obscured  in  the  embryos  of 
higher  vertebrates.  The  process  has  been  profoundly  modified  by  the 
accumulation  of  yolk  in  the  ovum  and  the  precocious  development  of  the 
embryonic  membranes.  The  embryonic  plate  situated  on  the  archenteron 
(Fig.  37)  may  be  regarded  as  the  modified  gastrula  stage  of  amphioxus 
and  the  primitive  streak  as  a  modified  blastopore.     We  shall  see  that 


PRIMITIVE  STREAK,  NOTOCHORD  AND  SOMITES         39 

some  of  the  primary  processes  of  development  are  initiated  at  the  margins 
of  the  primitive  streak.^ 

Origin  of  the  Mesoderm  and  Coelom. — In  the  developing  ova  of 
higher  vertebrates  the  mesoderm  is  known  to  originate  at  each  side  of  the 
primitive  streak,  but  it  is  difficult  to  follow  the  exact  manner  of  its  de- 
velopment (Fig  34,  G).  In  amphioxus  it  arises  as  a  bilateral  series  of 
diverticula  from  the  margin  of  the  gastrular  mouth  or  blastopore,  along 
the  line  at  which  the  ectoderm  and  entoderm  are  continuous  (Fig.  38,  C). 
The  diverticula  expand  and  their  cavities  fuse  together  between  the  two 
primary  layers  to  form  the  coelom  ;  the  right  and  left  series  of  diverticula 
meet  below  the  archenteron  and  form  a  ventral  median  mesentery  (Fig. 
38,  C).  In  higher  vertebrate  ova,  the  ectoderm  and  entoderm  are  fused 
together  along  the  primitive  groove  just  as  round  the  primitive  mouth  of 
a  Hydra.  The  mesoderm  arises,  as  we  have  seen,  from  the  line  of  union, 
and  spreads  outwards  between  the  two  primary  layers.  The  coelom  is 
formed,  not  as  a  diverticular  cavity,  but  by  a  cleavage  of  the  mesoderm, 
into  outer  and  inner  layers.  In  the  human  embryo  the  mesoderm  appears 
at  an  extremely  early  stage  ;  long  before  the  primitive  streak  has  been 
formed  mesoderm  appears  in  the  human  blastocyst  (see  Fig.  15,  p.  13). 
The  coelom  appears  first  as  a  cleavage  of  the  blastocystic  mesoderm  (Fig. 
16,  p.  14),  so  altered  have  developmental  processes  become  owing  to  the 
early  formation  of  the  ectodermal  wall  in  the  human  ovum  yet  the  primi- 
tive streak  remains  the  chief  site  of  mesodermal  production. 

Differentiation  of  Mesoderm. — In  Fig.  39,  a  diagrammatic  representa- 
tion is  given  of  the  parts  into  which  the  mesoderm,  and  the  cavity  of  the 


NEURAL. 

CANAL 

PA/^AXIAL   MESODERM          \ 

MYOCOEL. 

INTERMEDIATE 
SOMATOPLEUFIE.     1 
AMNIO-CHOR  I 

MASS  1         1 

^ 

1     notochord 

/    /nephrocoel 

Jy       /embryonic 

^5^    /      COELOM 

SPLANCHNO-  PUEURE  EXTRA-EMB.  COELOM 


Fig.  39. — Diagrammatic  section  across  a  vertebrate  embryo  to  show  the  parts  of  tlie 
mesoderm,  of  the  coelom,  and  also  the  origin  of  the  neural  canal  and  notochord. 

mesoderm — the  coelom — become  differentiated.     Along  each  side  of  the 
medullary  tube  lies  the  paraxial  mesoderm  ;  that  part  we  shall  see  becomes 

^  For  details  relating  to  the  nature  of  the  blastopore  see  Text-Booh  of  Embryology . 
Vol.  I.  Invertebrata,  by  Prof.  E.  W.  MacBride.  Vol.  II.  Vertebrata,  by  J.  Graham  Kerr, 
1919  ;  Profs.  J.  T.  Wilson  and  J.  P.  Hill,  PJiil.  Trans.  1908,  Ser.  B,  vol.  199,  p.  31  ; 
The  late  Dr.  E.  Assheton,  Quart.  Journ.  Mic  Sc.  1910,  vol.  54,  pp.  221,  631, 


40      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

divided  into  somites  and  gives  rise  to  muscles  and  vertebrae  ;  lateral  to 
the  paraxial  mass,  comes  the  intermediate  cell  mass  ;  then,  lateral  to  the 
intermediate  mass  in  which  the  urino-genital  glands  are  formed,  the 
mesoderm  is  cleft  into  an  outer  and  inner  layer^one  joining  the  ectoderm 
to  form  the  somatopleure,  the  other,  the  entoderm,  to  form  the  splanch- 
nopleure.  The  cleft  between  these  laminae  is  the  coelomic  space  ;  part 
becomes  enclosed  within  the  embryo  to  form  the  pericardial,  pleural  and 
peritoneal  cavities  ;  the  extraembryonic  part  (Fig.  39)  is  carried  away 
and  obliterated  between  the  foetal  membranes.  Within  the  inter- 
mediate cell  mass  and  within  the  paraxial  mass  there  are  extensions 
of  the  original  coelom — known  as  the  nephrocoel  and  myocoel.  From 
the  mesoderm  arise  the  great  mass  of  tissues  which  constitute  the 
human    body — the    tissues    of    locomotion — muscles,    bones,    ligaments 

NOTOCHORU 

BRAIN  "^''"^  /  NEUf^AL    CANAL 

°""  PRIMITIVE    GUT        I  I 

\  \  I  11  CAUDAL    CAMBIUM 


POST   ANAL    GUT 


Fig.  40. — A.  Diagrammatic  longitudinal  section  of  a  larval  Polypterus — a  ganoid 
fish — to  show  the  relations  of  the  notochord.     (After  Graham  Kerr.) 

B.  The  larval  form  of  Lepidosteus,  another  ganoid  fish,  to  show  the  segmented 
vertebral  musculature  covering  the  notochord.     (After  Graham  Kerr.) 

and  connective  cells.  And  also  the  circulatory  and  mobile  systems — 
the  heart,  blood  vessels,  blood  cells  of  all  kinds  and  all  forms  of  moving 
tissue  cells.  To  this  latter  element  of  the  mesoderm — the  cells  which 
form  vessels,  blood,  connective  tissue  and  mobile  cells  is  given  the  name 
of  Mesenchyme. 

Notochord. — In  its  origin  the  notochord,  the  forerunner  of  the  spinal 
column,  is  closely  related  to  the  primitive  streak  (see  Fig.  40,  A).  Amongst 
the  structures  produced  at  its  anterior  end  where  the  ectoderm  turns  in 
to  join  the  entoderm  is  a  plate  of  cells  which  comes  to  lie  along  the  median 
line  on  the  roof  of  the  archenteron  or  primitive  gut-cavity  (Fig.  35). 
Presently  the  plate  becomes  folded  ofi  from  the  roof  of  the  archenteron 
(Fig.  39)  to  form  a  rod  of  peculiar  cells — the  notochord.  The  posterior 
part  of  the  notochord  never  forms  part  of  the  gut-cavity,  but  is  developed 
from  the  lateral  margins  of  the  primitive  streak.     It  will  thus  be  seen  that 


PRIMITIVE  STREAK,  NOTOCHORD  AND  SOMITES  41 

the  first  representation  of  a  skeleton  is  produced  at  an  extremely  early 
date,  and  that  it  appears  as  a  support  for  the  medullary  plate  when  that 
plate  is  folded  in  to  form  a  tube.  Its  continuity  with  the  primitive  gut 
seems  accidental,  for  it  is  hard  to  believe  that  a  mesodermal  skeletal 
structure  such  as  the  notochord  could  have  been  evolved  from  the 
alimentary  system. 

Segmentation. — We  have  seen  that  the  medullary  folds  rise  up  to- 
wards the  end  of  the  third  week  of  development  when  the  embryonic 
plate  is  only  about  1"5  mm.  in  length.  No  sooner  do  they  commence  to 
fuse  and  thus  enclose  the  neural  plate  than  that  part  of  the  mesoderm 
which  has  been  laid  down  by  the  side  of  the  neural  tube — the  paraxial 
mesoderm  (Fig.  39)  begins  to  be  divided,  from  before  backwards,  into 
segmental  blocks  or  somites.  Segmentation  which  begins  at  what  will 
become  the  occipital  region  of  the  head,  is  confined  to  the  paraxial  meso- 
derm. In  the  embryo  shown  in  Fig.  19,  p.  17,  five  somites  have  been 
formed  ;  by  the  end  of  the  fourth  week,  when  the  embryo  has  grown  to  a 
length  of  about  3  mm.  (Fig.  20)  the  process  has  reached  the  first  caudal  or 
coccygeal  segment,  there  being  at  this  time  3  occipital  and  30  body  somites. 
Thereafter  segmentation  proceeds  slowly  in  the  caudal  region,  there  being 
8  or  10  caudal  somites  at  the  end  of  the  sixth  week,  when  the  tail  has 
reached  its  maximum  development  and  the  embryo  is  about  11  mm. 
long. 

To  understand  the  meaning  of  segmentation  we  must  again  appeal  to 
comparative  anatomy.  Segmentation  marks  the  onset  of  vertebral 
characterization  in  the  human  embryo.  In  Fig.  40,  A  a  diagrammatic 
longitudinal  section  of  a  fish  larva  is  reproduced  to  show  the  relations  of 
the  notochord  ;  it  and  the  neural  tube  we  have  seen  are  formed  first  in 
the  head  region  and  then  grow  backwards.  In  Fig.  40,  B  another  fish 
larva  is  depicted,  with  the  notochord  clothed  with  muscle  segments  or 
myotomes.  A  mere  glance  at  such  diagrams  shows  that  the  notochord 
or  primitive  vertebral  column  and  the  segmented  spinal  musculature 
represent  a  great  sculling  apparatus — the  locomotory  machine  of  the 
lowest  and  oldest  vertebrates.  Gill  arches  also  appear  in  the  human 
embryo  very  soon  after  segmentation  has  commenced  (see  Fig.  20),  but 
even  without  their  guidance  one  would  infer,  on  the  evidence  of  segmenta- 
tion alone,  that  the  human  embryo  in  the  fourth  week  is  passing  through 
a  fish  stage  and  that  our  vertebral  column  and  spinal  musculature  represent 
a  former  locomotory  system.  The  gill-segmentation  is  different  and 
apparently  older  than  the  body-segmentation  ;  and  as  we  shall  see,  the 
gills  are  not  fashioned  out  of  the  paraxial  mesoderm. 

Experimental  Embryology. — In  recent  years  those  who  study  the 
development  of  the  body  have  resorted  to  experiment  in  order  to  obtain 
a  more  direct  knowledge  of  the  laws  and  conditions  of  development. 
Loeb  has  shown  that  the  ova  of  some  invertebrate  animals  may  be  stimu- 
lated to  development  by  chemical  substances  which  thus  simulate  the 
action  of  spermatozoa.  Dareste,  fifty  years  ago,  discovered  that  eggs 
hatched  at  abnormal  temjDeratures  often  gave  rise  to  malformed  embryos. 
In  more  recent  years  it  has  been  discovered  that  the  addition  of  certain  salts 


42      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

produces  one  form  of  malformation,  while  another  group  of  salt  solutions 
added  to  the  water,  in  which  the  embryos  of  invertebrate  animals  are  being 
hatched,  will  produce  another  set  of  abnormalities.  Stockard  ^  discovered 
that  the  addition  of  magnesium  chloride  to  the  sea  water  in  which  fish- 
embryos  are  being  hatched  will  lead  to  half  of  the  larvae  becoming  cyclops. 
It  has  been  found  that  embryonic  structures  can  be  transplanted  or  grown 
on  artificial  media.  The  original  experiments  of  Dr.  Ross  Harrison,^  in 
which  he  transplanted  and  studied  parts  of  the  living  embryonic  spinal 
cord,  did  much  to  open  up  this  method  of  enquiry.  In  embryonic  struc- 
tures thus  transplanted  the  development  of  nerve  and  other  cells  has  been 
successfully  studied.  It  has  also  been  found  that  by  dividing  the  ovum 
at  an  early  stage  after  fertilization,  or  by  separating  the  cells,  it  is  possible 
to  produce,  in  lower  animal  forms,  an  embryo  from  each  part  or  cell 
separated,  but  the  embryos  so  produced  are  small  in  size,  and  do  not 
reach  adult  life.  In  other  cases  the  cells  thus  separated  only  produce  part 
of  an  embryo.  Those  who  wish  to  obtain  information  on  this  important 
branch  of  embryology  will  find  some  of  the  more  recent  papers  by 
Ross  Harrison,  W.  H.  Lewis  and  others  in  the  American  Journal  of 
Anatomy  and  Anatomical  Record. 

Twins,  Perfect  and  Imperfect.^ — The  study  of  early  stages  in  the 
development  of  the  ovum  throws  some  light  on  the  manner  in  which, 
twins  arise,  and  especially  on  the  production  of  human  monsters  by  the 
incomplete  separation  of  twins.  Three  theories  are  held  concerning  the 
production  of  Twins  :  (1)  There  may  be  two  or  more  ova  shed  and  fertil- 
ized. (2)  That  each  of  the  cells  produced  by  the  first  division  of  the  ovum 
gives  rise  to  an  embryo.  Assheton  found  two  inner  cell  masses  in  the 
blastocyst  stage  (Stage  II.)  of  a  sheep,  each  of  which  would  have  formed  an 
embryo.  A  blastocyst  is  necessarily  the  product  of  one  ovum.  (3) 
Beard  regards  the  cells  formed  by  the  early  divisions  of  the  ovum  as 
indeterminate  in  nature — a  thaUus  from  which  a  brood  of  germinal  cells 
are  produced.  One  of  these  germinal  cells  becomes  the  embryo,  in  the 
genital  ridges  of  which  the  remaining  germ  cells  find  a  nidus  and  form  ova 
or  spermatozoa.  If  two  of  these  germinal  cells  become  embryos,  twins 
are  produced,  if  three,  triplets.  Twins  are  produced  once  in  every  89 
births,  but  it  is  probable  that  twin  pregnancies  are  more  frequent  than  is 
suggested  by  birth  statistics.  Dr.  Streeter  found  a  vestigial  twin  in  a 
human  pregnancy  of  the  third  week."^  Dr.  Crawford  Watt  ^  has  described 
a  normal  twin  pregnancy  of  the  fourth  week. 

1  Journ.  Exper.  Zool  1909,  vol.  6,  p.  285. 

2  Ibid.  1907,  vol.  4,  p.  239. 

*  For  literature  on  malformations  of  the  body  see  :  J.  W.  Ballantyne,  Antenatal 
Pathology,  London,  1904 ;  Die  Morphologie  der  Missbildungen  des  Menschen  und  der 
Tiere,  edited  by  Ernst  Schwalbe,  Jena,  1906-1912  ;  Prof.  F.  P.  Mall,  Journ.  of  Mor- 
phology, 1908,  vol.  19,  p.  3  (Description  of  a  large  collection  of  malformed  human 
embryos  with  references  to  the  more  recent  literature  on  the  causation  of  the  various 
kinds  of  maldevelopment).     Also,  later,  Amer.  Journ.  Anat.  1917,  vol.  22,  p.  49. 

*  Contributions  to  Embryology,  1920,  vol.  9,  p.  389. 
'Ibid.  1915,  vol.  2,  p.  5. 


PRIMITIVE  STREAK,  NOTOCHORD  AND  SOMITES 


43 


"  Identical  "  twins  ^  are  iiroduced  by  the  division  of  a  single  ovum. 
They  are  contained  Avithin  the  same  enveloping  membranes,  are  of  the 
same  sex  and  so  alike  in  features  that,  to  the  casual  observer,  they  are 
hard  to  distinguish.  In  the  production  of  identical  twins,  the  embryonic 
plates  (see  Figs.  A,  B,  C)  may  remain  unseparated,  and  in  this  manner 
most  of  the  numerous  forms  of  human  monsters  are  produced.  The 
embryos  may  remain  attached  to  a  common  yolk  sac,  thus  forming  a 
"  Siamese  "  twin — the  two  individuals  remaining  attached  in  the  region 
of  the  umbilicus.  The  union  may  affect  only  the  lower  body  and  limbs,  or 
only  the  upper  part  and  arms.  All  kinds  and  degrees  of  union  occur — 
head  to  head,  buttocks  to  buttocks,  but  the  most  common  is  a -ventral 
union  effected  through  a  common  yolk  sac.  In  some  cases  one  twin 
becomes  a  "  joarasite,"  and  dependent  on  the  other — the  "  host  "  twin 
— for  its  circulation  and  nourishment.     Only  part  of  the  parasitic  twin 


A.  B.  C 

Fig.  41.^ — Division  of  tiie  Embryonic  Plate,  forming  imperfect  twins. 
A,  Anterior  dichotomy  ;  B,  posterior  dichotomy  ;  C,  intermediate  union. 

may  develop,  and  then  remains  attached  as  an  appendage  to  the  body  of 
the  host  twin.  At  an  early  stage  of  development  the  parasitic  twin, 
arrested  and  delayed  in  development,  may  become  included  within  the 
body  of  the  host  twin.  There  are  two  examples  of  this  condition  in  the 
Museum  of  the  Royal  College  of  Surgeons,  England. 

Duplication  and  Atrophy  of  Parts. — Parts  of  the  body,  such  as  a 
digit  or  the  penis  may  be  duplicated.  In  such  cases  we  suppose  that  the 
group  of  cells  which  give  rise  to  the  part  undergo  a  division  or  dichotomy, 
just  as  the  growing  point  of  a  plant  stem  may  undergo  branching.  Those 
parts  of  the  body  which  arise  as  outgrowths,  such  as  the  nasal  processes, 
the  extremities,  or  segments  of  the  extremities,  may  be  partially  or  com- 
pletely arrested  at  a  very  early  stage  of  development.  The  embryo  itself 
may  be  retarded  in  development  or  completely  blasted  while  the  membranes 
go  on  developing,  giving  rise  to  the  developmental  product  known  as  a 


1  D.  Berry  Hart,  Proc.  Roy.  Soc.  Edin.  July  1909  ;    J.  F.  Gemmill,  Teratology  of 
Fishes,  Glasgow,  1912  ;    G.  W.  Tannreuther,  Anat.  Rec.  1919,  vol.  6,  p.  355. 


44      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

"  mole."  It  is  known  that  if  eggs  are  incubated  in  abnormal  conditions  as 
regards  temperature  or  atmosphere,  such  malformations  occur  more  fre- 
quently than  usual.  Practically  nothing  is  known  of  the  circumstances 
or  influences  which  give  rise  to  abnormalities  in  the  Human  Embryo.  We 
know  that  such  abnormalities  tend  to  occur  in  certain  families  ;  they 
are  hereditary,  but  we  do  not  know  the  circumstances  which  give  rise 
to  them. 

Knowledge  relating  to  deformed  and  monstrous  foetuses  is  known  as 
Teratology.  Mention  will  be  made  of  various  congenital  deformities  as 
we  proceed. 


CHAPTER  IV. 
THE  AGE  CHANGES  IN  THE  EMBEYO  AND  FOETUS. 

In  Chapter  I.,  having  followed  the  developmental  changes  in  the  human 
embryo  during  the  first  five  weeks,  when  it  had  reached  a  crown-rump 
length  of  5  mm.  {}  in.)  and  the  condition  of  parts  shown  in  Fig.  42,  we  had 
to  break  away  in  Chapters  II.  and  III.  to  note  the  manner  in  which  it 


^•''.••VE.NTRICLE 


5. MM 


riG.  42. — Outline  of  a  Human  Embryo  5  mm.  in  length,  and  in  the  5th  week  of 
development.    (Reconstructed  by  Professor  Keibel  and  Dr.  Elze.) 

efiected  a  lodgment  in  the  uterus  and  to  examine  certain  processes  which 
give  rise  to  fundamental  parts  of  the  embryonic  body.  In  this  chapter 
we  return  to  trace  the  further  history  of  the  embryo,  to  watch  it  becoming 
transformed  into  a  foetus  and  to  register  the  subsequent  changes  during 
the  nine  months  it  spends  in  its  mother's  womb. 

In  recent  years  our  knowledge  concerning  the  rate  at  which  the  human 
embryo  grows  and  the  stages  through  which  it  passes  week  by  week  has 
become  more  accurate.  This  is  largely  due  to  the  work  accomplished  by 
the  late  Prof.  Mall  ^  who  collected  facts  relating  to  all  cases  where  the  age 
of  an  embryo  had  been  ascertained  and  by  tabulating  his  data  was  able  to 
estimate  the  size  and  stage  of  development  reached  by  an  average  human 
embryo  week  after  week.  His  main  results,  so  far  as  concern  the  first 
two  months,  are  set  out  in  Fig.  43,  taken  from  an  article  written  by  his 

1  See  his  last  paper  on  this  subject,  Amer.  Jouni.  of  Anat.  1918,  vol.  23,  p.  397. 

45 


46 


HUMAN  EMBEYOLOGY  AND  MORPHOLOGY 


distinguished  pupil — Prof.  Herbert  Evans.  Six  stages  of  development  are 
represented  :  at  the  end  3rd,  4th,  5th,  6th,  7th  and  8th  weeks.  Under 
each  embryo  is  given  the  mean  length  it  should  reach  at  a  certain  date, 
but  it  has  to  be  remembered  that  the  rate  of  growth  will  vary  in  embryos 
and  foetuses  just  as  in  children,  and  that  some  will  be  precocious  while 
others  will  be  backward.  The  measurements  relate  to  fresh  specimens, 
for  when  embryos  are  preserved  and  prepared  for  microscopic  examination 
they  shrink  in  size.  It  is  convenient  to  regard  3  mm.  as  measured  from  the 
crown  to  the  rump  of  the  embryo  as  marking  the  end  of  the  fourth  week,  and 
5  mm.  as  an  index  of  the  end  of  the  fifth  week  of  development.  In  the 
6th,  7th,  and  8th  weeks  the  embryo  adds  almost  one  millimetre  to  its 


G  5         4-.    3. 

1/ mm.     5-5mw.2S   -s 

Fig.  43. — Series  of  six  drawings  illustrating  the  stages  of  growth  from  the  end  of 
the  3rd  week  to  the  end  of  the  8th.  In  a  corner  of  the  figure  is  a  diagram  to 
illustrate  the  rate  of  growth  of  the  chorionic  vesicle  at  corresponding  dates. 
(Prof.  H.  M.  Evans.) 

length  daily,  being  about  25  mm.  (1  inch)  at  the  end  of  the  8th  week. 
Hence  we  may  readily  estimate  the  age  of  an  embryo  or  foetus  under  25  mm. 
in  length,  by  regarding  the  first  5  mm.  as  representing  35  days'  growth, 
and  adding  a  day  for  every  additional  millimetre  of  its  length.  For 
example,  the  age  in  days  of  an  embryo  measuring  15  mm.  in  length  would 
be  estimated  thus  :  5  mm.  =  35  days  + 10  for  the  additional  10  mm.  =  45 
days.  In  the  9th,  10th,  11th,  12th,  13th  and  14th  weeks — up  to  the  end 
of  the  3rd  month — when  the  crown-rump  length  amounts  to  100  mm. 
(4  in.)^ — the  daily  rate  of  growth  is  approximately  1"5  mm. 

External  Changes  in  the  6th  week.^ — As  may  be  seen  by  comparing 
Figs.  42  and  44,  the  6th  week  constitutes  a  period  of  rapid  transformation. 

^  See  specimens  described  by  J.  L.  Bremer,  Amer.  Journ.  Anat.  1905,  vol.  5,  p.  459  ; 
C.  Elze,  Anat.  Hefte,  1907,  vol.  35,  p.  409  ;  N.  W.  Ingalls,  Archiv.  f.  mikros.  Anat.  und 


AGE  CHANGES  IN  EMBRYO  AND  FOETUS 


47 


Not  only  does  the  length  of  the  embryo  increase  from  5  mm.  to  11  mm. 
but  there  are  very  definite  changes  in  the  form  of  its  external  parts.  At 
the  end  of  the  5th  week  the  gill-arch  system  of  the  primitive  pharynx  is 
at  its  height,  four  arches  being  distinguishable  ;  in  the  6th  week  the 
3rd  and  ith  arches  sink  into  a  pit  in  the  neck — the  cervical  sinus — (Fig.  44), 
while  the  2nd  or  hyoid  arch  grows  backwards  over  the  pit  and  thus  hides 
the  hinder  arches.  Here  we  are  witnessing  the  closing  in  or  operculation 
of  the  branchial  arches — as  it  takes  place  in  gill-bearing  vertebrates. 
Even  at  the  close  of  the  6th  week  the  face  is  represented  merely  by  a 


4-*VENTRtCLE 


1*?  CLEFT 


pERVIC  . 
SINUS 


TML 


+    5  TIMES 


Fig.  44. — Outline  of  a'Human  EiiQbryo'10.4  mm.  long  and  in  the  6th  week  of  develop- 
ment.   (After  Broman.)    (Magniilc.  x  5.) 

forehead  filled  out  by  the  relatively  small  f orebrain  vesicle ;  behind  and 
under  the  forehead  are  seen  the  nasal,  maxillary  and  mandibular  processes 
which  will  give  rise  to  the  face  proper.  All  of  these  elements  have  made 
headway  during  the  6th  week  (Figs.  42,  43,  44).  The  head  region  even  in 
the  6th  week  is  still  tubular  in  form  ;  the  mid  and  hind  brains  form  the 
greater  part  of  the  central  nervous  system,  for  the  cerebral  vesicles  have  as 
yet  only  begun  to  grow  out  from  the  fore  brain.  The  limb  buds,  which 
in  the  5th  week  were  still  undemarcated  into  segments,  now  show  their 
three  primitive  parts — upper  arm  and  thigh,  forearm  and  leg,  and  a  plate- 
like hand  and  foot.     In  point  of  differentiation  the  forclimb  is  always  in 

Entwick.  1907,  vol.  70,  p.  506  ;  L.  Frassi,  Ibid.  p.  492.  Dr.  H.  L.  Bamiville  gives  a 
full  description  of  an  8-5  mm.  Embryo,  Journ.  Anat.  1915,  vol.  49,  p.  1.  Dr.  F.  W. 
Thyng  gives  excellent  figures  of  one  measuring  17-8  mm.,  Amcr.  Jottrn.  of  Anat.  1915, 
vol.  17,  p.  51. 


48      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

advance  of  the  hind.  At  the  close  of  the  6th  week  the  first  appearance  of 
jvebbed  digits  can  be  detected,  and  at  the  same  time,  when  the  length  of 
the  embryo  is  about  11  mm.  the  tail  reaches  its  maximum  development 
(Fig.  44)  ;  in  the  7th  week  retrogression  has  already  set  in.  The  um- 
bilical cord  becomes  lengthened  and  more  clearly  differentiated  in  the 
6th  week ;  between  the  attachment  of  the  cord  to  the  ventral  wall  of  the 
embryo  and  the  gill-formation  of  the  primitive  pharynx  is  seen  the 
bulging  eminence  of  the  heart  (Fig.  42) ;  below  the  heart  eminence,  as 
may  be  seen  in  Fig.  44,  there  appears  in  the  6th  week  a  second  eminence, 
that  caused  by  the  developing  liver. 

Embryo-Foetus. — During  the  7th  week  the  embryo  becomes  a  foetus 
— the  transformation  being  well  shown  in  Fig.  43.  In  its  crown-rump  length 
the  embryo  expands  from  11  to  17  mm.,  but  the  characteristic  changes 

4*  VENTRICLE 


CEREBRAL  VESICLE 


NASAL    PROCESS 
NASAL     I 
MAX:  PROCESS 
MAND;  PROCESS 


Fig.  45. — Outline  of  an  embryo,  although  only  11  mm.  long,  yet  showing  changes 
characteristic  of  an  early  stage  of  the  7th  week.    (After  Broman.) 

The  line  C-R  indicates  the  manner  in  which  the  crown-rump  diameter  is  measured  ; 
the  line  A-R  shows  the  neck-rump  length,  A  being  found  by  drawing  a  line  back- 
ward through  the  rudiments  of  the  eye  and  ear. 

are  seen  in  the  face,  head  and  limbs.  An  early  stage  of  the  7th  week  is 
shown  in  Fig.  45  ;  the  basal  parts  of  the  face  are  being  laid  down.  Under 
the  eye  are  seen  the  nasal  processes  carrying  the  open  nasal  cavities  back- 
wards into  the  region  of  the  mouth,  while  growing  forwards  beneath  the 
eye  are  to  be  observed  the  maxillary  processes  which  will  provide  the  bases 
of  the  upper  jaw.  Still  further  back  in  the  pharynx  (Fig.  45)  are  seen 
two  comparatively  small  processes — the  mandibular  (1st  arch)  and  hyoid 
(2nd  arch).  Behind  the  hyoid  arch  there  is  a  depression  marking  the 
cervical  sinus.  By  the  end  of  the  7th  week  (Fig.  43)  the  nasal,  maxillary 
and  mandibular  processes  have  united  to  form  a  relatively  small  face  ; 
at  the  upper  end  of  the  postmandibular  cleft  has  appeared  the  rudiment 
of  an  ear.  The  changes  in  the  head  itself  are  also  apparent ;  at  the  end 
of  the  7th  week  the  cylindrical  cranial  form  is  being  replaced  by  one  more 


AGE  CHANGES  IN  EMBRYO  AND  FOETUS 


49 


distinctly  globular  ;  the  forehead  in  particular  has  become  enlarged. 
These  changes  are  due  to  the  rapid  expansion  of  the  cerebral  vesicles 
during  the  7th  week.  The  changes  in  the  limbs  are  also  very  evident ; 
they  are  now  folded  on  the  belly-wall,  palm  towards  palm  and  sole  towards 
sole  ;  the  digits  are  demarcated.  The  tail  is  disappearing.  The  head  is 
no  longer  bent  forwards  with  the  forehead  touching  the  root  of  the  um- 
bilical cord,  but  is  lifted  up,  for  the  embryonic  flexure  of  the  cervical  region 
is  being  undone  and  a  narrowing  of  the  post-cranial  region  to  form  a  neck 
becomes  apparent.  The  heart  is  now  comjDletely  divided  into  right  and 
left  chambers  and  the  growth  of  the  neck  is  lifting  the  pharyngeal 
region  away  from  the  heart.  With  these  changes  in  the  facial  region, 
in  the  head,  neck,  limbs  and  heart  the  embryo  of  the  6th  week  becomes  the 
foetus  of  the  7th.  One  other  very  important  event  also  characterises  this 
stage  of  transformation :  the  cellular  blastema  of  the  skeleton  begins  to 
change  into  cartilage  and  into  bone.  It  also  becomes  possible  to  dis- 
tinguish the  ovary  from  the  testicle. 

Changes  in  the  8th  week. — At  the  end  of  the  8th  week  the  crown- 
rump  diameter  measures  about  25  mm.  (1  inch).  The  changes  of  this 
week  are  a  continuation  of  those  we  have  just  described  (Fig.  46)  ;    the 


4"  VES/TRlCLe 


fJASAL.  PROCESS 


MAX     PROCESS 
I^AND     PROCESS 


tBOUT     2}i    T 


Fig.  46. — Outline  of  a  Foetus  22  mm.  long,  and  at  the  end  of  the  2nd  month  of 
development.     (After  Broman.) 


nasal  and  maxillary  processes  have  fused  to  form  the  upper  face  ;  the 
upper  lip  is  completed,  but  the  palatal  processes  have  not  yet  separated 
the  buccal  from  the  nasal  cavities.  The  cerebral  vesicles  are  expanding 
rapidly  backwards  ;  the  neck  is  being  difEerentiated  and  the  limbs  are 
making  progress.     The  rudiment  of  the  external  genital  organs  is  ap- 


50      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

parent,  but  as  yet  gives  no  clue  to  sex.  The  intestinal  loop  lies  within  the 
root  of  the  umbilical  cord.  Henceforward,  until  the  end  of  gestation, 
the  chief  changes  are  those  of  growth.^ 

The  Full-Time  Foetus.- — We  speak  of  the  period  of  human  gestation — 
the  time  spent  by  the  human  young  in  the  uterus  of  the  mother,  preparatory 
to  an  independent  existence — as  being  one  of  9  months,  and  if  by  a  month 
we  mean  30  days — 270  days  in  all — we  are  as  near  the  truth  as  our  present 
evidence  will  take  us.  Medical  men  can  seldom  discover  the  exact  date  of 
conception  and  hence  to  get  a  fixed  point  for  a  reckoning  they  begin  their 
estimate  counting  from  the  first  day  of  the  mother's  last  menstrual  period, 
and  taking  this  day  as  a  fixed  point,  count  that  parturition  will  take  place 
280  days  hence.  Observations  made  on  cases  where  the  date  of  impregna- 
tion may  be  inferred  show  that  the  actual  mean  period  of  gestation  is  270 
days.  The  270th  day  is  the  bull's-eye  at  which  Nature  aims,  but  even  the 
best  of  marksmen  make  "  inner s  "  and  "  outers,"  and  it  is  so  in  all  of 
Nature's  shootings.  She  is  ever  subject  to  the  law  of  chance  ;  hence  in 
all  developmental  and  growth  manifestation  we  meet  with  variation  round 
a  mean. 

By  the  270th  day  the  foetus  has  attained  a  weight  of  about  7  lbs.  and  a 
height,  if  we  measure  from  crown  to  rump  (sitting  height),  of  336  mm.,  but 
if  we  include  the  lower  extremities  (standing  height)  the  measurement  is 
500  mm.  (nearly  20  inches).  It  sometimes  happens  that  birth  takes  place 
at  the  end  of  the  7th  month  when  the  foetus  weighs  between  4-5  lbs.  and  in 
its  standing  height  measures  400  mm.  or  less,  its  sitting  height  being  then 
about  265  mm.  In  such  premature  children,  who  have  always  a  defective 
heat-regulating  mechanism,  it  will  be  observed  that  the  tips  of  the  nails 
just  reach  the  ends  of  the  nail  beds,  whereas  in  the  full-time  child  the  nail 
edges  are  free  and  projecting.  The  full-time  child  has  also  an  outcrop 
of  hair  on  the  head  ;  lanugo — foetal  hair — can  be  detected  on  various  parts 
of  the  body.  The  hair  tips  which  break  on  the  surface  of  the  skin  about 
the  end  of  the  4th  month  may  be  plentiful  on  the  scalp  at  the  end  of  the 
7th,  but  the  skin  is  then  of  bright  lobster-red,  the  subcutaneous  tissue  is 
less  stored  with  fat  and  the  sebaceous  covering,  known  as  the  vernix  caseosa, 
forms  a  thin  and  unequal  coating. 

1  For  changes  in  9th  week  see  F.  E.  Blaisdell,  Journ.  Anat.  1914,  vol.  48,  p.  182. 


AGE  CHANGES  IN  EMBRYO  AND  FOETUS 


51 


Table  of  Growth. — It  is  impossible  for  anyone  to  remember  the  dimen- 
sions reached  during  the  various  stages  of  foetal  development  and  growth, 
but  it  is  often  convenient  to  have  a  table  of  measurements  for  reference. 
The  one  given  here  was  prepared  by  the  late  Prof.  Mall  ^  : 


Crown -Rump 

Standing 

Age  in 

Age  in 

Length. 

Height. 

Weeks. 

Days. 

•5  mm. 

— 

3 

21 

2-5  „ 

— 

4 

28 

5-5  „ 

— 

5 

35 

11 

— 

6 

42 

17 

— 

7 

49 

25 

— 

8 

56 

32   „ 

. 

9 

63 

43   „ 

— 

10 

70 

53   „ 

— 

11 

77 

68    ,; 

— 

12 

84 

81 



13 

91 

100 

149 

14 

98 

end  of  3rcl  month 

111 



15 

105 

121 

— 

16 

112 

134   „ 

— 

17 

119 

145 

223 

18 

126 

end  of  4th  month. 

157   „ 

. 

19 

133 

167   „ 

— 

20 

140 

180 

— 

21 

147 

192 

295 

22 

154 

end  of  5th  month 

202 



23 

161 

210 

— 

24 

168 

220 

— 

25 

175 

230   „ 

331 

26 

182 

end  of  6th  month. 

237 



27 

189 

245 

— 

28 

196 

252 

— 

29 

203 

265 

400 

30 

210 

end  of  7th  month. 

276 

. 

31 

217 

284   „ 

— 

32 

224 

293   „ 

— 

33 

231 

301 

443 

34 

238 

end  of  8th  month. 

310   „ 

. 

35 

245 

316   „ 

— 

36 

252 

325 

— 

37 

259 

336 

500 

— 

270 

end  of  9th  month. 

^  See  reference,  p.  45. 


CHAPTEE  V. 
THE   SPINAL   COLUMN  AND   BACK. 

Stages  in  the  Development  of  the  Spinal  Column. — In  previous 
cLiapters  tlie  main  facts  relating  to  the  development  of  the  human  body 
during  the  first  and  second  months  have  been  briefly  sketched.  We  now 
turn  to  the  consideration  of  particular  parts  of  the  human  body,  and 
naturally  take  up  first  the  vertebral  column — the  main  axis  of  the  body. 
The  most  primitive  form  of  axial  support — the  notochord — appears  in 
the  embryo  during  the  third  week.  In  amphioxus  the  notochord  forms  a 
permanent  structure  ;  in  all  vertebrate  animals  it  is  replaced  by  a  seg- 
mented or  vertebral  axis.  In  the  evolution  of  the  spinal  column  three 
stages  are  recognized  :  (1)  one  in  which  the  skeletal  segments  were  composed 
of  cellular  or  mesenchymatous  tissue  ;  (2)  a  cartilaginous  stage,  in  which  the 
cells  of  the  mesenchyme  (see  p.  40)  become  modified  into  cartilage-forming 
or  chondrogenous  cells  ;  (3)  a  final  stage  where  the  cartilage  is  replaced  by 
bone.  In  the  human  embryo  we  see  those  three  stages  appear  in  succession  ; 
at  the  beginning  of  the  second  month  the  membranous  foundation  of  the 
vertebra  is  being  laid  down  ;  in  the  middle  of  that  month  the  cartilaginous 
change  has  commenced  ;  by  the  beginning  of  the  third  month  ossification 
has  commenced.  In  only  certain  groups  of  fishes  is  the  cartilaginous  stage 
a  permanent  one. 

Stages  in  the  Evolution  of  the  Human  Spinal  Column. — We  have 
already  seen  that  the  vertebral  column  and  its  muscles  appear  first 
as  a  great  flexible  scull  for  driving  the  animal  forwards  (p.  41),  but  in 
nearly  all  mammals  the  vertebral  column  comes  to  serve  as  a  horizontal 
axis  or  arch,  which  is  supported  on  the  fore  and  hind  limbs.  In  a  small 
group,  however,  which  includes  the  anthropoid  apes  and  man,  the  spinal 
column  no  longer  forms  a  horizontal  but  a  vertical  axis  or  column.  These 
higher  primates  are  upright  or  orthograde  when  they  move,  in  contra- 
distinction to  the  ordinary  four-footed  mammals  which  are  pronograde. 
There  is  no  doubt  that  the  orthograde  posture  was  evolved  from  the 
pronograde.  Although  the  anthropoid  apes  are  orthograde,  yet  they  use 
their  arms  in  locomotion,  to  assist  their  lower  extremities  in  supporting 
the  weight  of  their  bodies.  Man  is  also  orthograde,  but  he  differs  from  the 
anthropoids  in  supporting  the  weight  of  his  body  entirely  on  his  lower 
extremities.  Hence  we  find  that  the  spinal  column  of  man,  although 
similar  to  that  of  the  anthropoids,  shows  many  j)eculiar  adaptations  to  his 
manner  of  locomotion.     These  adaptations  become  especially  manifest  as 

52 


THE  SPINAL  COLUMN  AND  BACK 


53 


O.VII. 
D.L 


Cisruical  pyramid 


upper  dorsal 


sternum 


dorso-lumbar 


the  child  learns  to  walk,  and  are  best  realized  by  a  survey  of  the  pyramids 
and  curves  of  the  spine. 

The  Pyramids  of  the  Spine. — The  spine,  when  viewed  from  the 
front,  is  seen  to  be  made  up  of  four  pyramids  :  (1)  Cervical ;  (2)  upper 
dorsal ;  (3)  dorso-lumbar ;  (4)  sacro- 
coccygeal (Fig.  47).  The  bases  of  the 
two  upper  pyramids  meet  at  the  disc 
between  the  7th  cervical  and  1st  dorsal 
vertebrae ;  the  bases  of  the  lower  two 
at  the  disc  between  the  5th  lumbar  and 
1st  sacral  vertebrae.  The  apices  of  the 
two  middle  pyramids  meet  at  the  disc 
between  the  4th  and  5th  dorsal  vertebrae, 
which  have  therefore  the  narrowest  bodies 
of  the  vertebral  series.  The  narrowing 
in  the  upper  dorsal  region  is  due  to  the 
fact  that  the  weight  of  the  upper  half  of 
the  trunk  is  partly  borne  by,  and  trans- 
mitted to,  the  lower  dorsal  region  by  the 
sternum  and  ribs  which  thus  relieve  the 
spine  to  some  extent  (Fig.  47).  At  the 
sacrum  the  weight  is  transferred  to  the 
pelvis  and  lower  limbs,  hence  the  rapid 
diminution  of  the  sacrum  and  coccyx. 
A  well-marked  thickening  or  bar  in  each 
ilium  runs  from  the  auricular  surface  to 
the  acetabulum  and  transmits  the  weight 
to  the  femora. 

The  Curves  o£  the  Spinal  Column. — 
There  is  only  one  curve — an  anterior 
concavity — until  the  3rd  month  (Fig. 
48,  A).     About  the  beginning  of  the  4th 

month  the  sacro-vertebral  angle  forms  between  the  lumbar  and  sacral 
regions  (Fig.  48,  B).  At  birth  the  cervical  and  sacral  curves  have  appeared, 
but  the  sacral  not  to  a  pronounced  extent  (Fig.  48,  C).  The  lumbar  curve 
appears  as  the  child  learns  to  walk.  It  is  produced  to  allow  the  body  to 
be  brought  vertically  over  the  lower  extremities.  The  sacral  and  cervical 
curves  also  become  at  that  time  more  marked  (Fig.  48,  D).  The  dorsal 
curvature  and  the  sacro-vertebral  angle  are  the  primitive  curves  and  are 
present  in  all  mammals.  The  others  are  adaptations  to  the  upright 
posture.  The  lumbar  curve  is  most  pronounced  in  the  highly  civilized 
races. 

Proportion  of  Cartilage  and  Bone. — The  intervertebral  discs  form 
one-third  of  the  total  height  of  the  spine  ;  the  proportion  of  cartilage  is 
greater  in  the  lumbar  than  in  the  dorsal  region  and  greater  in  the  dorsal 
than  in  the  cervical.  The  lumbar  and  cervical  curvatures  are  due  chiefly 
to  the  shape  of  the  discs  (H.  Morris).  In  the  lumbar  region,  which  is  con- 
vex forwards,  only  the  lower  three  vertebrae  are  deeper  in  front  than 


Fia.  47.- 


sacral 


-Diagram  of  the  Pyramids 
of  the  Spine. 


54 


HUMAN  EMBRYOLOaY  AND  MORPHOLOGY 


behind.  This  is  true  only  for  the  higher  races  of  mankind,  for  as  Cunning- 
ham has  shown,  in  lower  races,  as  in  the  gorilla,  only  the  last  lumbar 
vertebra  is  deeper  in  front  than  behind,  and  thus  helps  to  maintain  the 
lumbar  curvature. 

Unstable  Regions  of  the  Spine.^^ — In  about  90  %  of  men  there  are  7 
cervical,  12  dorsal,  5  lumbar,  5  sacral  and  4  caudal  vertebrae,  making  33 
in  all.  In  the  remaining  10  %  there  is  some  departure  from  the  normal 
arrangement  and  these  departures  afiect  certain  definite  regions.  The 
regions  affected  are  those  which  lie  at  the  junction  of  one  section  of  the 
spine  with  another — at  the  cervico-dorsal,  dorso-lumbar  and  lumbo- 
sacral junctions.     At  an  early  stage  of  development  all  the  vertebrae 


1,^,  ~dorso-lumbar 
g]      curue 


-ceruical 


iorso-lumbar 
curue 


sacro-uert. 
angle 


■ — dorsal  curve 


'eruic.  curue        l$ceruic.  curue 


Iw- sacro-uert.  angle 


■dorsal  curue 


r  lumbar  curve 


-sacral  curve 


B 


Fig.  48. — Diagram  of  the  Curves  of  the  Spinal  Column. 
A.  At  the  6th  week  of  foetal  life.     B.  At  the  4th  month  of  foetal  life, 
present  at  Birth.    B.  Curves  present  in  the  Adult. 


C.  Curves 


are  of  the  same  generalized  type  ;  at  a  later  stage  the  vertebra  of  each  body- 
segment  assumes  its  peculiar  form,  but  it  is  not  uncommon  for  one  vertebra 
to  assume  some  or  all  of  the  characters  of  the  one  before  it  or  behind  it. 
These  variations  represent  the  normal  error  in  developmental  markmanship; 
if  need  arises  the  developmental  aim  can  be  altered.  Such  vertebral 
variations  are  frequent,  and  are  often  of  clinical  importance. 

I.  The  sacro-lumbar. — The  25th  vertebra  in  95  %  of  people  forms  the 
1st  sacral ;   in  1  %  the  24th,  and  3  %  the  26th.     These  percentages  are 

1  For  literature  on  variations  of  vertebrae  :  Bardeleben,  Ergebnisse  der  Anat.  1905, 
vol.  15,  p.  119  ;  1906,  vol.  16,  p.  141  ;  1908,  vol.  18.  p.  71.  A.  Fischel,  Anat.\Hefte, 
1906,  vol.  31,  p.  459.  E.  Rosenberg,  Morph.  Jahrbuch,  1907,  vol.  36,  p.  609.  F.  Wood 
Jones,  Journ.  Anat.  and  Physiol.  1910,  vol.  44,  p.  377  (Influence  of  Nerve -plexuses  in 
determining  Development  of  Costal  Processes).  C.  R.  Bardeen,  A^ner.  Journ.  Anat. 
1905,  vol.  4,  p.  163  (Development  of  Vertebrae).  E.  Barclay  Smith,  Journ.  Anat.  and 
Physiol.  1911,  vol.  45,  p.  144.  T.  Manners  Smith,  Journ.  Anat.  and  Physiol.  1909, 
vol.  43,  p.  146.  A.  F.  Le  Double,  Bull,  et  Mem.  Soc.  d'Anthrop.  1911,  Ser.  6,  vol.  2, 
p.  413  (Lumbar  Ribs) ;  p.  428  (Cervical  Ribs).  F.  Wood  Jones,  Journ.  Anat.  and  Physiol. 
1911,  vol.  45,  p.  249  (Cervical  Ribs).  T.  W.  Todd,  Journ.  Anat.  and  Physiol.  1912, 
vol.  46,  p.  244  (Cervical  Ribs).  J.  C.  Brash  (Anomalous  Spines),  Journ.  Anat.  1915, 
vol.  49,  p.  243.     M.  F.  Lucas-Keen,  Journ.  Anat.  1915,  vol.  49,  p.  336. 


THE  SPINAL  COLUMN  AND  BACK 


55 


drawn  from  the  observations  of  Paterson,  Rosenberg,  and  others  who 
have  made  researches  on  this  subject.  The  vertebral  formula  is  not 
fixed.  Rosenberg's  investigations  showed  (Fig.  49)  that  it  is  the  26th 
vertebra  that  forms  the  first  of  the  sacral  series  in  the  early  embryo  ; 
later  the  25th  throws  out  great  lateral  masses,  and  thus  forms  a  connection 
with  the  ilia.  Bardeen  has  not  been  able  to  confirm  Rosenberg's  observa- 
tions ;  he  found  that  the  vertebra  which  was  to  form  the  first  sacral — 
whether  it  was  the  24:th,  25th  or  26th  in  the  vertebral  series — took  on  a 
predominance  at  its  earliest  appearance.  In  the  lower  primates  (monkeys) 
the  27th  forms  the  1st  sacral ;  with  the  evolution  of  man  the  26th,  then 
the  25th  underwent  sacral  modifications,  the  trunk  being  correspondingly 
shortened.  The  lumbar  region  of  the  human  spine  elongates  much  more 
rapidly  after  birth  than  either  the  cervical  or  dorsal  region,  in  order  to 
form  an  elongated  flexible  pillar  for  the  support  of  the  upper  part  of  the 


2.4' VERT.    5'^'^LUM. 


J^SAC. 


zn"  coc 


Fig.  49. — A  Section  of  the  Lumbo-sacral  Region  of  the  Spine  in  a  Foetus  at  the 
end  of  the  2nd  month,  sho-ning  the  26th  vertehra  forming  the  1st  Sacral.  (After 
Rosenberg.) 

body.  In  the  anthropoid  apes  the  lumbar  region  is  relatively  short  as  in 
the  child  at  birth.  It  will  be  seen  that  the  number  of  lumbar  vertebrae 
in  man  is  not  definitely  fixed.  The  anterior  point  of  attachment  of  the 
ilium  fluctuates  from  the  24th  to  the  26th  vertebra.  With  the  sacral 
transformation  of  the  25th  and  26th  (lumbar)  vertebrae,  there  is  a  cor- 
responding movement  forwards  of  the  sacral  plexus. 

II.  Sacro-coccygeal. — The  30th  vertebra  forms  the  1st  coccygeal ; 
not  uncommonly  this  vertebra  is  sacral  in  type  and  forms  part  of  the 
sacrum.  On  the  anterior  or  pelvic  aspect  of  the  1st  coccygeal  vertebra 
a  rudiment  of  the  haemal  arch  is  usually  to  be  found  during  foetal  life. 
The  haemal  arches  are  well  developed  on  the  proximal  caudal  vertebrae 
of  tailed  monkeys,  and  represent  developments  from  the  hypochordal  or 
intercentral  element  of  a  vertebra.  Variations  at  the  distal  end  of  the 
coccyx  are  dealt  with  later  (p.  65). 

III.  Dorso-Iumbar  region. — Tins  region  is  also  liable  to  variation  ; 
the  20th  vertebra  instead  of  forming  the  1st  lumbar,  may  simulate  the 
|ast  dorsal  in  the  type  of  its  articular  processes,  and  may  bear  ribs,  probably 


56 


HUMAN  EMBRYOLOGY  AND  MOEPHOLOGY 


a  reversion  to  an  ancestral  condition,  or,  on  the  other  hand,  the  12th 
dorsal  vertebra  (19th)  may  not  carry  ribs.  About  2  %  of  bodies  show  the 
latter  kind  of  variation — a  reduction  of  the  costal  series,  and  about  6  to 
8  %  the  former  kind,  in  which  the  costal  series  is  increased  (see  also  Chap. 
XIX.). 

IV.  Dorso-cervical. — The  7th  vertebra  may  carry  ribs  ;  rarely  the  8th 
vertebra  has  no  ribs  attached  to  it  and  is  cervical  in  type. 

In  Fig.  50  is  represented  the  condition  of  the  seventh  cervical  vertebra, 
as  seen  in  72  human  skeletons.  In  the  foetus,  the  costal  element  is  always 
apparent ;  in  the  adult  it  may  vanish  or  fuse  with  the  transverse  process. 
In  about  1  %  of  individuals  it  assumes  the  development  shown  in  Fig. 
50,  E  ;  it  may,  in  occasional  cases,  assume  all  the  characters  of  a  first 
dorsal  rib,  with  its  anterior  end  implanted  on  the  presternum.  Recently 
Prof.  Wingate  Todd  has  published  a  series  of  observations,  which  are 


Fig.  50. — Diagram  showing  the  variation  in  the  development  of  tlie  costal  element 
of  the  seventh  Cervical  Vertebra  in  72  skeletons.  In  A  and  B  the  costal  element 
is  partly  fused  with  the  transverse  process  ;   in  C,  D  and  E  it  remains  free. 

confirmed  by  the  statements  made  here.  A  cervical  rib  may  fuse  with 
the  costal  element  of  the  first  dorsal  vertebra,  thus  giving  rise  to  a  bicipital 
rib  (Dr.  Wood  Jones).  The  lower  trunk  of  the  bracbial  plexus  crosses  a 
cervical  rib,  and  hence  in  such  cases  symptoms  of  nerve-pressure  may  arise. 

V.  Cervico-occipital  Region.^ — The  occipital  or  posterior  part  of  the 
skull  represents  three  united  vertebrae.  Very  rarely  the  last  of  these  may 
partly  assume  a  vertebral  form,  but  it  is  by  no  means  rare  to  see  the  atlas 
or  first  cervical  vertebra  partly  fused  with  the  occipital  bone,  representing 
a  tendency  to  add  a  fourth  vertebra  to  the  occipital  series. 

The  Notochord. — In  its  primitive  form,  this  predecessor  of  the  vertebral 
column  is  well  seen  during  the  larval  stage  of  certain  fishes  (Fig.  40,  A). 
Its  manner  of  origin  in  the  human  embryo  has  been  mentioned  already 
(p.  36).  The  notochord  when  first  laid  down  under  the  neural  plate  of 
the  embryo  is  hollow — the  hinder  end  of  the  canal  opening  at  first  at  the 
neurenteric  canal.  Later  the  notochordal  tube  is  produced  in  the  primitive 
streak  and  later  still  at  the  growing  point  of  the  tail  (Fig.  63).     Afterwards 

^  For  reports  of  cases  of  fusion  of  atlas  :  Schumacher,  Anat.  Anz.  1907,  vol.  31, 
p.  145  (Homologies  of  Occipital  Bone) ;  K.  Weigner,  Anat.  Hefte,  1911,  vol.  45, 
p.  81  (Assimilation  of  Atlas)  :  Glaesmer,  Anat.  Anz.  1910,  vol.  36,  p.  129  ;  Le  Double, 
Bull,  et  Mem.  Anat.  1912,  p.  20  ;  G.  Elliot  Smith,  Brit.  Med,  Jgurn,  1908,  2,  p.  594  . 
R.  J.  Gladstone,  Journ.  Anat.  1915,  vol,  49,  p.  190,  ' 


THE  SPINAL  COLUMN  AND  BACK 


57 


the  notocliord  becomes  a  solid  rod  composed  of  cells  of  a  peculiar  type. 
A  sheath  is  formed  round  it  by  cells  of  the  paraxial  mesoblast  (Fig.  51), 

neural  crest 

neural  canal 

muscle  plate 

cutis  plate 
sclerotome 


notochord 
aorta 


muscle  plate 

intermedA, 
cell  mass) 

limb,  bud 

amnion 

umb.  uein 

Fig.  51. — A  Schematic  Section  of  an  Embryo  to  show  the  sclerotome,  muscle  plate 
and  skin  plate  which  arise  from  each  segment  of  the  paraxial  mesoblast.  (Com- 
pare with  Fig.  65,  p.  67.) 

which  grow  inwards  and  surround  it.  These  cells  form  the  sclerotome  and 
spring  from  the  inner  parts  of  the  primitive  segments  or  somite  into  which 
the  paraxial  mesoblast  is  divided  (p.  51). 
At  the  same  time  the  cells  of  the  sclero- 
tome also  grow  up  and  gradually  sur- 
round the  neural  tube.  From  these  cells 
which  grow  inwards  and  surround  the 
notochord  and  neural  canal,  the  mem- 
branous basis  of  the  spinal  column  is 
formed  and  also  the  basi-occipital  and 
part  of  the  basi-sphenoid  bones  of  the 
skull  (Fig.  52). 

What  becomes  of  the  Notochord.^ — 

In  the  second  month  of  foetal  life  the 
notochord  begins  to  disappear ;  the 
bodies  of  the  vertebrae  and  parachordal 
cartilages  form  round  its  sheath  and 
constrict  it.  The  parachordal  cartilages 
are  transformed  into  the  basi-occipital  / 

and  part  of  the  basi-sphenoid — the  basal       ^'' 
part  of  the  skull — behind  the  pituitary     f 

fossa.     The  notochord  disappears  in  the    A-, .nnrnux 

basilar  cartilage  of  the  skull.     Eternod,    tM 
however,  found  the  anterior  j^art  of  the     ^ 

v^^4-^«'U^^j     „,,     •i-'U^     J„  „„i    „,„ii     ^t    ^-^^  FiO.  52. — Where  remnants  of  the  Noto- 

notochord    on    the    dorsal    wall    of    the  chord  may  occur  in  the  Adult. 

^Papers  on  notochord:  A.  Bruni,  Anat.  Hefte,  1912,  vol.  45,  p.  307  (Involu- 
tion of  Notochord)  ;  A.  Linck,  Anat.  Hefte,  1911,  vol.  42,  p.  607  (Dev.  of  Notochord) ; 
L,  W.  Williams,  Amer,  Journ.  Anat.  1908,  vol.  8,  p.  251. 


pituitary 

basi-sphen, 

basi-occip. 

suspensory  lig. 
—axis. 

interuert.  disc, 
centre  of  diso 


i 


58 


HUMAN  EMBRYOLOGY  AND  MOEPHOLOGY 


pharynx  in  the  human  embryo ;  Robinson  has  shown  that  in  man 
the  parachordal  cartilages  are  developed  in  part  on  its  dorsal  aspect  (Fig. 
53).  The  odontoid  process  represents  the  body  of  the  atlas,  and  the 
suspensory  ligament  the  disc  between  the  occipital  bone  and  atlas.  A 
remnant  of  the  notochord  is  enclosed  in  the  suspensory  ligament.  The 
centrum  or  body  of  each  vertebra  is  formed  round  the  notochord  (Fig.  56, 
F),  but  only  between  the  centra,  where  the  intervertebral  discs  are  formed, 
does  this  primitive  structure  persist.  In  the  discs  the  notochord  swells 
out  and  forms  a  considerable  part  of  the  central  mucoid  core  which  each  disc 
contains. 

Primitive  Segments  or  Somites. — Somites,  or  protovertebrae  as  they 
were  formerly  named,  are  not  the  forerunners  of  the  vertebrae  ;  they  are 


notochord  in  rooj 
of  phar 


:  c. 


Fig.  53. 


atlas 
axis 


-The  relationship  of  the  Notochord  to  the  basilar  or  parachordal  cartilage  of 
the  human  embryo.     (Arthur  Robinson.) 


the  primitive  segments  into  which  the  mass  of  mesoderm  at  each  side  of 
the  neural  canal  and  notochord  divides  (Fig.  51,  also  Fig.  19),  The  process 
of  division  or  segmentation  begins  at  the  occipital  region  towards  the  end 
of  the  third  week,  and  spreads  backwards  until  35  or  more  body  segments 
or  somites  are  isolated.  Each  segment  thus  separated  forms  its  own 
muscles  (from  its  muscle  plate  or  myotome),  has  its  own  nerve  (spinal  nerve), 
its  own  artery  (intercostal),  its  own  cutis  plate  or  dermatome,  and  the  basis 
for  its  skeletal  tissue  (sclerotome)  (Fig.  51).  The  intersegmental  septum 
separates  one  somite  from  another.  Ribs,  transverse  and  spinous  processes,, 
are  formed  in  the  intersegmental  septa.  Hence  an  intercostal  space  with 
its  muscles,  vessels,  and  nerves,  with  the  corresponding  intervertebral 
structures,  represents  a  differentiated  somite.  In  the  ventral  aspect  of 
the  neck  and  loins,  some  of  the  intersegmental  septa  disappear. 

Morphological  Parts  of  a  Vertebra. — The  constituent  parts  of  a 
vertebra,  although  much  modified,  may  be  best  recognized  in  the  altas 
(see  Fig.  54).     These  parts  are  (1)  the  centrum,  which  forms  the  odontoid 


THE  SPINAL  COLUMN  AND  BACK 


59 


process  ;  (2)  the  right  and  (3)  the  left  half  of  the  neural  arch  ;  (4)  the 
hypochordal  part,  which  forms  the  anterior  arch  or  bow.  Besides  the  four 
chief  elements  there  are  three  secondary  processes  or  levers,  all  of  which 
spring  from  the  neural  arch.     These  are  (a)  spinous,  (6)  transverse,  (c) 

spinous  pr. 
spinal  canal 
heck  lig.  (disc) 
trans,  lig.  (disc) 
eural  arch 


'transu.  pr. 
'cost  pr. 
odontoid  (centrum) 
hypo-chordal  part. 

Fig.  54. — The  Morphological  Parts  of  the  first  Cervical  Vertebra. 

costal  processes.  In  the  dorsal  region  the  costal  processes  become  separated 
from  the  neural  arches  by  articulations  ;  in  other  vertebrae  they  retain 
their  continuity  with  the  arch. 

Development     of     a     Typical    Vertebra— the    6th     Dorsal.^— (1) 
Membranous   Stage   (5th  and  6th  weeks).     The  vertebra  then  consists 


notochord 


neural  canal 


yerteb. 
bow 


-notochord 
sheath 


hypochord.  boi/j 


B 


liypoch.  bouf 

~^body  of  uert. 

myotome 

septum 

rib 
_^ myotome 

^^ — hypochord.  bow 
body  of  uert. 
septum 

rib 

=  myotome 

hypochord.  bow 


notochord 


Fig.  55. — The  development  of  the  Membranous  Basis  of  a  Vertebra. 
A.  In  transverse  section.    B.  In  horizontal  section  showing  the  relation  of  the 
vertebra  to  the  Primitive  Segments.    The  section  is  viewed  from  the  dorsal 
aspect. 

of  1st  a  centrum  surrounding  the  notochord,  formed  from  its  sheath  (Fig. 
55,  A),  and  2nd  a  horse-shoe  shaped  vertebral  bow  (Fig.  55,  A  and  B). 
The  bow  consists  of  the  right  and  left  limbs  which  become  corresponding 

^  For  an  account  of  the  differentiation  and  development  of  vertebrae  :  C.  R.  Bardeen, 
Amer.  Journ.  Anat.,\Q05,  vol.  4,  p.  163  (Thoracic  Vertebrae)  ;  also  p.  265  ;  1908,  vol. 
8,  p.  181  (Cervical  and  Occipital  Regions). 


60 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


parts  of  the  neural  vertebral  arch  and  the  hypochordal  bow  which  unites 
the  neural  arch  limbs  ventral  to  the  centrum. 

(2)  Cartilaginous  Stage  (Fig.  56)  commences  in  the  6th  week  when  the 
embryo  is  9  to  10  mm.  in  length.  The  fibrous  basis  of  the  whole  vertebra 
is  transformed  into  cartilage.  In  each  lateral  half  of  the  cellular  basis  of  a 
vertebra  three  centres  of  chondrification  appear — one  for  the  neural  arch, 
one  for  the  costal  process,  and  one  for  each  half  of  the  centrum,  but  those 
of  the  centrum  soon  fuse.  In  the  process  of  chondrification  the  cells 
derived  from  the  sclerotome  are  directly  transformed  into  cartilage  cells. 
In  the  atlas  the  hypochordal  part  of  the  bow  becomes  cartilaginous  and 
subsequently  ossified  ;    in  all  the  other  vertebrae,  excepting  the  cervical 


neural  canaL 


neun  arch 


hypoch.  bow^  notoch. 

epiph.  centres 
costal  facet 


centrum 

plate 
centrum 


centre 
for  arch 

neuro-cent. 
sut. 


centres  for  body 


neurO'Cent  sut. 


Opiate 


cavity 


Fig.  56.— Showing  the  Stages  in  the  Development  of  a  Vertebra. 
A.  In  the  Membranous  Stage.    B.  In  the  Cartilaginous  Stage.     C.  The  appearance 
of  Ossiflc  Points.    D.  The  appearance  of  Secondary  Ossiflc  Centres.    E.  The 
epiphyseal  plates  of  the  centra.    F.  Section  of  an  amphicoelous  vertebra. 

segments  just  behind  the  atlas  (Fig.  58),  this  element  never  passes  beyond 
the  membranous  stage  of  development.  It  should  be  noticed  (Fig.  55,  B) 
that  the  vertebral  bodies  are  formed  round  the  notochord,  opposite  each 
intersegmental  septum.  Hence  each  centrum  must  be  regarded  as  the 
product  of  two  somites.  The  intervertebral  disc  is  situated  opposite  the 
middle  of  a  segment  (Ebner).  The  lateral  limbs  of  the  cartilaginous  bow 
meet  behind  (dorsal  to)  the  neural  canal  in  the  4th  month,  thus  completing 
the  neural  arch.     At  the  site  of  a  spina  bifida  (see  p.  83)  this  union  fails. 

(3)  Bony  Stage. — The  centrum  and  neural  arch  elements  of  the  carti- 
laginous vertebra  fuse  and  give  rise  to  the  condition  shown  in  Fig.  56,  C. 
In  the  7th  week  two  centres  of  ossification  appear  in  the  centrum,  but 
quickly  fuse  ;  one  appears  in  each  limb  of  the  neural  arch  (8th  week)  ; 
at  birth  the  ossific  centres  of  the  centrum  and  neural  arch  have  met.  The 
central  and  neural  ossifications  meet  at  the  neuro-central  suture,  and 
unite  at  the  4th  or  5th  year,  the  body  being  formed  by^(l)  the  centrum, 
(2)  basal  parts  of  the  neural  arch  (Figs.  54,  56).     The  neural  ossifications 


THE  SPINAL  COLUMN  AND  BACK  61 

fuse  behind  (where  the  spinous  process  is  produced)  in  the  1st  year.  The 
spinous  and  transverse  processes  are  formed  by  outgrowths  of  cartilage 
into  the  septa  between  the  somites  or  primitive  segments,  where 
they  serve  as  levers  on  which  the  spinal  musculature  acts.  The  ribs  are 
also  formed  by  outgrowths  from  the  vertebrae.  In  the  cervical,  lumbar 
and  sacral  regions  they  fuse  with  the  transverse  processes,  but  in  the 
dorsal  region  they  remain  as  separate  elements.  In  typical  ribs  the  head 
corresponds  to  the  intervertebral  disc,  because  according  to  Gadow  the 
rib  was  originally  evolved  from  an  intervertebral  element — the  inter- 
central  or  hypochordal.  In  atypical  ribs — ^the  1st,  11th  and  12th — the 
head  of  the  rib  articulates  only  with  the  vertebra  behind  its  own  disc. 
Epij)hyseal  centres  for  the  ossification  of  the  transverse  and  spinous  pro- 
cesses appear  about  puberty. 

The  Bodies  of  Mammalian  Vertebrae  are  peculiar  (1)  in  the  development 
of  an  upper  and  lower  epiphyseal  plate  ;    (2)  in  that  no  trace  of  the  noto- 

7^-i^  Cent  in,6tl]  0.  Vert      

rT.i:(:|HHHtl:lHglthl«l->|.i.»|.>hJn--|-.|     II     I    I 

Axis  in  4th  month      < —      7th  week >         5th  Sac.  in  5th  month 


1st  cent  in  Atlas. 


6m 


■r 

7th  weeli     >  o  3rd  Sac.  in  7th  month 

a. 

Fig.  57. — The  Order  in  which  the  Centres  of  Ossification  appear  in  the  Bodies  (,A) 
and  in  the  Nemral  Arches  (iJ)  of  the  Spinal  Column. 

chord  remains  within  them.  In  Fishes,  as  in  the  early  human  or  mam- 
malian foetus,  the  bodies  are  hour-glass  shaped  (amjDhicoelous,  Fig.  56,  F)  ; 
in  Amphibians  they  may  retain  a  concavity  in  front  (procoelous)  or  behind 
(opisthocoelous),  but  in  mammals  both  ends  are  filled  up. 

It  will  be  observed  (Fig.  57,  B)  that  the  centres  of  ossification  ^  for 
the  neural  arches  appear  first  in  the  anterior  end  of  the  spine  (1st  cervical), 
the  date  becoming  later  the  more  posterior  the  vertebra.  In  the  1st 
sacral  they  appear  about  the  4th  month  ;  in  the  2nd  sacral,  in  the  5th 
month  or  later  ;  in  the  3rd  they  may  not  appear.  In  the  4th  and  5th 
sacral  and  1st  coccygeal  vertebrae  only  vestiges  of  the  neural  arches  are 
formed.  These  vertebrae  retain  the  early  foetal  type  shown  in  Fig.  56,  B. 
In  the  remaining  coccygeal  vertebrae  only  the  centres  for  the  bodies 
appear.  The  centres  for  ossification  of  the  bodies  of  the  vertebrae  appear 
first  in  the  mid-dorsal  region  (6th  dorsal).  From  that  point  they  spread 
forwards  and  backwards,  the  centres  for  the  odontoid  process  appearing 
at  the  4th  month,  and  that  for  the  5th  sacral  at  the  5th  month,  while  the 
coccygeal  do  not  appear  until  about  birth. 

1  See  F.  P.  Mall,  Amer.  Journ.  Anat.  1906,  vol.  5,  p.  433  (Centres  of  Ossification 
before  end  of  2nd  month);  E.  Fawcett,  Journ.  Anat.  and  Physiol.  1911,  vol.  45, 
p.  172  (Costal  Epiphyses).     Prof.  F,  Dixon,  Journ.  A^iat.  1921,  vol.  55,  p.  38. 


62 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


The  Atlas  and  Axis. — The  atlas  represents  the  completed  bow  of  the 
1st  cervical  vertebra  (Fig.  54).  The  body  of  the  vertebra  fuses  with  the 
body  of  the  2nd,  and  forms  the  odontoid  process.  A  remnant  of  the  disc 
between  the  1st  and  2nd  vertebrae  can  sometimes  be  seen  when  the  odon- 
toid is  split  open.  The  suspensory  and  check  liagments  are  the  repre- 
sentatives of  the  disc  between  the  last  occipital  segment  and  the  1st  cervical 

term,  of  notoch. 
basi-occip. 
susp.  lig. 
1st  hypoch.  bow  (atlas) 

1st  centrum 

•2nd  hypoch.  boiu. 

2nd  centrum 

J  3rd  bow  and 
\  centrum 

notochord 

Fig.  58. — A  Diagrammatic  Section  of  the  Foetal  Axis,  Atlas,  and  Basi-occipital. 

(Figs.  54  and  58).  A  nodule  in  the  suspensory  ligament  may  represent  an 
occipital  centrum  (see  p.  144). 

Occipito-atlanto-axial  Articulations.^ — In  the  intervertebral  discs  of 
the  cervical  region  there  is  at  each  side,  between  the  lateral  lips  of  the 
vertebral  bodies,  a  small  articular  cavity  (Fig.  59).  It  is  situated  between 
the  part  of  the  body  formed  by  the  neural  arches  and  lies  in  front  of  (ventral 

occip. 

artic. 

1st  neural  arch 

artic. 

2nd  neural  arch 

artic. 

3rd  neural  arch 

3rd  centrum 

FiQ.  59. — The  nature  of  the  Atlanto-axio-occipital  Articulations. 

to)  the  issuing  spinal  nerves.  Between  the  axis  and  atlas  this  articulation 
is  greatly  enlarged.  Here  the  rotatory  movements  of  the  atlas  on  the 
axis  take  place.  The  atlanto -occipital  joint,  which  separates  the  atlas 
and  the  last  occipital  segment,  is  of  the  same  nature.  The  atlas  has 
neither  the  upper  nor  the  lower  articular  processes  of  the  other  vertebrae. 
Hence  the  1st  and  2nd  cervical  nerves  appear  to  issue  behind  the  articular 
processes.  At  one  time  the  single  median  occipital  condyle  seen  in  birds 
and  reptiles  was  regarded  as  very  different  in  nature  from  the  double 

1  0.  Jaekel,  Anat.  Anz.  1912,  vol.  40,  p.  609  (Morphology  of  Atlas). 


rib 


'      THE  SPINAL  COLUMN  AND  BACK  63 

condyles  of  mammals.  Eecently  Symington  has  sliown  that  in  the  lowest 
mammals  (mouotremes),  the  occijHtal  condyles  are  fused  in  the  middle 
line,  and  that  all  foetal  mammals  also  show  this  condition.  The  articular 
facets  on  the  upper  surface  of  the  atlas  are  also  continuous  over  the  hypo- 
chordal  element.  In  the  human  skull  a  remnant  of  this  median  fusion  of 
the  condyles  is  frequently  seen  on  the  anterior  margin  of  the  foramen 
magnum  ;  it  is  named  the  third  or  median  occipital  condyle. 

The  Ribs  are  developed  as  outgrowths  of  the  membranous  vertebrae 
into  the  septa  between  the  primitive  segments  of  the  thoracic  region  of 
the  embryonic  body.  In  lower  vertebrates  (birds,  reptiles,  etc.)  each  rib 
has  two  heads,  a  dorsal  and  ventral  (Fig.  60) .  The  tuberosity  of  the  human 
rib  represents  the  dorsal  head  ;  the  ventral  head  is  well  developed  in  man, 
as  in  mammals  generally.  The  rib  articulates  with  the  neural  arches  only 
(Fig.  56,  B).  The  conjugal  ligament  is  made  up  of  fibres  which  cross  in 
the  posterior  aspect  of  the  intervertebral  disc  and  unite  the  heads  of  the 

snim     ,  , 

'  '         imns.  proc. 

^''""^-  P'''^^-~^-^M3^  ^dors.  head 

costal  facet 

centrum^  ,i.    ,„„  . 

(jsnt  head 

Fig.  60. — The  Bicipital  Rib  of  a  Lower  Vertebrate  (crocodile). 

corresponding  right  and  left  rib.  The  conjugal  ligament  which  is  strong 
in  some  animals  is  weak  in  man  (Bland-Sutton).  The  transverse  ligament 
of  the  atlas  may  belong  to  the  conjugal  series. 

Vestigial  Ribs. — Although  the  ribs  are  only  fully  developed  in  the 
dorsal  region,  yet  a  representative — a  costal  element — is  present  in  every 
vertebra.  In  the  cervical  vertebrae  (Fig.  54)  the  anterior  part  of  the 
transverse  processes  represents  a  costal  process,  but  only  in  the  6th  (some- 
times) and  7th  is  the  costal  process  formed  by  a  separate  centre  of  ossifica- 
tion. The  costal  process  of  the  7th,  usually  represented  by  a  mere  vestige, 
may  develop  into  a  rudimentary  or  even  a  fully  formed  rib  which  reaches 
the  sternum.  In  the  lumbar  vertebrae  only  the  first  shows  a  separate  centre 
for  the  formation  of  the  costal  process  ;  it  fuses  with  the  transverse  process 
in  the  later  months  of  foetal  life  ;  in  the  other  lumbar  vertebrae  the  tips 
or  perhaps  the  whole  of  the  transverse  processes  represent  costal  processes. 
The  12th  dorsal  rib  varies  greatly  in  size  ;  it  may  be  six  or  ten  inches  long 
or  reduced  to  a  mere  vestige.  In  quite  40  %  of  women  the  12th  rib  cannot 
be  palpated  because  it  does  not  project  beyond  the  outer  border  of  the 
erector  spinae. 

In  the  1st,  2nd  and  3rd  sacral  vertebrae  the  costal  processes  are  large 
and  have  their  own  centres  of  ossification.^     Their  cartilaginous  bases 

1  References  to  papers  on  sacrum  :    E.  Fawcett,  A7iat.  Anz.  1907,  vol.  30,  p.  414 
(Sacral  Costal  Epiphyses) ;    Otto  Petersen,  Anat.  Anz.  1905,  vol.  26,  p.  521  ;    D.  E. 
Derry,  Journ.  Anat.  and  Physiol.  1911,  vol.  45,  p.  202  (Sacral  Accessory  Articulations) 
L.  Bolk,  Anat.  Anz.  1912,  vol.  41,  p.  54. 


64 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


fuse  early  to  form  the  greater  part  of  ttie  lateral  masses  of  tlie  sacrum. 
The  part  of  the  lateral  mass  formed  by  the  costal  processes  is  shown  in 
Fig.  62.  The  costal  processes  are  absent  in  the  4th  and  5th  sacral  and  in 
all  the  coccygeal  vertebrae.  The  two  lateral  epiphyseal  plates  on  each 
side  of  the  sacrum  are  new  and  independent  formations. 

The  Accessory  Processes  are  found  in  the  lumbar  and  lowest  two  dorsal 
vertebrae.  They  are  developed  at  the  base  of  the  transverse  processes  and 
are  for  the  attachment  of  slips  of  the  longissimus  dorsi.  The  mammillary 
processes  are  developed  on  the  articular  processes  of  the  lower  two  or  three 
dorsal  and  all  the  lumbar  vertebrae.  They  give  attachment  to  tendons 
of  origin  of  the  multifidus  spinae.     In  a  paper  {Journ.  Anat.  and  Physiol. 


MAMMILLARY    PROC 
TRANSVERSE  PROC: 


COSTAL 
ELLEMENT 


neural  arch 
trans,  proc. 
cost  proc. 

epiphysis 


centrum 


CENTRUM 


Fig.  61. — Half  of  a  first  Lumbar  Vertebra  showing  a  separate  costal  element. 
Fig.  62. — A  Section  to  show  the  Nature  of  the  Elements  composing  the  Sacrum. 


1912,  vol.  47,  p.  118)  by  Dr.  Wood  Jones,  it  is  pointed  out  that  these  two 
muscular  processes,  the  mammillary  and  accessory,  are  fused  together 
in  the  dorsal  region,  but  in  the  lumbar  region  they  are  separated  by  a 
groove  containing  the  inner  branch  of  the  posterior  division  of  the  corres- 
ponding spinal  nerve. 

The  Transverse  and  Spinous  Processes  grow  out  from  the  vertebral 
bow  (Fig.  56,  A)  into  the  septa  between  the  primitive  segments.  Each 
transverse  process  is  pierced,  while  still  in  the  fibrous  condition,  by  a 
branch  of  the  corresponding  segmental  (intercostal)  artery.  In  only 
the  cervical  region  do  those  perforating  arteries  and  their  foramina  persist. 
In  that  region  the  perforating  arteries  anastomose,  and  out  of  the  chain 
thus  formed  is  developed  the  vertebral  artery.  Thus  the  foramina  for  the 
vertebral  artery  are  formed  independently  of  the  costal  element  in  each 
cervical  transverse  process.  The  spines  are  absent  on  the  1st  cervical, 
4th  and  5th  sacral  and  coccygeal  vertebrae.  They  are  slightly  developed 
and  united  by  ossification  of  the  interspinous  ligament  in  the  2nd  and  3rd 
sacral  vertebrae.     The  2nd,  3rd,  4th,  5th,  and  6th  cervical  spines  are 


THE  SPINAL  COLUMN  AND  BACK 


65 


bifid  in  Europeans  ;   but  in  lower  races,  as  in  anthropoids,  the  5th  and  6th 
spines  are  usually  undivided. 

Caudal  or  Coccygeal  Vertebrae.^At  the  end  of  the  6th  week,  the 
body  of  the  embryo  being  then  11  mm.  in  length,  the  human  tail  reaches 
its  maximum  growth — projecting  as  a  conical  process  fully  1  mm.  in  length 
and  equal  to  about  one-tenth  of  the  long  diameter  of  the  embryonic  body. 
En  the  adult  body  the  30th  vertebra  is  usually  the  first  of  the  coccygeal 
series.     In  the  fifth  week  the  growing  caudal  point,  at  which  neural  canal, 


5  yveeMs. 


SEGMENT  20 


Fig.  63. — A  series  of  four  figures  showing  tlie  conlition  of  tlie  liuman  caudal  or  coccy- 
geal region  at  the  stages  indicated  on  the  drawings  (after  Kunitomo). 


notochord,  sclerotomes,  and  cloaca  are  all  being  extended  in  a  backward 
direction,  has  reached  and  produced  the  30th  segment  (Fig.  63)  ;  at  the 
6th  week,  ten  or  twelve  caudal  segments  have  been  laid  down.  There- 
after retrogression  sets  in  ;  by  the  end  of  the  8th  week  (Fig.  63)  only  the 
caudal  tip  j)rojects  and  the  coccygeal  vertebrae  have  been  reduced  to 
4  or  5,  while,  by  the  13th  week,  a  depression  or  pit  marks  the  site  where  the 
tip  disappeared.  The  coccygeal  part  of  the  neural  canal  is  atrophied  and 
the  distal  part  of  the  whole  cord  is  retracting  in  a  cranial  direction.^ 

^  See  Prof.  G.  L.  Streeter,  Amer.  Journ.  Ayiat.  1919,  vol.  25,  p.  1  ;   Dr.  Kanae  Kun- 
itomo, Contrib.  to  Embryology,  1918,  vol.  8,  p.  161. 


CHAPTER  VI. 


THE   SEGMENTATION   OF  THE   BODY. 

At  the  end  of  the  3rd  week,  as  we  have  already  seen  (p.  18),  the  paraxial 
mesoderm,  lying  at  each  side  of  the  neural  tube,  becomes  divided  from 
before  backwards  into  somites  or  primitive  segments,  their  demarcation 
becoming  evident  first  in  the  occipito-cervical  region  of  the  body.  By  the 
end  of  the  4th  week  the  process  has  reached  the  1st  coccygeal  segment, 
there  being  then  3  occipital  and  30  body  somites.  The  occipital  somites 
disappear  prematurely  but  those  of  the  body,  although  they  become 
specialized  and  broken  up,  can  still  be  recognized  in  the  adult.     In  the 

nth  interc.  nerve. 

/lUh  rib.  ^^^^fj^ 

int.  ob. 

umb.  cord 


12th  rib. 


lat.  cut. 


ant  cut 


FiC.  64. — Some  of  the  structures  derived  from  the  11th  Dorsal  Segment  of  the 
Right  Side. 

preoccipital  region  of  the  head,  parts  are  also  arranged  on  a  segmental 
plan,  one  which  is  older  than  the  vertebrate  segmentation  of  the  trunk  and 
can  best  be  identified  by  the  visceral  or  gill  arch  system  of  the  pharynx. 

Segmentation  of  the  Body.i— The  human  body  or  trunk  consists  of 
33  or  34  segments.  Each  segment  is  fundamentally  of  the  same  type, 
but  the  resemblance  is  obscured  owing  to  extensive  modifications  which  the 
somites  undergo  to  form  the  cervical,  dorsal  (thoracic),  lumbar  (abdominal), 
sacral  (pelvic)  and  caudal  regions  of  the  body.  The  outgrowth  of  the 
limbs  also  renders  it  difficult  to  recognize  in  the  adult  the  simple  system 
of  segments  which  can  be  seen  in  the  embryo  at  the  end  of  the  third  week 
(Fig.  19,  p.   17). 

^  For  papers  on  segmentation  see  :  G.  van  Rynberk,  Ergebnisse  der  Anat.  1908, 
vol.  18,  p.  353  ;  A.  L.  J.  Sunier,  Onderzoekingen  verricht  in  het  Zoolog.  Lab.  Univ. 
Groningen,  191 1,  Leyden  (Differentiation  of  Myotome).  See  also  references  : — Bar  leen, 
p.  54  ;   under  W.  H.  Lewis,  p.  426  ;   Watt,  p.  42;   Barniville,  p.  47. 

66 


THE  SEGMENTATION  OF  THE  BODY 


67 


Until  lately  the  segmentation  of  the  human  body  was  a  matter  of  only 
speculative  importance,  but  recent  advances  in  our  knowledge  of  the 
distribution  of  nerves  have  shown  that  it  has  a  direct  bearing  on  diagnosis 
and  treatment. 

Constitution  of  a  Typical  Segment  (11th  Dorsal). — It  is  better  to 
study  the  development  of  a  typical  body  segment,  and  from  that  the 
student  will  be  able  to  note  for  himself  the  modifications  which  have  taken 
place  in  the  more  highly  differentiated  segments  of  the  body.  As  already 
explained,  the  process  of  segmentation  affects  chiefly  the  paraxial  block 
of  mesoderm  which  lies  on  each  side  of  the  neural  canal  and  notochord, 


lot.  dorsi 


quad.  lumb. 

lumb.  aponeur. 

Kidney 
(intermed. 
cell-mass) 

nf.  ob. 
somato-pleure 


ilio-costalis 

vertebra  {sclerotome} 

^neural  canal 
-post,  root  gang. 

^muscle  plate 
-skin  plate 
■  sclerotome 
h^^Xf^i-z^^i  ^ — notochord 


-aorta 


{intermed. 

\  cell-mass. 

splanchno-pleure 

somato-pleure 

coelom 
gut 


rectus 

Fig.  C5,  A. — A  Transverse  Section  showing  tlie  Elements  of  the  1st  Lumbar  Segment 

in  the  Adult. 
B. — A  corresponding  Section  of  an  Embryo  about  the  end  of  the  4th  week 
(diagrammatic). 

and  also,  to  a  lesser  degree,  the  intermediate  cell  mass.  In  Figs.  65,  A,  B, 
a  body-segment  is  represented  in  the  adult  and  in  the  embryonic  condition. 
The  following  elements  make  up  the  11th  dorsal  segment  :  (1)  Its 
skeletal  basis  ;  (2)  Muscular  element  ;  (3)  Renal  element ;  (4)  Vessels  ; 
(5)  Nerves  ;  (6)  Neural  segment ;  (7)  Cutis  plate.  Although  the  ecto- 
derm and  entoderm  are  never  segmented,  yet  a  definite  area  of  each  is 
associated  with  every  body  segment.  The  origin  of  each  element  will 
be  taken  separately. 

I.  The  skeleton  of  the  11th  dorsal  segment  is  represented  by  the  adjacent 
halves  of  the  11-12  dorsal  vertebrae  and  the  disc  between  them,  for,  as 
already  pointed  out,  the  vertebrae  are  intersegmental  in  their  development 
(Fig.  55,  B).  The  transverse  processes,  the  spinous  processes  and  11th  and 
12th  ribs  are  also  formed  in  the  septa  in  front  of  and  behind  the  11th 


68      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

segment  (Fig.  64).  The  septum  in  the  rectus  muscle  a  little  below  the 
umbilicus  represents  the  intersegmental  septum  corresponding  to  the  11th 
rib.  Sometimes  another  septum  occurs  in  the  rectus,  midway  between 
the  pubes  and  umbilicus,  marking  the  lower  limit  of  the  11th  segment. 
The  linea  alba  separates  the  segments  of  the  two  sides. 

In  the  linea  alba  or  ventral  median  line  of  the  thoracic  region,  the 
sternum  is  developed.  The  intersegmental  septa  are  well  marked  in  the 
thoracic  region  ;  the  ribs  and  their  cartilages  are  developed  in  them. 
In  the  neck  the  septa  are  almost  lost ;  the  intermediate  tendon  of  the 
omo-hyoid  and  the  septa  occasionally  found  in  the  sterno-hyoid  and 
-thyroid,  complexus  and  trachelomastoid  muscles  are  the  only  repre- 
sentatives of  them  in  the  cervical  region. 

II.  The  Muscles  of  the  11th  Dorsal  Segment. — All  the  muscles  of  this 
segment  are  developed  from  the  muscle  plate  (myotome)  of  the  primi- 
tive segment  (see  Figs.  64  and  65).  There  is  a  cavity,  which  probably 
arises  as  a  diverticulum  of  the  coelom,  in  each  primitive  segment  (Fig. 
39,  p.  39).  The  cells  of  the  mesoderm  on  the  inner  side  of  the  segmental 
cavity  become  columnar  and  form  the  muscle  plate  (Figs.  51,  64).  Each 
segment  has  its  own  muscle  plate.  The  cells  or  myoblasts  of  each  plate 
increase  rapidly  in  number,  forming  a  fused  mass  or  syncytium  ;^  they 
spread  into  the  somatopleure,  and  form  the  muscles  of  the  body  wall  and 
limbs.  In  the  myosyncytium  fibrillae  and  fibres  are  formed  ;  each  fibre 
becomes  elongated  and  directed  across  its  segment  from  septum  to  septum. 
The  intercostal  muscles  retain  this  arrangement,  but  in  the  abdominal 
region  the  fibres  fuse  with  those  of  neighbouring  segments  to  form  muscular 
sheets — -the  external  oblique,  internal  oblique,  transversalis  and  rectus. 
In  the  foetus  of  the  fifth  month  traces  of  these  septa  may  be  seen  ;  Bardeen 
found  that  the  intercostal  nerves  retained  their  segmental  distribution 
in  the  muscles  of  the  belly  wall.  In  fishes  the  embryonic  segmental 
arrangement  of  the  musculature  persists.  The  manner  in  which  the 
final  groups  of  muscles  are  derived  from  the  muscle  plates  is  not  accurately 
known,  but  in  the  typical  segment  with  which  we  are  at  present  dealing 
it  will  be  seen  that  the  musculature  falls  into  two  groups  (see  Fig.  65,  A)  : 
(1)  epaxial,  the  erector  spinae,  etc.  ;  and  (2)  ventro-lateral  or  body-wall 
muscles  (intercostals,  rectus,  oblique  muscles,  etc.).  The  musculature 
of  the  limbs  is  derived  from  the  ventro-lateral  group  (Figs.  446,  p.  423 ; 
455,  p.  432). 

The  ventro-lateral  sheet  separates  into  a  ventral  longitudinal  band 
and  a  lateral  transverse-oblique  stratum.  Each  of  these  divides  into 
an  inner  and  outer  primary  layer ;  the  outer  and  inner  secondary  layers 
arise  as  delaminations  of  the  primary  layers,  thus  making  four  in  all. 
The  internal  oblique  and  transversalis,  and  internal  intercostal  are  derived 
from  the  internal  primary  layer  ;  the  external  oblique  and  external  inter- 
costal from  the  external  primary  layer. ^  The  rectus  abdominis  represents 
the  deeper  of  the  two  layers  derived  from  the  external  primary.     Parts 

1  See  Prof.  J.  Cameron,  Trans.  Roy.  Soc.  Canada,  1918,  vol.  11,  p.  81. 

2  See  Prof.  T.  Walmsley,  Journ.  Anat.  1916,  vol.  50,  p.  165. 


THE  SEGMENTATION  OF  THE  BODY 


69 


of  the  deejjest  layer  of  the  lateral  sheet,  represented  in  the  adult  by  the 
transversalis,  have  migrated  inwards  to  form  the  subvertebral  or  hypaxial 
muscles — the  quadratus  lumborum,  crura  of  the  diaphragm,  longus  colli, 
rectus  capitis  anticus  major  and  minor,  and  the  levator  ani.  When 
muscles  migrate  they  invariably  carry  with  them  the  nerves  of  the  body 
segments  in  which  they  are  developed.  Hence  the  nerve  supply  affords 
the  clue  to  the  segments  from  which  a  muscle  or  part  of  a  muscle  arises. 
The  middle  layer  of  the  lumbar  fascia  is  developed  between  the  epaxial 
and  ventrolateral  musculatures. 

Many  of  the  ventro-lateral  muscles  (trapezius,  rhomboids,  and  latissimus 
dorsi),  migrate  dorsalwards  over  the  epaxial  muscles,  and  take  origin  from 
the  spines  of  the  vertebrae  (Fig.  65,  A). 


\o  ^^^intercost.  artery 


dorsal  br. 


aorta 


ventral  anastom. 


Fig.  66. — The  distribution  of  a  typical  Segmental  Artery. 

Muscular  fibrillae  begin  to  form  in  the  5th  week,  appearing  in  the  proto- 
plasmic matrix,  in  which  the  nuclei  of  the  myoblasts  are  embedded.  The 
fibrillae  group  themselves  in  bundles  or  muscle  fibres,  the  nuclei  with  some 
of  the  myoplasm  being  applied  to  the  surface  of  the  completed  fibre.  New 
fibre  production  goes  on  rapidly  until  the  5th  month,  when  the  complement 
for  each  muscle  is  nearly  complete.  Thereafter  muscles  grow  in  size, 
chiefly,  it  is  believed,  by  an  increase  in  the  size  of  the  individual  fibres. 
Although  voluntary  muscle  fibres  atrophy  when  their  nerve  is  cut,  yet 
myoblasts  will  develop  into  muscles  when  separated  from  nerve  cells 
(Ross  Harrison),  or  when  grown  in  artificial  media  outside  the  body. 

III.  The  Arteries  of  the  11th  Segment  ^  (Fig.  66).— The  11th  intercostal 
is  the  artery  of  the  segment.  It  gives  off  a  dorsal  branch  to  supply  the 
epaxial  muscles,  the  spinal  column,  spinal  cord  and  membranes,  and  skin. 

^  For  segmentation  origin  of  arteries  see  :  I.  Broman,  Ergebnisse  der  Anat.  1906, 
vol.  16,  p.  639. 


70      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

The  segmental  artery  joins  at  its  termination  with  a  ventral  longitudinal 
vessel,  the  deep  epigastric.  The  primitive  arrangement  in  vertebrates 
appears  to  have  been  one  with  a  dorsal  and  ventral  longitudinal  vessel, 
the  segmental  artery  passing  from  the  dorsal  to  the  ventral  vessel.  The 
vertebral,  ascending  cervical,  deep  cervical,  ascending  lumbar  and  lateral 
sacral  arteries  are  examples  of  the  anastomoses  that  may  arise  between 
segmental  arteries. 

Segmental  arteries  also  arise  from  the  aorta  to  supply  the  structures 
formed  from  the  intermediate  cell  mass  (the  kidney,  testis,  ovary,  etc., 
Fig.  66).  As  a  rule  only  one  renal  segmental  artery  persists,  but  frequently 
accessory  renals  are  seen.  These  may  be  persistent  embryonic  vessels 
of  several  segments  of  the  intermediate  cell-mass  in  which  the  Wolffian 
body  and  kidney  arise.  The  splanchnopleure  shows  no  certain  traces  of 
segmentation  ;  hence  its  vessels  (coeliac  axis  and  mesenteric)  if  of  segmental 
origin  have  become  profoundly  modified.  Lately  Broman  has  demon- 
strated that  the  splanchnic  arteries  have  the  appearance  of  a  segmental 
arrangement  in  the  embryo  (Fig.  25).  During  the  4th  week  there  are  right 
and  left  aortae,  each  giving  o£E  splanchnic  branches  ;  in  the  5th  week 
fusion  of  the  aortic  trunks  sets  in  ;  later  the  right  and  left  splanchnic 
branches  unite. 

IV.  The  Nerve  Elements  of  the  11th  Segment  (Fig.  67).— Although  the 
spinal  cord  during  development  of  the  human  embryo  shows  no  clear  sign 
of  being  definitely  divided  into  segments  corresponding  to  those  of  the 
body,  yet  from  what  we  know  of  its  condition  in  embryos  of  other  animals 
and  from  clinical  evidence  there  can  be  little  doubt  that  such  a  segmenta- 
tion does  take  place,  and  that  it  possesses  segments  corresponding  to  those 
of  the  body.  From  each  segment  four  groups  of  cells  arise  :  (1)  Somatic 
motor,  (2)  somatic  sensory,  (3)  splanchnic  motor,  (4)  splanchnic  sensory. 
The  motor  groups  for  the  greater  part  remain  within  the  spinal  cord,  but 
many  enter  the  sympathetic  ganglia  ;  the  sensory  groups  form  ganglia 
outside  the  cord.  The  nerve  fibres  connected  with  the  somatic  groups 
have  a  diameter  varying  from  9-lSfx  ;  those  with  the  splanchnic,  2-9/>t. 
The  somatic  motor  group,  in  the  anterior  horn,  sends  out  processes  to  all 
the  muscles  of  the  primitive  body  segment  in  which  it  is  situated.  The 
anterior  root  of  a  spinal  nerve  is  formed  by  the  somatic  motor  fibres. 
The  splanchnic  motor  cells,  in  the  lateral  horn,  send  out  processes  within 
the  splanchnopleure  which  reach  viscera  through  the  white  rami  com- 
municantes  and  sympathetic  system  (Fig.  67,  A).  It  is  probable  that,  as 
Elliot  Smith  has  suggested,  some  of  the  splanchnic  motor  cells  emigrate 
from  the  cord  and  take  up  a  position  in  the  prevertebral  ganglia. 

At  the  point  where  the  medullary  plates  are  cut  ofi  from  the  ectoderm 
to  form  the  neural  canal,  a  crest,  the  neural  crest,  grows  out  on  each  side 
(Fig.  67,  B)  composed  of  the  cells  which  formed  the  junctional  line  between 
medullary  plates  and  ectoderm.  A  group  of  these  neuroblasts— the 
somatic  sensory  group — grows  into  each  segment  and  forms  the  posterior 
root  ganglion.  Each  neuroblast  within  the  ganglion  sends  out  a  process 
which  bifurcates,  one  branch  or  fibre  growing  into  the  cord  and  ending 
in  the  posterior  column  and  cells  of  the  posterior  horn,  the  other  passing 


THE  SEGMENTATION  OF  THE  BODY 


71 


to  the  skin,  muscles,  etc.,  of  the  segment.  The  posterior  nerve  root  is  thus 
formed  by  the  ingrowing  processes  from  the  cells  of  the  posterior  root  gang- 
lion, and  the  body  segment  in  which  the  outgrowing  processes  are 
distributed  is  thereby  brought  into  sensory  communication  with  the  central 
nervous  system  (see  also  p.  84).  The  anterior  and  posterior  roots  unite 
to  form  a  spinal  or  segmental  nerve.  Like  the  artery,  it  divides  into  a 
posterior  division  for  the  epaxial  part  of  the  segment  and  an  anterior  for 
the  ventro-lateral  part  (Fig.  67,  A).  The  splanchnic  sensory  ^  groups  are 
situated  in  the  posterior  root-ganglia,  and  probably  also  in  the  various 
ganglionic  masses  of  the  sympathetic  system.  These  sympathetic  cells 
are  derived,  with  the  posterior  root  ganglion,  from  the  neural  crest,  and  at 


POST.    DIV. 

POST.  ROOT  GANG. 


ECTODERM 

um\mm\\ 


POST.  ROOT 
GANG. 


NEUP -CANAL 


VERT.  GANG. 


RENAL  GANG- 


SEMILUNAR 
GANG. 


UERBACHS   PLEK. 


(A) 


VERT.  OANG- 


RENAL  QANO. 


FIG.   67.   A. 
B. 


SEN1ILUNAR   GANG. 


-Diagram  of  the  Nerve  System  of  the  11th  Dorsal  Segment. 
-A  diagram  showing  the  derivation  of  the  Parts  of  the  Nerve  System 
of  the  11th  Segment  in  the  Embryo. 


first  form  a  continuous  paravertebral  column  (in  5th  week).  From  the 
paravertebral  column  are  differentiated  : 

(a)  The  prevertebral  ganglion  situated  on  the  vertebra  (in  the  gangliated 
chain),  ventral  to  the  exit  of  the  spinal  nerve  ; 

(&)  A  group  to  the  intermediate  cell  mass  (renal  ganglion  and  adrenal 
body)  ; 

(c)  Another  to  the  splanchnopleure  (in  the  semilunar  ganglia)  ; 

{d)  To  the  viscera  (cells  of  Auerbach's  plexus,  etc.). 

Groups  (c)  and  {d)  show  no  trace  of  segmentation  in  their  arrangement, 
but,  clinically,  evidence  is  to  be  found  that  every  viscus  or  part  of  a  viscus 
is  connected  with  certain  segments  of  the  spinal  cord.  The  cells  of  the 
sympathetic  ganglia  throw  out  axis-cylinder  processes,  which  are  connected 
with  the  spinal  cord  by  fibres  in  a  white  ramus  communicans  and  posterior 
root,  and  act  as  sensory  pathways  from  the  viscera.  The  distal  end  of  the 
axis-cylinder  process  terminates  in  a  viscus.  In  this  manner  certain 
segments  of  the  spinal  cord  are  brought  into  touch  with  certain  parts  of 

^  See  Gaskell's  original  paper  in  Journ.  of  Phy.siol.  1886,  vol.  7,  p.  1. 


72 


HUMAN  EMBEYOLOGY  AND  MORPHOLOGY 


the  viscera.     The  vaso-motor  supply  of  each  body  segment  passes  to  it 
from  the  sympathetic  ganglion  by  a  grey  ramus  communicans. 

It  will  thus  be  seen  that  all  the  parts  of  a  segment— body  wall  (somato- 
pleure),  kidney  (intermediate  cell  mass),  and  a  part  of  the  abdominal  or 
thoracic  viscera  (splanchnopleure)  are  connected  by  nerves  to  a  corre- 
sponding segment  of  the  spinal  cord.  In  diseased  conditions  of  any  part  of 
a  body  segment,  the  corresponding  spinal  segment  of  the  cord  is  disturbed. 
Such  a  disturbance  is  referred  along  the  somatic  sensory  fibres,  for  the 
brain  has  no  power  to  assign  to  their  source,  impressions  travelling  inwards 
by  the  splanchnic  sensory  fibres.  Thus,  for  instance,  a  stone  in  the  pelvis 
of  the  kidney  (which  is  supplied  from  the  10th,  11th,  and  12th„ dorsal 
segments)  is  frequently  accompanied  by  pain  which  the  brain  refers  along 
the  11th  and  12th  intercostal  nerves.     The  skin  supplied  by  these  nerves 


::J-ri.Cervic. 
n 


Fig.  68.- — Cervical  and  dorsal  parts  of  the  Spine  of  a  Human  Foetus  showing 
irregularities  of  segmentation. 

may  become  hyper-aesthetic.  In  the  central  nerve  system  as  in  the 
muscular,  the  primary  simple  segmental  arrangement  has  been  disturbed 
by  enormous  changes  which  have  occurred  in  the  process  of  evolution. 
In  order  to  secure  a  harmonious  co-operation  of  the  various  segments  of 
the  body,  communications  have  been  established,  by  means  of  nerve 
tracts,  between  the  various  segments  of  the  spinal  cord  and  between  the 
segments  of  the  cord  and  the  higher  centres  of  the  brain.  These  com- 
munications have  obliterated  well  nigh  all  traces  of  the  primitive  segments, 
and  yet  we  see  in  the  ganglia  of  the  posterior  roots  and  in  the  prevertebral 
ganglia  of  the  sympathetic  chain  clear  evidence  that  each  segment  of  the 
body  was  originally  provided  with  its  own  semi-automatic  nerve  mechanism. 
Clinical  observation  has  supplied  evidence  that  certain  viscera — -such  as 
the  heart,  the  liver,  the  kidneys — have  a  nervous  correlationship  with 
certain  segments  of  the  body,  and  we  may  infer  that  these  organs  have 
been  evolved  in  connection  with  certain  definite  segments  of  the  body. 

Segments     from    which    Splanchnic    Fibres    Escape. — The    small 
meduUated  or  splanchnic  fibres  dp  not  arise  from  every  spinal  segment. 


THE  SEGMENTATION  OF  THE  BODY  73 

Bishop  Harman  found  that  in  man  such  fibres  escape  only  by  the  roots  of 
the  dorsal  nerves  and  first  lumbar  ;  occasionally  splanchnic  fibres  come  out 
in  the  roots  of  the  last  cervical  and  second  lumbar.  These  fibres  enter 
the  gangliated  chain,  and  are  distributed  to  the  viscera.  Splanchnic  fibres 
also  escape  by  the  3rd  sacral,  frequently  too  from  the  2nd  or  4th,  to  form 
the  nervi  errigentes  for  the  pelvic  viscera.  The  greater  part  of  the  9th, 
lOtli  and  11th  cranial  nerves  is  made  up  of  splanchnic  fibres.  There  are 
thus  three  visceral  areas — an  anterior  or  medullary,  a  middle  or  thoracic, 
and  a  posterior  or  sacral.  How  these  centres  came  to  be  thus  separated 
is  not  known.  It  is  also  remarkable  that  the  nerve  centres  which  regulate 
or  constrict  arterioles  are  situated  in  the  middle  or  thoracic  area. 

Abnormal  Segmentation. — In  certain  pathological  conditions  the 
process  of  segmentation  is  disturbed,  with  the  result  that  an  irregular 
and  asymmetrical  separation  of  the  segments  takes  place.  In  Fig.  68  part 
of  the  spinal  column  and  ribs  are  shown  of  a  foetus  in  which  the  effects  of 
such  an  irregularity  are  well  illustrated.  The  vertebrae  of  the  3rd  and  4th 
cervical  segments  are  fused  on  the  left  side  ;  the  succeeding  segments  show 
many  abnormalities  of  a  similar  kind.  The  bodies  of  the  1st  and  2nd  ribs 
of  the  right  side  are  fused.  In  the  same  foetus  the  pectoral  muscles  were 
imperfectly  developed.  In  such  foetuses  one  or  both  of  the  shoulders  are 
placed  high  in  the  neck  (congenital  elevation  of  the  scapula).  Imperfect 
separation  of  two  adjacent  vertebrae  or  ribs  is  occasionally  seen — abnor- 
malities due  to  a  lesser  irregularity  of  segmentation. 


CHAPTER  VII. 


CENTRAL  NERVOUS  SYSTEM— DIFFERENTIATION  OF  THE 
SPINAL  CORD. 

Evolution  of  the  Central  Nervous  System. — To  students  who  are 
familiar  with  the  extraordinary  complexity  of  the  central  nervous  system 
of  man  it  must  seem  incredible  that  it  arose  by  the  specialization  of  an 
area  of  the  ectoderm  or  covering  of  the  body.  It  is  only  on  such  a  hypo- 
thesis that  we  can  explain  the  fact  that  the  medullary  plates,  out  of  which 
the  whole  central  nervous  system  of  the  body  is  developed,  are  exposed 
on  the  surface  of  the  embryo  during  the  greater  part  of  the  3rd  week  of 
development.     It  occasionally  happens  that  children  are  born,  in  which 


NEURAL    ARCH 


'SPINAL    NERVE 


Fig.  69. — Diagrammatic  Section  across  the  Back  of  an  Anencephalic  Child  in  which 
the  medullary  plates  were  exposed  on  both  head  and  spine. 

Fig.  70. — Diagram  to  show  how  the  ectodermal  cells  of  the  Medullary  Plates  are 
differentiated  into  nerve  cells  or  neuroblasts  and  supporting  cells  or  spongioblasts. 
(After  Prenant.)  The  central  canal  is  being  enclosed  by  upgrowth  of  the  medul- 
lary plates.  B,  B,  ectoderm  ;  C,  sensory  cell  in  ectoderm  ;  D,  D,  cells  which 
become  enclosed  in  posterior  root  ganglion  ;  E,  E,  nerve  cells  which  connect  the 
sensory  and  motor  cells  ;  F,  F,  motor  cells  in  anterior  horn  ;  G,  G,  muscle  plates. 

the  medullary  plates  are  exposed  along  the  head  and  back  as  they  are 
during  very  early  embryonic  life.  The  condition  is  shown  in  Fig.  69, 
and  it  is  impossible  to  explain  its  occurrence  except  by  supposing  the 
medullary  plates  to  be  modified  parts  of  the  ectoderm.  When,  however, 
one  remembers  the  condition  in  the  lower  invertebrates,  such  as  is  seen  in 
the  organization  of  the  Hydra,  the  explanation  becomes  more  acceptable. 
The  ectodermic  cells  of  Hydra  are  not  only  protective  and  secretory  in  func- 
tion, but  they  also  serve  the  purposes  of  nerve  cells  and  muscle  cells.  One 
can  understand  how  a  specialization  of  function  in  the  ectodermal  cells  may 
have  occurred — some  becoming  purely  contractile,  others  purely  sensory, 
or  secretory,  or  protective.  In  the  cells  of  the  medullary  plate  we  see  a 
further  specialization  (see  Fig.  70)  ;  cells  are  specialized  to  connect  the 
sensory  with  the  contractile  or  muscle  cells.     Those  connected  with  the 

74 


THE  BRAIN  AND  SPINAL  CORD 


75 


sensory  cells — the  posterior  root  ganglia — arise  near  the  lateral  margins  of 
the  medullary  plates  ;  those  connected  with  the  muscle  cells  arise  near 
their  mesial  margins.  If  this  hypothesis  is  true,  then  the  central  canal 
is  merely  an  enclosed  tube  of  ectoderm  and  filled  with  fluid,  because  the 
form  of  animal  in  which  the  medullary  plates  were  evolved  was  a  water- 
living  form.  Dr.  W.  H.  Gaskell  has  advanced  the  view  that  the  central 
canal  represents  a  former  alimentary  tube  round  which  nerve  cells  have 
gathered.  While  Dr.  Gaskell's  hypothesis  explains  many  facts,  it  leaves 
many  more  unexplained — especially  the  manner  in  which  the  central 
nervous  system  is  develoj)ed. 

Formation  of  the  Central  Canal. — The  medullary  plates  of  ectoderm, 
which  form  the  spinal  cord  and  brain,  rise  up,  meet,  and  enclose  a  canal — 
the  central  canal  of  the  spinal  cord  and  brain  (Fig.  71).     The  lips  of  the 


amnion  rent 
open 


ceph.  end 

f-med.  groove 
~f — med.  fold 


-z^r^^j')P^Z^~~C9'' 


blastopore 
primitive  groove 
body  stalk 

— chorion 


Fig.  71. 


-Medullary  Folds  uniting  to  form  the  Neuxal  Tube  in  a  Human  Embryo  in 
the  3rd  weeli  of  development.     (After  Graf  Spee.) 


medullary  plates  meet  alnd  fuse  together  in  the  cervical  region  first,  the 
process  of  union  spreading  forwards  and  backwards,  the  last  parts  to  be 
enclosed  being  the  cephalic  and  caudal  extremities.  The  opening  at  the 
anterior  extremity — the  neuropore — and  the  posterior  or  caudopore  close 
towards  the  end  of  the  4th  week,  the  neuropore  closing  first.  The  optic 
vesicles  begin  to  grow  out  from  the  medullary  plates  before  these  have 
united  to  enclose  the  cavity  of  the  fore-brain.  It  will  be  thus  seen  that  the 
optic  vesicle,  which  becomes  the  retina  and  optic  nerve,  is  developed  as  a 
part  of  the  medullary  plate. 

Division    of    the    Neural    Canal    (Figs.  72,  73).— At  the  end  of  the 
4th  week  the  neural  tube  is  divided  into  four  parts.     They  are  : 

(1)  An  anterior  dilatation,  the  fore-brain,   which  forms  the  3rd  and 
lateral  ventricles  and  their  walls. 

(2)  The  mid-brain,  which  becomes  transformed  into  the  aqueduct  of 
Sylvius,  corpora  quadrigemina  and  crura  cerebri. 


76 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


(3)  Tlie  hind-brain,  the  basis  of  the  4th  ventricle,  pons,  cerebellum  and 
medulla. 

(4)  The  central  canal  and  spinal  cord. 

The  Spinal  Cord.— The  Spinal  Cord  at  first  extends  throughout  the 
whole  length  of  the  spinal  column.  At  the  end  of  the  3rd  month  the  spinal 
column  and  canal  grow  more  rapidly  than  the  cord,  and  at  birth  its  lower 
end  has  become  withdrawn  to  the  level  of  the  3rd  lumbar  vertebra.^    By 


3rd  uent. 
aqueduct  Sy/ufus 

4th  uent. 


central  canal.  % 


fore-brain 
optic  uesic. 

id-brain 


-iiind  brain 


-spinal  cord.  ^^u^ 


NOTOCHORO 


riG.  72.— Diagram  of  the  Four  Primary  Divisions  of  the  Neural  Tube. 
Fig.  73.- — Lateral  View  of  the  Central  Nerve  System  of  a  Human  Embryo  of  the  4th 
week —  2'6  mm.  long.    (Dr.  Low.) 

the  third  year  it  usually  terminates  opposite  the  disc  between  the  1st  and 
2nd  lumbar  vertebrae,  but  it  may  stop  at  the  lower  border  of  the  2nd 
lumbar  or  rise  as  high  as  the  middle  of  the  12th  dorsal  vertebra.^  The 
results  of  this  inequality  of  growth  are  : 

(1)  The  roots  of  the  lumbar  and  sacral  nerves  become  enormously 
elongated,  forming  the  cauda  equina  ;  all  the  nerves  are  more  or  less  drawn 
up,  except  the  1st  and  2nd  cervical ;  the  origins  of  the  lower  cervical 
nerves  are  drawn  up  2  vertebrae  (as  indicated  by  the  position  of  their 
spines)  ;   the  upper  dorsal,  3  ;   the  lower  dorsal,  4  ;   the  lower  lumbar,  5  ; 

^  G.  L.  Streeter,  Amer.  Journ.  Anat.  1919,  vol.  25,  p.  1. 
2  R.  E.  McCotter,  Anat.  Rec.  1916,  vol.  10,  p.  559. 


THE  BRAIN  AND  SPINAL  CORD 


77 


the  coccygeal,  10.     These  statistics  represent  a  broad  expression  of  the 
observations  made  by  Professor  R.  W.  Reid. 

(2)  The  caudal  part  of  the  spinal  cord  is  the  last  part  of  the  neural  tube 
to  be  formed  (see  Fig.  63).  Its  fate  has  been  recently  investigated  by 
Professor  Streeter.^  Even  in  the  9th  week  (Fig.  73,  A)  the  caudal  segment 
is  still  represented  over  the  coccyx,  ending  in  a  subcutaneous  vesicle,  but 
already  retrogression  has  set  in,  the  coccygeal  ganglia  have  disappeared 
and  the  neural  canal,  immediately  distal  to  the  origin  of  the  5th  pair  of 
sacral  nerves,  is  becoming  dilated  to  form  the  terminal  ventricle.  By  the 
12th  week  (Fig.  74),  when  retraction  has  set  in,  we  see  that  the  caudal 
segment  has  become  differentiated  into  a  distal  or  extradural  part,  which 


SPINfit-  CORD 


SAC  1 


QQQOOQOoogg 


30  mm   9^''  week. 


SAC  t 


Fig.  73,  A. — Showing  the  differentiation  of  the  terminal  part  of  the  neural  tube  into 
the  coccygeal  thread  and  fllum  terminale.     (Streeter.) 

is  drawn  out  to  form  the  coccygeal  thread,  while  the  intradural  part  is 
being  stretched  and  will  become  the  filum  terminale. 

Differentiation  of  the  Spinal  Cord. — As  the  neural  plate  is  folded 
in  towards  the  end  of  the  3rd  week,  the  single  layer  of  columnar  epithelium 
of  which  it  is  composed  is  already  undergoing  certain  changes.  Three 
stages  in  its  differentiation  are  shown  in  Fig.  74  ;  in  Stage  I.,  the  single  layer 
of  ill-defined  columnar  cells  is  shown  ;  the  bases  of  the  cells  are  directed 
towards  the  central  canal,  resting  on  a  delicate  internal  limiting  membrane  ; 
their  outer  ends,  appearing  on  the  surface  of  the  neural  tube,  are  bounded 
by  the  external  limiting  membrane.  In  Stage  II.  there  has  been  an  active 
proliferation  of  the  cells  and  an  increase  in  the  thickness  of  the  neural  wall ; 
the  cell  bodies  have  fused  to  form  a  cytoplasmic  syncytium,  in  which  the 
nuclei  are  spread  between  the  inner  and  outer  limiting  membrane.  In 
Stage  III.  (Fig.  74),  which  is  reached  about  the  close  of  the  4th  week, 
the  wall  has  made  a  further  increase  in  thickness  ;  in  the  common  C3rto- 
plasm   a   fibrillar   meshwork — a   myelospongium — has   been   laid   down  ; 

^G.  L.  Streeter,  Amer.  Journ.  Anat.  1919,  vol.  25,  p.  1. 


78 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


three  zones  can  be  distinguislied,  a  middle  or  mantle  zone  in  whicli  most 
of  the  nuclei  are  contained,  which  will  become  the  grey  substance  of  the 
cord  ;  an  outer  or  marginal  zone  made  up  of  myelospongium  into  which 
the  fibre-tracts  of  the  cord  will  grow  ;    an  inner  or  ependymal  zone,  not 


INT.   LIM.   MEMB. 

rEXT.  LIM.  MEMB. 


XT'.  LIM.  MEMB. 
STAGE  H. 


£"XT.  LIM.  MEMB. 

PIAL  SHEATH 


T' 

MARG.  ZONE 


'-'"-^  MAf^TLE  ZONE 

EPENO.  "ZONE 

Fig.  74. — Three  stages  in  the  early  differentiation  of  the  wall  of  the  spinal  neural 
tube.  Stage  I.,  single  layer  of  ill-differentiated  columnar  epithelium  ;  Stage  II., 
in  which  the  single  layer  has  been  transformed  into  a  nucleated  syncytium ; 
Stage  III.,  in  which  three  zones  begin  to  be  apparent.     (After  Streeter.) 

distinctly  demarcated,  but  characterized  by  the  presence  of  actively 
dividing  large  nuclei — germinal  cells.  The  nuclei,  with  their  surrounding 
protoplasm,  are  becoming  differentiated  into  neuroblasts — the  producers 
of  nerve  cells,  and  neuroglial  or  supporting  cells.     The  neuroglial  fibres 


OORiAL.    PLATE 


ALAR.  LAMII^A 


POST.    ROO 


EPENDVMAL 
■ZONE 

MAUTLE  ZONE 


-pxo    7r; Section  across  the  developing  Spinal  Cord  at  the  beginning  of  the  5th  week. 

(After  His.) 


are  laid  down  in  the  cell-protoplasm  of  neuroglial  cells.     The  ependymal 
cells  which  line  the  central  canal,  are  derived  from  the  inner  zone. 

A  section  across  the  embryonic  spinal  cord  at  the  beginning  of  the  5th 
week  (Fig.  75)  brings  out  certain  instructive  features  :  the  central  canal  is 
cofiin-shaped  in  section  ;  the  epithelium  in  its  roof  and  floor — forming  the 
roof  and  floor  plates — increases  but  slightly  in  thickness  ;  its  side  walls, 
walls,  or  lateral  plates,  which  become  the  cord  or  medulla,  are  indistinctly 


THE  BRAIN  AND  SPINAL  CORD 


79 


separated  into  a  ventral  part  or  basal  lamina,  from  which  the  anterior  root 
fibres  emerge  and  winch  will  have  to  do  with  motor  functions  and  a  dorsal 
part — the  alar  lamina,  into  which  the  fibres  of  the  posterior  root  grow 
and  which  will  have  to  do  with  sensory  functions.  The  three  zones  in  each 
lateral  plate  are  distinct ;  the  inner  or  ependymal  zone  increases  in  breadth 
as  it  is  followed  from  the  floor  plate  to  the  roof  plate,  whereas  the  middle 
or  mantle  zone  does  the  opposite  ;  it  diminishes  as  it  passes  into  the  alar 
lamina.  In  the  ventral  part  of  the  middle  zone  the  anterior  grey  column 
or  horn  is  quite  apparent,  whereas  the  posterior  horn  is  just  beginning  to 
form.  We  must  suppose  that  the  inner  or  ependymal  zone  is  one  of 
production  or  proliferation  and  that  its  cells  are  becoming  differentiated 
and  added  to  the  middle  zone.     The  anterior  or  motor  column  is  demarcated 


EPENDYMAL    ZONE 
NUCLEAR    ZONE 
MARGINAL 


POST.  ROOT 


FUN    GRACILIS 
POST   SEPT.  \      ruNCUNEATUS. 


ANT.  ROOT 


CENTRAL  CANAL 


CENTRAL   CANAL 


(A)rsm:m'7'^''week.    (B)30m.m.  ^^bvJ6ek.  fcJtSrm.m.  ii^f'week.      (D)eom.m.  isthweek. 

Fig.  76. — Sliowing  the  progressive  differentiation  of  tlie  spinal  cord  during  the  second 
and  third  months  of  development.     (After  Streeter.) 

before  the  posterior  or  sensory  horn.  In  each  lateral  plate  the  neuroblasts 
become  grouped  thus  from  anterior  to  posterior  horn  :  (1)  somatic  motor, 
(2)  splanchnic  motor  (both  in  the  basal  lamina),  (3)  splanchnic  sensory, 
(4)  somatic  sensory  (both  in  the  alar  lamina).  This  order  of  grouping  holds 
true  from  end  to  end  of  the  neural  tube.  The  fibres  from  the  cells  in  the 
anterior  horn  begin  to  emerge  as  the  anterior  root  in  the  latter  part  of  the 
4th  week  ;  the  processes  from  the  ganglion  cells  of  the  posterior  root 
commence  to  enter  the  marginal  zone  of  the  alar  lamina  at  the  same 
time.  As  is  diagrammatically  represented  in  Fig.  75  neuroglial  fibres 
pass  from  the  lining  ependyma  of  the  central  canal  to  the  external 
limiting  membrane. 

The  developmental  changes  in  the  cord  during  the  2nd  and  3rd  months 
are  set  out  in  a  semi-diagrammatic  manner  in  Fig.  76,  A,  B,  C,  D.  We 
may  centre  our  attention  first  on  three  structures — the  central  canal,  the 
inner  or  ependymal  zone  and  the  posterior  median  septum,  for  all  three  are 
closely  correlated.     In  the  7th  week  (Fig.  76,  A)  the  central  canal  still 


80      HUMAN  EMBEYOLOGY  AND  MORPHOLOGY 

retains  its  coffin-shaped  section,  the  ependymal  zone  is  still  extensive,  and 
although  the  roof  plate  has  thickened  there  is  still  no  posterior  median 
septum,  for  the  posterior  funiculi  or  conducting  tracts  (f .  gracilis  and  f. 
cuneatus)  have  scarcely  appeared  in  the  dorsal  marginal  zone.  In  the 
9th  week  changes  are  in  progress  :  the  dorsal  part  of  the  central  canal  is 
being  obliterated  by  the  apposition  of  the  lateral  plates  (Fig.  76,  B) ;  the 
ependymal  zone  is  reduced  ;  the  posterior  funiculi  are  being  formed  in 
the  dorsal  marginal  zone  and  the  posterior-median  septum  is  formed  between 
the  right  and  left  posterior  funiculi  developing  on  each  side  of  the  original 
roof  plate.  In  the  11th  week  the  central  canal  and  ependymal  zone  are 
further  reduced  ;  the  posterior-median  septum  has  increased  in  depth 
owing  to  the  rapid  growth  of  the  posterior  funiculi ;  the  middle  or  mantle 
zone  is  now  differentiated  into  the  anterior  and  posterior  columns  of 
grey  matter.  In  the  13th  week  the  adult  condition  is  reached  ;  the  cord 
is  reduced  to  its  final  size,  the  ependymal  zone  now  forms  merely  a  lining 
to  the  canal ;  the  anterior  and  posterior  horns,  with  their  various  groups 
of  nerve  cells,  are  reaching  their  final  form,  while  in  the  marginal  zone  the 
great  connecting  and  association  tracts  of  white  matter  have  arisen  or  are 
arising.  There  is  now  a  deep  posterior  median  septum  and  an  open 
anterior  median  fissure,  formed  during  the  development  of  the  ventral 
funiculi  in  the  anterior  part  of  the  marginal  zone. 

Spinal  Tracts. — With  the  formation  of  the  posterior  columns,  the  grey 
matter  of  the  dorsal  laminae,  at  first  united  by  the  roof  plate,  becomes 
widely  separated  to  form  the  posterior  horns  (Fig.  77).  At  the  same  time 
part  of  the  gelatinous  tissue  of  the  inner  zone  is  separated  to  form  a  cap  on 
the  posterior  horns  (Fig.  76).  In  the  gelatinous  tissue  congenital  cysts 
may  occur.  The  columnar  cells  which  line  the  central  canal  are  ciliated. 
Thus  by  the  end  of  the  3rd  month  the  nerve  cells  have  taken  up  their 
permanent  stations  in  the  grey  columns  of  the  spinal  cord.  The  cells  which 
have  to  do  with  the  reception  and  transmission  of  sensory  messages  are 
situated  in  the  posterior  root  ganglia  ;  those  which  have  to  do  with  the 
dispatch  of  motor  impulses  are  situated  in  the  anterior  and  lateral  horns  ; 
the  remainder  may  be  regarded  as  intercalated  or  shunt  cells,  and  are  con- 
cerned in  linking  up  or  associating  the  afferent  and  efferent  systems  and 
centres.  The  marginal  zone  provides  a  basis  into  which  the  nerve  pro- 
cesses— the  axons  which  are  to  connect  neuron  with  neuron  and  centre 
with  centre — may  grow  and  reach  their  destinations.  It  is  a  remarkable 
fact  that  the  lower  we  go  in  the  vertebrate  scale  the  more  automatic  or 
independent  do  the  nerve  centres  of  the  spinal  cord  become  ;  the  higher  we 
go  in  the  scale  the  more  they  become  dominated  by  and  dependent  upon 
nerve  centres  situated  in  the  hind-brain,  mid-brain  and  fore-brain.  Hence 
we  are  prepared  to  find  that  the  first  tracts  of  nerve  fibres  which  appear 
in  the  marginal  zone  are  those  which  link  together  the  nerve  centres  in 
the  spinal  cord  itself.  At  the  end  of  the  first  month  the  fibres  of  the 
posterior  root  have  entered  the  marginal  zone  on  the  dorsal  side  of  the  cord, 
and  have  thus  formed  the  rudiment  of  the  posterior  funiculi ;  these 
effect  connections  with  receptive  nuclei  in  the  posterior  horns.  At  the 
same  time  fibres  which  associate  neighbouring  or  allied  nuclei  or  centres 


THE  BRAIN  AND  SPINAL  CORD 


81 


of  the  cord  aj)pear  in  the  marginal  zone  of  the  lateral  and  anterior  parts 
of  the  cord.     These  may  be  described  as  inter-segniental  tracts. 

Later,  in  the  3rd  month,  commences  the  growth  of  fibres  within  the 
antero-lateral  marginal  zone  of  (1)  tracts  which,  arising  from  cells  in  the 
cord,  are  to  end  in  hind-brain,  mid-brain  and  fore-brain,  and  thus  supply 
these  higher  centres  with  afferent  impulses  which  are  reaching  the  spinal 
centres  ;  (2)  tracts  which,  commencing  in  the  hind  and  mid-brain,  grow 
down  to  permit  the  higher  centres  to  influence  the  lower  centres  in  the 
cord.  Lastly,  in  the  5th  month,  the  pallio-spinal  or  pyramidal  tracts 
commence  to  develop.  The  pyramidal  tracts  (crossed  and  direct)  grow 
down  from  the  cells  of  the  motor  cortex.  They  are  not  medullated  until 
soon  after  birth.  The  pyramidal  tracts  are  the  means  by  which  the  brain 
controls  the  motor  cells  of  the  cord.     In  man  these  tracts  are  remarkable, 


gelat  ^substance 

post.  horn. 


surface  stratum 


post.  coL 
roof  plate 
dorsal  lam. 

central  can 

ventral  lam.- 

floor  plate 


Fig.  77. — Diagrammatic  Section  of  the  developing  Spinal  Cord  to  show  (1)  the  Roof 

and  Floor  Plates  ;    (2)  the  Dorsal  (alar)  and  Ventral  (basal)  Laminae  ;    (3)  the 

Gelatinous  Tissue  between  the  Middle  and  Inner  Zones. 
Fig.  78. — Showing  Transformation  of  Cells  of  the  Ectoderm  to  Sense  Epithelium, 

Nerve  Cells  and  Supporting  Cells,  in  A,  the  Olfactory  Plate,   B,  the  Otocyst, 

C,  the  Retinal  Layer  of  Optic  Cup. 

not  only  for  their  great  size,  but  also  that  in  addition  to  the  crossed  lateral 
tract,  which  is  present  in  all  mammals,  there  is  also  an  anterior  or  direct 
tract.  The  anterior  tract  appears  to  be  a  recently  evolved  system  ;  it  is 
extremely  variable  in  size.  The  only  other  animals  which  possess  it  are 
those  nearest  allies  of  man — the  great  anthropoid  apes. 

The  myelinization  ^ — the  formation  of  medullary  sheaths  for  the  fibres 
of  nerve  tracts — commences  about  the  4:th  month  and  is  not  really  finished 
until  the  age  of  puberty  is  reached.  The  oldest  tracts — the  ones  which  are 
first  required  to  carry  messages — are  the  earliest  to  be  medullated.  The 
process  begins  in  the  oldest  part  of  the  fibre — the  part  nearest  the  parent 
nerve-cell — and  spreads  towards  the  growing  tip.  The  great  nerve  tracts 
are  ensheathed  at  different  dates  ;  hence  it  is  possible  to  distinguish  and 
unravel  one  tract  from  another  during  the  period  of  development. 

1  Florence  R.  Sabin,  Amer.  Journ.  Anat.  1911,  vol.  11,  p.  113  (Model  of  Tracts 
medullated  at  Birth). 

]? 


82      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

Segments  of  the  Spinal  Cord.— To  that  part  of  the  neural  tube  and 
neural  crest  which  corresponds  in  position  to  a  primitive  body-segment, 
the  name  of  Neuromere  is  given.  From  the  cells  of  a  neuromere  are 
produced  the  posterior  and  anterior  root  of  a  spinal  nerve  on  each  side. 
The  extent  of  each  neuromere  is  thus  marked  out  by  the  attachments  of 
its  nerve  roots.  At  no  time  are  the  medullary  plates  divided  into  embryo- 
logical  segments  in  the  same  sense  as  the  mesoderm  is  divided,  although 
the  neural  tube  probably  did  arise  from  the  fusion  of  a  series  of  neuro- 
meres  or  ganglia,  each  presiding  over  a  definite  segment  of  the  body, 
subsequent  evolutionary  changes  have  led  to  their  fusion.  Dr.  Watt 
observed  in  a  human  embryo  in  which  there  were  18  body-somites,  that  11 
segments  were  to  be  noted  in  the  spinal  cord.  These  changes  are  related 
to  the  combination  of  the  various  segments  and  systems  in  carrying  out 
the  functions  of  the  body.  The  cervical  and  lumbar  enlargements  of  the 
cord  appear  in  the  4th  month.  They  contain  the  neuroblasts  connected 
with  the  body-segments  which  gave  rise  to  the  upper  and  lower  extremities. 
The  neuroblasts  are  arranged,  not  according  to  the  original  neural  segments, 
but  rather  in  relationship  to  the  movements  of  the  limb.  The  group 
representing  the  hand  movements  lie  behind  (distal  to)  those  representing 
movements  of  the  forearm. 

Origin  of  the  Medullary  Plates  and  Nerve  Cells.— The  medullary 
plates  like  the  olfactory  plates  which  give  rise  to  the  sense-epithelium  of  the 
nose,  the  otocyst  from  which  the  auditory  organ  is  developed,  and  the 
retina  are  derived  from  the  ectodermal  covering  of  the  embryo.  The 
olfactory  plate  retains  to  the  greatest  extent  the  features  of  the  ectoderm 
(Fig.  78,  A).  Its  cells  are  of  three  kinds  :  (1)  protective,  (2)  secretory, 
(3)  sensory,  the  latter  being  essentially  surface  nerve  cells  in  nature.  A 
process  or  axis  cylinder  is  produced  from  each  sense  cell ;  from  its  opposite 
extremity  a  sensory  process  is  produced  (Fig.  78,  A).  In  worms,  sense 
epithelial  cells  sink  beneath  the  protective  and  secretory  cells,  the  sensory 
process  being  drawn  out  to  form  a  fibre.  In  the  otocyst,  the  sensory  cells 
produce  no  axis-cylinder  process,  but  a  ganglionic  cell — produced  from  the 
ectoderm  through  the  neural  crest — comes  into  connection  with  it  (Fig. 
78,  B).  From  the  ganglionic  cells  are  produced  (1)  a  chief  process  or  axis 
cylinder ;  (2)  a  branching  process  or  processes — dendrites — from  the 
opposite  pole,  which  end  in  an  arborescence  round  the  sensory  cells.  To  a 
nerve  cell  and  all  the  processes  developed  from  it  the  name  of  Neuron  is 
given.  In  the  retina,  as  in  the  olfactory  plate,  three  types  of  cells  are  seen  : 
(1)  protective  or  supporting  which  form  the  fibres  of  Miiller,  (2)  secretory 
over  the  ciliary  processes,  (3)  the  sensory  cells,  which  produce  an  axis 
cylinder  on  one  side,  and  a  rod  or  cone  on  the  other  (Fig.  78,  C).  Further, 
by  a  process  of  division,  bipolar  and  ganglionic  cells  are  produced  from  the 
retinal  sense  cells.  In  the  medullary  plates  of  the  spinal  cord  the  repre- 
sentatives of  the  original  ectoderm  form  the  ependymal  and  neuroglial 
cells,  the  first  of  which  may  be  regarded  as  both  secretory  and  supporting  ; 
the  neuroblasts  arise  by  a  process  of  division  from  the  primary  ectodermal 
cells.  Each  neuroblast  gives  rise  to  a  neuron.  Their  axons  or  axis 
cylinders  are  in  many  cases  two  feet  or  more  in  length  ;   for  instance,  the 


THE  BRAIN  AND  SPINAL  CORD 


83 


motor  and  sensory  fibres  which  pass  from  the  lumbar  enlargement  to  the 
muscles  and  skin  of  the  foot.  The  nerve  cells  in  the  basal  laminae  are 
peculiar  in  that  their  axis  cylinders  end  on  muscle  cells. 

Malformations  of  the  Neural  Canal.^ — The  fact  that  children  are 
occasionally  born  with  the  medullary  plates  open  and  exposed  on  the  head 
and  back  has  already  been  mentioned  (see  Fig.  69).  Total  Rachischisis, 
as  the  condition  is  named,  is  rare  ;  it  is  much  more  usual  to  find  only  one 
part  of  the  neural  tube  open — either  the  anterior  or  cephalic  part,  giving 
the  condition  known  as  Anencepiialy — absence  of  brain,  or  the  posterior 
or  lumbo-sacral  part,  giving  the  condition  known  as  cystic  spina  bifida. 
The  latter  condition  is  shown  in  Fig.  79.  As  the  spinal  cord  is  followed 
down,  it  is  seen  to  enter  a  cystic  structure  formed  by  a  dilatation  of  the 


□  URA 
.'  -  ARACHNOID 

SKIN 


Serous  zone 

CANAL 


^::'-'  SEROUS  zone 


Fig.  79. — Vertical  Section  of  the  Lumbar  Region  to  show  the  arrangement  of  parts 
in  a  typical  case  of  cystic  spina  bifida. 

subarachnoid  space,  across  which  the  roots  of  the  lumbo-sacral  nerves  pass. 
The  projecting  dome  of  the  cyst  is  formed  by  the  expanded  medullary 
plates  ;  hence  the  spinal  cord  appears  to  end  on  the  wall  of  the  cyst,  and 
spinal  nerves  to  actually  arise  from  it.  The  lumbo-sacral  parts  of  the 
neural  tube  and  of  the  spine  have  never  been  enclosed  ;  the  cerebro-spinal 
fluid  collects  in  the  subarachnoid  space,  and  the  unresisting  medullary 
plates  are  raised  up  to  form  part  of  the  wall  of  a  cystic  tumour.  Another 
form  of  pathological  dilatation  may  appear  after  the  neural  tube  is  com- 
pletely closed.  In  chicks  hatched  at  abnormal  temperatures  fluid  may 
collect  in  certain  parts  of  the  tube,  thus  dilating  it  and  giving  rise  to  cystic 
conditions. 

Membranes  and  Vessels  of  the  Cord.— When  the  neural  tube  is 
enclosed  towards  the  end  of  the  3rd  week  by  the  upgrowth  of  mesoderm 

1  J.  P.  Good,  Journ.  Anat.  1912,  vol.  46,  p.  391 ;  J.  Voigt,  Anat.  Hefte,  1906,  vol.  30, 
p.  393  ;  W.  M.  Baldwin,  Anat.  Record,  1915,  vol.  9,  p.  365 ;  Theodora  Wheeler,  Contrib. 
to  Embryology,  1920,  vol.  9,  p.  95  ;   E.  J.  Carey,  Anat.  Record,  1919,  vol.  16,  p.  45. 


84      HUMAN  EMBRYOLOaY  AND  MORPHOLOGY 

in  the  medullary  folds,  mesenciiymal  cells  become  applied  to  the  neural 
tube.  They  form  the  primary  sheath  of  the  neural  tube  (Fig.  74,  C). 
The  sheath  receives  a  vascular  supply  from  each  dorsal  branch  of  the 
segmental  arteries  and  veins.  Branches  of  the  vessels  perforate  the  nerve 
tissue,  and  thus  a  vascular  mesodermal  element  is  added  to  the  ectodermal 
neural  laminae.  By  the  middle  of  the  second  month  the  primary  sheath 
has  become  cleft  into  an  inner  or  pial  layer,  and  an  outer  or  arachno-dural 
layer.  The  cleft  becomes  the  subarachnoid  space,  which  is  apparently 
of  the  nature  of  a  lymphatic  space  (see  p.  90). 

Development  of  Nerves.^ — In  lower  fishes  Kupffer  found  that  nerve 
fibres  were  formed  by  the  union  of  a  chain  of  cells — probably  ectodermal 
in  origin,  and  many  suppose  that  the  nerve  fibres  of  all  vertebrates  are 
formed  in  this  manner,  the  nuclei  of  the  chain-cells  becoming  the  nuclei 
of  the  neurolemma.  On  the  other  hand  His  and  KoUiker  concluded  that 
every  nerve  fibre  is  produced  as  a  continuous  outgrowth  from  one  nerve 
cell,  and  that  the  cells  of  the  sheath  are  mesodermal  in  origin.  It  is  possible 
that  both  interpretations  of  the  apjDearance  presented  by  developing  nerves 
are  right,  and  that  one  kind  of  nerve  fibre  is  produced  in  the  first  manner 
and  another  kind  in  the  second  manner.  The  opinion  generally  held  at  the 
present  time  is  that  an  axis  cylinder  is  the  product  of  one  nerve  cell  or  neuro- 
blast, and  that  the  cells  which  surround  the  growing  fibres  and  form  their 
sheaths  are  derived  from  the  neural  crests,  and  are  therefore  ectodermal  in 
origin.  It  is  maintained  by  Dr.  John  Cameron  that  these  surrounding 
cells  assist  in  the  deposition  of  an  achromatic  substance  at  the  growing 
points  of  nerve  fibres  {Journ.  Anat.  and  Physiol.  1906,  vol.  41,  p.  8).  Dr. 
Ross  Harrison  found  that  when  small  parts  of  the  medullary  plates  of 
tadpoles  were  transplanted  or  maintained  alive  in  artificial  media  the 
outgrowth  of  the  neurons  as  processes  from  single  cells  could  be  witnessed 
{Anat.  Record,  1908,  vol.  2,  Nos.  9,  10). 

Another  theory  receives  support  from  Graham  Kerr's  recent  investiga- 
tions on  Lepidosiren,  viz.  that  nerve  fibres  are  formed  by  the  stretching 
of  protoplasmic  connections  which  originally  exist  between  nerve  and 
muscle  cells. 

^  Papers  on  histogenesis  of  nerves  :  R.  G.  Harrison,  Amer.  Journ.  Anat.  1906, 
vol.  5,  p.  121  ;  W.  H.  Lewis,  Amer.  Journ.  Anat.  1906,  vol.  6,  p.  461  ;  J.  Cameron, 
Journ.  Anat.  and  Physiol.  1907,  vol.  41,  p.  8  ;  Prof.  T.  H.  Bryce,  Quain's  Anatomy, 
1908,  vol.  1,  p.  94. 


CHAPTER  VIII. 
THE  MID-  AND  HIND-BRAINS. 

When  the  neural  tube  is  traced  forwards  into  the  head  region,  it  is  seen 
to  undergo  a  marked  change  in  form — a  transformation  due  to  a  change  in 
function.  In  the  spinal  cord  the  nerves  arose  in  two  rows — a  dorsal  sensory 
and  a  ventral  motor  ;  here  the  dorsal  and  ventral  series  are  still  repre- 
sented, but  a  third  or  intermediate  series  has  been  added.  This  series  is 
represented  by  the  spinal  accessory  (XI),  vagus  (X)  and  glossopharangeal 
(IX),  facial  (VII)  and  fifth  (V)  pairs  of  nerves.  They  arise  from  an  inter- 
mediate column  of  cells  representing  in  an  exaggerated  degree  the  splanchnic 
or  visceral  nerve  columns  of  the  spinal  cord.  Further,  the  central  canal 
becomes  enlarged  to  form  the  4th  ventricle.  Part  of  the  roof  of  the  neural 
tube  becomes  reduced  to  a  membranous  lamina,  forming  the  medullary 
velum  and  choroid  plexus — a  secretory  mechanism.  Part  of  the  roof  is 
specialized  to  form  a  complex  mechanism  (the  cerebellum)  for  the  co- 
ordination of  the  impulses  dispatched  to  the  motor  cells  of  the  spinal  cord. 
This  high  degree  of  specialization  almost  obliterates  the  original  simple 
nature  of  that  part  of  the  neural  tube  which  forms  the  mid-  and  hind- 
brain.  In  the  human  embryo  at  the  beginning  of  the  4th  week  it  is  seen 
that  this  part  of  the  central  nervous  system  retains  its  tubular  character, 
while  that  part  which  is  to  form  the  hind-brain,  even  at  this  early  stage, 
shows  an  imperfect  segmentation  into  nine  neuromeres.  Further,  the 
neural  tube  in  the  regions  of  the  mid-  and  hind-brain,  as  in  the  spinal  cord, 
lies  over  the  notochord  (Fig.  80).  The  notochord  ceases  at  the  junction 
of  the  mid-  and  fore-brain.  The  developing  walls  of  the  mid-  and  hind- 
brain  show  the  same  three  zones  as  were  seen  in  the  spinal  cord — inner  or 
ependymal,  middle  or  mantle  and  outer  or  marginal.  We  shall  find, 
too,  the  same  division  of  each  lateral  neural  plate  into  basal  and  alar 
laminae. 

A  reference  to  the  relationships  of  the  hind-brain,  during  the  4th  week 
of  develo]Dment  (Fig.  80),  serves  to  explain  why  the  vital  centres  of  the 
body — those  which  are  concerned  in  the  regulation  of  respiration,  circula- 
tion, deglutition  and  digestion,  come  to  be  placed  in  its  walls.  At  this 
time  the  hind-brain  lies  over  the  pharynx,  with  its  aortic  arches,  and  its 
gill-pockets — representing  the  breathing  mechanism  of  fishes.  When 
lungs  arise  the  control  of  respiration  still  lies  in  the  original  respiratory 
centres  of  the  hind-brain.  The  heart,  too,  lies  directly  under,  or  ventral  to, 
the  hind-brain  (Fig.  80)  ;  hence  the  centres  for  circulation  are  placed  there. 

85 


86      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

The  fore-gut,  from  which  the  mouth,  pharynx,  oesophagus,  trachea,  lungs, 
stomach  and  liver  are  to  arise  is  also  placed  in  the  territory  of  the  hind- 
brain.  Its  relationship  to  the  otocyst,  however,  is  to  prove  the  most 
important.  From  that  structure  is  to  arise  a  vestibular  or  balancing 
mechanism  designed  to  supply  information  concerning  the  position  and 
movements  of  the  head.  The  cerebellum  and  pons  which  so  transform 
the  simple  tubular  hind-brain,  arise  in  connection  with  the  vestibular 
nuclei.  One  other  point  may  be  noted  before  proceeding  to  follow  the 
transformation  of  the  hind-brain  into  medulla  oblongata,  cerebellum  and 
pons.  The  mid-brain  is  interpolated  between  the  spinal  cord  on  the  one 
side  and  the  mid-  and  fore-brain  on  the  other  ;  hence  it  becomes  the  great 

rOREBRAIN 


OPTIC    VESICLE 


ORAL    MBMB.-- 

/??  ARCH 

HEART _    _ 

IsC  POCKET--'  ~  I 

2nd  POCKET \ 

Zr'^  POCKET--  5^ 


-CAfrfG.  VII  -vni. 


Sept.transy.  ^'^'^^yW  sJi      "    *^  ^^*' 

-''  P  I'^CERVICAL. 

STOMACH  s  AEORAL.  CREST 

AORTA 

NOTOCRORD 

riG.  80. — Showing  the  tubular  form,  the  neuromeres  and  relations  of  the  Mid-  and 
Hind-Brain  in  a  Human  Embryo  in  which  there  were  18  body  somites — in  the 
4th  week  of  development.     (Crawford  Watt.) 

highway  for  the  nerve  tracts  which  are  developed  to  link  brain  and  spinal 
cord  into  a  functional  whole.  Throughout  the  greater  part  of  the  second 
month  the  hind-brain  forms  a  little  less  than  half  of  the  total  neural  tube. 

The  Fourth  Ventricle. — The  cavity  or  neural  canal  of  the  hind-brain 
becomes  the  fourth  ventricle.  In  its  floor  are  developed,  out  of  the  basal 
or  ventral  and  alar  or  dorsal  laminae  (Fig.  81)  of  the  neural  plates,  the  pons 
and  medulla.  In  its  roof  are  developed  the  cerebellum,  the  superior  and 
inferior  medullary  vela. 

Basal  and  Alar  Laminae  of  the  Medulla. — The  basal  and  alar 
laminae  of  the  neural  tube  become  flattened  out  to  form  the  floor  of  the 
hind-brain.  At  the  end  of  the  4th  week  each  medullary  plate  shows 
three  zones  :  an  inner  or  ependymal  where  new  cells  are  being  produced  ; 
a  middle  or  mantle  zone  in  which  neuroblasts,  neuroglial  fibres  and  young 
nerve  fibres  are  being  differentiated  and  an  outer  or  marginal  zone.  By 
the  6th  week  the  disposition  of  the  nuclei  connected  with  the  cranial 
nerves  in  the  mantle  zone  can  be  made  out.     The  grouping  of  the  nuclei 


THE  MID-  AND  HIND-BRAINS 


87 


as  seen  in  a  diagrammatic  section  across  the  hind-brain  is  shown  in  Fig. 
82.  In  the  mantle  zone  of  the  basal  lamina  are  three  columns  of  motor 
cells — the  columns  being  much  interrupted  as  they  are  traced  from  the 
lower  to  the  upper  end  of  the  hind-brain.  These  are  :  (1)  the  somatic 
motor,   continuing  upwards  the  somatic  cells  of  the  anterior  horn  and 


ROOF  PLATE  (inf.  MED    VELUMJ 


EPBNDfMA 


INNER  ■ZONE 

MIDDLE   ZONE 

OUTER  ZONE 


TAENIA 
ALAR  LAMINA 
SOL. TRACT 


BASAL    LAMINA 


Fig.  81. — Section  across  the  Hind-Brain  of  a  Human  Embryo  in  the  6th  week. 

supplying  muscles  derived  from  the  body  somites  ;  from  this  column 
arise  the  XII  and  VI  nerves.  (2)  The  lateral  somatic  motor,  supplying 
striped  muscle  which  was  first  evolved  for  the  movement  of  gill-arches  ; 
from  this  column  arise  motor  fibres  of  XI,  X,  IX,  VII  and  V.  The 
nucleus  ambiguus  forms  part  of  the  column.  (3)  The  splanchnic  motor 
nuclei,  giving  origm  to  fibres  distributed  to  the  musculature  of  the  heart, 


fICOF  PLATE 

»                SKIN 

SOMATIC                             \l. 

\x 

LAM  ALAR  -— — iC^^S^E/ 

^\^ 

^^^\      £■'*"   I'^S 

.     ^tJ>^       \      SENSORY   G-ANi 

LAM    BA3-r'        7      ^^7^/        \\  / 

\             %            ' 

SOMATIC  ,  -^■^-'M'jW^  ^            VI 

(Efferent)  tTnT3o^iiM»>^           V 

^-^ 

FLOOR  PLATE         ^^^^~j/-J~\^ 

~ STRIPED  MUSCLE 

SYM     QANQ. '~^'~^^  /            \ 

M 

UNSTRIPED            ''•*^^^^^^J>tv(ffl  L 

Vmj        BRANCHIAL  MUSCLE 
^ STRIPED 

MUSCLE                     jijjj^scaTiryiirraJSjgv 

Vise.   MUC.  MEMB 

Fig.  82. 


-Diagrammatic  section  across  the  Hind-Brain  to  show  the  grouping  of 
cranial  nerves  and  their  nuclei.     (After  Elliot  Smith.) 


lungs  and  alimentary  canal — represented  by  the  dorsal  nuclei  of  IX  and 
X.  In  the  alar  lamina  are  differentiated  two  main  groups  or  columns 
of  sensory  or  reception  nuclei :  (1)  splanchnic,  which  receive  the  ingrowing 
fibres  of  the  IX-X  nerves — and  therefore  are  in  connection  with  the  pharynx, 
heart,  lungs  and  alimentary  canal,  receiving  afferent  impulses  from  all 
including  those  of  taste  ;  (2)  somatic,  corresponding  to  the  posterior 
horn  cells  of  the  spinal  cord  and  receiving  fibres  in  series  with  the  posterior 
roots  of  spinal  nerves.     The  posterior  root  fibres  in  the  cranial  series  are 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


represented  by  tlie  sensory  root  of  the  Vth  and  by  tbe  Vlllth  nerve — botb 
vestibular  and  cocblear  divisions.  The  Vlllth  nerve  and  its  ganglia 
were  probably  derived  from  the  same  system  as  gave  rise  to  the  complex 
sensory  organs  of  the  lateral  line  of  fishes,  and  should  be  distinguished  from 
the  ordinary  somatic  group.  We  have  seen  how  the  posterior  funiculi 
are  formed  in  the  marginal  zone  of  the  spinal  cord  by  fibres  of  the  posterior 
roots.  The  sensory  fibres  of  the  cranial  nerves  also  form  tracts  in  the 
marginal  zone  ;  the  solitary  tract  is  formed  by  fibres  of  the  IXth  and  also 
of  the  Xth  and  Vllth.  The  vestibular  and  fifth  nerves  also  form  tracts 
• — the  latter  being  particularly  extensive.  At  first  these  tracts  lie  near  the 
surface  of  the  hind-brain,  but  in  the  sixth  week  they  become  overwhelmed 
and  buried  by  vast  migrations  of  neuroblasts. 

Neurobiotaxis.     In  Fig.  83,  B  is  given  a  diagrammatic  section  across  the 
right  half  of  the  neural  plate  of  the  hind-brain  at  the  sixth  week  of  develop- 


chor.  plex 
rest  body 

V: 

dese.  root 


solit     alar,  larn 
basal  lam. 


oliu.  body 


.roof  plate 


-fas.  solit 


A. 


B. 


Fig.  83,  A. — Diagrammatic  Section  of  a  Foetal  Medulla  to  show  the  relative  positions 
of  the  Nuclei  connected  with  the  Somatic  and  Splanchnic  Nerves,  and  the  Origin 
of  the  Olivary  Body.  The  Motor  Nerves,  both  Somatic  and  Splanchnic,  are 
represented  black.  The  arrow  indicates  the  route  of  migration  of  the  Cells  of 
the  Olivary  Body. 

Fig.  83,  B. — The  Alar  and  Basal  Laminae  of  the  Hind-Brain  at  the  beginning  of  the  6th 
week  to  show  the  superficial  position  of  the  sensory  root  of  the  Vagus.  (Compare 
with  A.)  The  rhombic  lip  and  the  point  at  which  the  root  of  the  5th  Nerve  and 
Restiform  Body  will  be  formed  is  indicated  by  an  asterisk.    (After  His.) 

ment,  showing  the  solitary  tract  in  the  marginal  zone  of  the  ventral  surface  ; 
in  Fig.  83,  A  is  given  the  condition  in  the  8th  week,  showing  the  fasciculus 
solitarius  buried  deeply — much  nearer  the  dorsal  than  the  ventral  aspect 
of  the  medulla.  What  has  happened  is  this  :  swarms  of  neuroblasts  have 
been  produced  in  the  ependymal  zone  near  the  dorsal  margin  of  the  alar 
lamina — at  the  rhombic  lip  marked  by  an  asterisk  in  Fig.  83,  B.  In 
the  spinal  cord  the  posterior  horn  is  the  latest  site  of  neuroblastic  pro- 
duction ;  in  the  hind-brain  this  tendency  to  new  production  of  neuroblasts 
in  the  dorsal  margin  of  the  alar  lamina  has  become  enormously  heightened. 
The  arrow  in  Fig.  83,  A  shows  the  direction  of  the  swarm  ;  they  invade  the 
marginal  zone,  burying  the  solitary  tract,  and  group  themselves  as  they 
approach  the  floor  plate  in  the  middle  line,  to  form  the  inferior  olivary 
body  ;  the  superior  olivary  body  and  the  great  terminal  or  receptive 
nuclei — the  gracile  and  cuueate  nuclei — are  formed  in  the  same  way. 


THE  MID-  AND  HIND-BEAINS  89 

Dr.  Aliens  Kappers  ^  in  his  studies  on  the  medulla  in  1907  was  struck  by 
the  apparent  evolutionary  and  developmental  movement  of  the  nuclei 
of  motor  nerves  ;  they  were  drawn  towards  the  terminal  nuclei  from  which 
they  received  their  chief  incoming  stimuli  or  messages. 

For  example  he  noted  a  forward  movement  of  the  motor  nucleus  of  the 
Xllth  towards  the  receptive  nuclei  of  the  IXth  and  Xth  ;  of  the  Vllth 
towards  the  descending  root  of  the  Vth,  while  the  nuclei  of  the  Xlth  for 
the  sternomastoid  and  trapezius  tend  to  spread  backwards  in  the  spinal 
cord  towards  the  receptive  nuclei  of  the  neck  and  shoulder.  To  the  law 
or  force  which  regulates  the  mass-movement  or  migration  of  neuroblasts 
Kappers  gave  the  name — Neurobiotaxis.  We  have  just  mentioned  the 
migrations  which  give  rise  to  the  olivary  nuclei  of  the  medulla,  but  we  shall 
find,  as  we  ascend  the  brain  stem — to  cerebellum  and  pons,  to  mid-brain 
and  basal  ganglia  and  particularly  to  the  cerebrum  itself — that  neuroblastic 
migration  is  the  basal  principle  of  development  and  transforms  the  simple 
embryonic  neural  tube  into  the  complexities  of  the  adult  brain.  In  the 
spinal  cord  neuroblasts  are  confined  to  the  mantle  zone,  but  in  the  hind-, 
mid-  and  fore-brains  they  invade  the  marginal  zone  and  there  establish 
their  chief  centres.  The  cortex  of  the  cerebellum  and  cerebrum  are  pro- 
duced by  a  neuroblastic  invasion  of  the  marginal  zone.  Nor  are  the  mass- 
migration  of  nerve-cells  really  different  from  other  manifestations  of  living 
cells.  Outgrowing  processes  from  the  neuroblasts  of  the  spinal  ganglia 
and  spinal  cord  spread  into  the  limb  buds  and  reach  their  destinations 
unerringly — drawn  and  regulated  by  some  obscure  force ;  Dr.  Eoss 
Harrison  found  that  if  a  limb-bud  was  transplanted,  the  strange  nerve 
fibres  which  entered  it  were  attracted  and  moulded  to  a  normal  supply 
by  some  influence  in  the  tissues  of  the  bud.  The  force  which  attracts  the 
wandering  defensive  cells  of  the  body  to  a  site  of  infection  is  probably  of 
the  same  nature  as  that  which  regulates  the  migration  of  neuroblasts. 

Inferior  Medullary  Velum. — When  a  section  is  made  across  the  hind- 
brain  of  an  embryo  in  the  6th  week  of  development,  the  same  parts  are  seen 
as  in  the  spinal  cord  except  that  the  roof  plate  has  become  enormously 
expanded  to  form  the  inferior  medullary  velum.  The  extent,  shape  and 
attachments  of  the  roof  plate  are  shown  in  Fig.  84  ;  it  is  diamond  shaped, 
its  hind  angle  being  continuous  with  the  roof  plate  of  the  spinal  cord,  its 
front  angle  with  the  roof  plate  of  the  mid-brain,  while  its  lateral  angles 
mark  the  sites  of  the  two  lateral  recesses  of  the  4th  ventricle.  Its  upper 
lateral  margin  is  attached  to  the  border  of  that  part  of  the  alar  lamina  in 
which  the  cerebellum  is  to  arise  ;  its  lower  lateral  margin  is  attached  to  the 
rhombic  lip,  the  dorsal  border  of  the  medullary  part  of  the  alar  lamina. 
This  border  is  folded  outwards  (Fig.  81).  The  shape  of  the  roof  plate,  or 
inferior  medullary  velum,  is  altered  by  remarkable  changes  which  set  in 
during  the  6th  week  (Fig.  84)  ;  growth  changes  cause  the  hind-brain  to  be 
folded,  producing  the  pontine  bend  and  bringing  the  cerebellar  part  of  the 
hind-brain  against  the  medullary.     The  inferior  medullary  velum  becomes 

^  See  his  more  recent  statement,  Journ.  of  Nerv.  and  Mental  Diseases,  1919,  vol.  50, 
p.  1.  Also  Dr.  Davidson  Black,  Jonrn.  Comp.  New.  1917,  vol.  27,  p.  467  ;  vol.  28, 
p.  379. 


90 


HUMAN  EMBKYOLOGY  AND  MORPHOLOGY 


drawn  out  transversely.  It  is  at  this  time  that  choroidal  villi  are  produced 
on  its  ventricular  surface,  first  in  a  transverse  row  extending  from  lateral 
recess  to  lateral  recess  and  subsequently  over  its  entire  surface.  At  the 
same  time  secretion  of  cerebro-spinal  fluid  commences  ^  ;  the  fluid  per- 
colates through  the  velum  at  three  places — middle  of  the  roof  and  at  the 
lateral  angles.  At  a  later  date  (3rd  month)  the  foramen  of  Magendie  and 
the  openings  of  the  lateral  recess  appear  at  the  points  of  percolation.  The 
subarachnoid  spaces  begin  to  form  at  the  sites  of  escape  and  from  there 
extend. 

As  shown  in  Fig.  88,  the  velum  is  continuous  with  the  cerebellum  above 
and  the  roof  of  the  central  canal  of  the  cord  below.  In  the  posterior  margin 
of  the  cerebellar  plates  are  developed  :    (1)  the  nodule,  (2)  the  flocculus. 


MID    BKAIN 


MID    BRAIN 


LAT  RECE-SS 


UAT    RECESS 
INF    MED  VSL 
[/   RHOMBIC  I.IR 


MID    BRAIN 


CEREBELLUM 


CERVICAL  BEND 


'^''_</veeH 


Qth  weex 


Fig.  84. — Showing  the  origin  of  the  Inferior  Medullary  Velum  from  the  roof  plate  of 
the  Hind-Brain. 


(3)  the  peduncle  of  the  flocculus  between  1  and  2  (Figs.  89,  90).  Hence 
the  inferior  medullary  velum  ends  above  in  these  structures.  The  obex 
and  ligula,  thickenings  or  ridges  found  on  the  margins  of  the  4th  ventricle, 
mark  the  attachment  of  the  roof  plate  or  velum  to  the  rhombic  lip  of  the 
medullary  plates.  They  represent  the  attached  margin  of  the  velum. 
The  velum  is  also  attached  to  the  restiform  body  which  is  developed  in 
the  upper  margin  of  the  alar  lamina.  Over  the  opening  of  the  central 
canal  of  the  spinal  cord  into  the  4th  ventricle  there  is  often  a  fold  formed 
by  the  union  of  the  alar  laminae  (see  J.  T.  Wilson,  Journ.  Anat.  and  Physiol. 
1906,  vol.  40,  p.  210). 

The  velum  is  to  be  regarded  as  a  part  of  the  neural  tube,  specially  modi- 
fied for  the  purpose  of  secreting  the  cerebro-spinal  fluid  which  fills  the 
central  canal  and  subarachnoid  systems.  This  fluid  may  help  to  support 
the  central  nervous  mass  in  a  mechanical  sense,  but  its  rapid  secretion,  its 
circulation   and   chemical   composition   point   to   some   more   important 

^  I  have  followed  the  account  given  by  Dr.  Lewis  H.  Weed  for  the  developing  pig. 
See  Anat.  Rec.  1916,  vol.  10,  p.  256. 


THE  MID-  AND  HIND-BRAINS 


91 


nutritive  or  regulatory  influence  on  the  neural  centres.  The  ectodermal 
cells  retain  the  primitive  columnar  type,  and  form  an  epithelial  covering 
over  inflections  and  processes  of  the  pia  mater  which  is  derived  from  the 
mesodermal  covering  of  the  neural  tube. 

Cerebellum.^ — At  the  beginning  of  the  2nd  month  the  cerebellum  is 
still  represented  by  simple  right  and  left  alar  plates  (Figs.  85  and  93) 

rudiment  of  cerebellum 

inf.  med.  uel. 


ceph.  flex. 


ligu/a 

^obex 
nuchal  flex. 


olf.  lobe 


restiform  body 


Fio.  85. — Lateral  View  of  the  Cephalic  Part  of  the  Neural  Tube  in  a  5th  week 
Human  Embryo.     (After  His.) 

which  show  the  usual  triple  stratification — an  internal  proliferating  epen- 
dymal  zone,  a  middle  neuroblastic  and  an  outer  marginal  meshwork. 
In  the  frog  a  plate-like  cerebellum  is  retained  (Fig.  86),  for  the  amphibia 
have  but  an  imperfect  power  for  sustained  co-ordination  of  their  limbs 


Corp.  bigem. 

cerebellum 


inf.  med.  uelum. 


3rd  uent 


^r^entral  canal 


aqueduct      ^t^  "^"i>^'ole 

Fig.  86.— Median  Section  of  the  Cerebellum  and  4th  Ventricle  of  a  Frog. 

during  locomotion  on  land.  By  the  end  of  the  2nd  month  (Fig.  87)  there 
has  been  an  active  proliferation  of  neuroblasts  in  the  cerebellar  plates  ; 
they  fuse  in  the  middle  line  to  form  the  vermis  or  median  lobe,  and  now 
bulge  into  the  4th  ventricle,  much  as  they  do  in  the  frog.  What  has 
happened  may  be  best  gathered  from  Fig.  87.  The  reception  nucleus  for 
the  Vlllth  nerve  is  developed  in  the  rhombic  lip  near  the  lateral  recess  ; 

^  I  have  followed  the  accounts  given  of  the  cerebellum  by  Elliot  Smith  (See  Cunning- 
ham's Text-Book  of  Anatomy,  1913) ;  and  by  Streeter  (see  Keibel  and  Mall's  Manual 
of  Human  Embryology,  1912).     See  also  Dr.  Sven  Ingvar,  Folia  Neurobiologica,  1918. 


92 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


througli  the  vestibular  fibres  of  the  Vllltli  nerve  this  nucleus  will  receive 
impulses  which  make  it  the  chief  recipient  of  messages  needed  for  the 
co-ordination  of  muscles.  Elliot  Smith  regards  the  cerebellum  as  a  pro- 
duct of  the  vestibular  nucleus.  Hence  the  proliferation  of  neuroblasts 
at  the  rhombic  lip  and  their  spread  into  the  cerebellar  plates.  In  the 
3rd  month  neuroblasts  invade  the  marginal  zone  of  the  plates  and  lay  the 
basis  of  the  molecular  layer  of  the  cortex.  The  cells  of  Purkinje — although 
not  fully  differentiated  until  after  birth — take  up  their  stations  at  the 
junction  of  the  mantle  and  marginal  zones. 

At  the  time  the  cerebellar  plates  are  being  thus  invaded  in  the  3rd 
month,  in  this  way  a  cellular  basis  for  the  cortex  being  laid  down,  other 
cells,  arising  in  the  rhombic  lip,  invade  the  adjacent  basal  laminae — the 
parts  which  will  become  the  pons  (Fig.  87).  There  they  lie  in  the  path 
of  fibres  descending  from  the  frontal  cortex  and  thus  bring  the  cerebellum 


CEREREi-UUM 


ATTACH.    OF 
INF.  M£0.  VEL. 


I. AT.  PLATE. 


CAVITY    OF 
I  V'*V£NTRICLE 


RHOMBIC  LIP 
AT  LATERAL    RECESS 


NUCLEI    PONTIS 


Fig.  87. — The  Human  Cerebellum  at  the  end  of  the  2nd  month  of  development. 
(After  Streeter.)  The  arrows  show  the  direction  of  the  migration  of  the  Pontine 
and  Cerebellar  Nuclei. 

into  touch  with  the  cerebrum.  The  restiform  body  begins  to  form  in  the 
second  month,  and  by  this  means  the  cerebellum  is  placed  in  connection 
with  the  recipient  nuclei  of  the  cord  and  medulla.  The  dentate  and  other 
central  cerebellar  nuclei  are  isolated,  the  dentate  nuclei  being  linked  with 
the  red  nuclei  of  the  mid-brain  by  the  superior  peduncles.  In  the  differ- 
entiation of  the  cerebellum  are  to  be  seen  numerous  illustrations  of  the  law 
of  neurobiotaxis  enunciated  by  Kappers. 

Differentiation  of  Lobes.— At  the  end  of  the  3rd  month  (Fig.  88)  the 
cerebellum  has  assumed  a  dumb-bell  form — ^the  lateral  elevations  representing 
the  right  and  left  lobes  which  are  united  by  a  median  plate — the  vermis. 
The  cortex  has  already  commenced  to  expand,  as  may  be  seen  by  the  early 
appearance  of  transverse  fissures  on  the  vermis.  It  is  at  this  period  that 
the  cerebellar  plate  becomes  demarcated  into  anterior,  middle  and  posterior 
primary  lobes,  these  being  separated  by  two  transverse  grooves  or  fissures — 
the  first  and  second  fissures  (Elliot  Smith).  Since  these  three  primary 
divisions  are  to  be  recognized  in  nearly  all  mammalian  cerebelli,  they  must 
be  of  fundamental  importance.     Quickly  succeeding  these  two  primary 


THE  MID-  AND  HIND-BRAINS 


93 


fissures  there  appear  two  others,  one  which  divides  the  median  part  of  the 
posterior  lobe — the  post-nodular  fissure — and  the  other  the  anterior  lobe 
(Figs.  89,  90).  The  post-nodular  fissure  may  appear  in  the  human  brain 
before  the  fissura  secunda.  Thus,  at  the  end  of  the  fourth  month  four 
fissures  are  seen  to  be  developed  in  the  human  cerebellum  (Fig.  89).     The 


uermis 


nodule 


fat.  lobe 
cerebellum 


ligula 


lot.  lobe 


for.  Majendie 


lat  recess 
inf.  med.  velur.<i 

obex. 

clam 

T/'~~fwcleus.  cuneatus 

"nucleus,  gracilis 


Fig.  88. — Diagram  of  the  Cerebellum  and  of  the  Attachments  of  the  Inferior  Medullary- 
Velum  at  the  end  of  the  3rd  month  of  development.     (After  Kollmann.) 

rapid  growth  of  the  cerebellum,  with  the  pressure  of  the  cerebrum  above  or 
in  front,  and  the  resistance  of  the  occipital  bone  below  or  behind  cause  the 
plate-like  form  to  be  replaced  by  one  which  is  wedge-shaped  in  section, 
with  an  upper  and  lower  surface.  The  minor  sulci  and  fissures  of  the 
cerebellum  appear  between  the  5th  and  7th  months  of  foetal  life. 


mid.  brain 


fissura  prima 

suprapyrfis. 

pyramid 

fis.  secunda 
nodule 
inf.  med.  uelum. 


Fig.  89. — Diagrammatic  Section  of  the  Cerebellum  of  a  Human  Foetus  earlj'  in  the 
4th  month,  showing  the  folding  of  the  Cerebellar  Plate.  (After  Kuithan  and 
Elliot  Smith.) 

Parts  derived  from  the  Posterior  Primary  Lobe  (Figs.  89,  90,  A,  B). 

— From  the  median  part  arise  the  nodule  and  uvula  separated  by  the 
post-nodular  fissure.  From  the  lateral  parts  arise  the  flocculus  and  para- 
flocculus,  which  represent  the  oldest  of  all  the  distinctive  parts  of  the  cere- 
bellum, and  the  first  to  become  differentiated  in  the  human  organ.     The 


94 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


para-flocculus,  part  of  which  fills  the  subarcuate  fossa  in  the  temporal 
bone  (p.  236),  becomes  reduced  to  a  vestige  in  man  and  the  anthropoids 
(Fig.  90,  A). 

Parts  derived    from    the    Anterior    and    Middle  Primary    Lobes 

(Figs.  89,  90,  A,  B). — From  the  anterior  primary  lobe  arise  the  lingula, 
central  lobe,  and  alae,  the  culmen  and  the  anterior  crescentic  lobes.  The 
rest  of  the  cerebellum,  comprising  by  far  its  greater  part,  arises  from  the 
middle  lobe.  It  represents  an  addition  to  the  older  and  more  primitive 
parts  represented  by  the  anterior  and  posterior  lobes,  and  hence  has  been 
named  the  neocerebellum.  The  median  part  forms  the  pyramid  and  the 
clivus,  separated  by  a  deep  fissure.  The  lateral  parts  undergo  an  enormous 
development  in  higher  primates.     In  man  the  tonsillar  and  biventral 

A.  & 

lingula. 
central 

crese. 


grt  horiz.  fissure 
paraflocc. 


horiz.  fio. 


para  flocculus.^ 


flocculus 


flocculus 


Fig.  90,  A. — Left  half  of  the  Cerebellum  of  a  Foetus  of  5  months,  seen  on  its  inferior 
aspect.  Only  the  middle  and  posterior  primary  lobes  are  exposed. 
The  parts  forming  the  posterior  lobe  are  stippled.  (After  Elliot  Smith.) 
B. — Right  half  of  a  typical  Mammalian  Cerebellum,  spread  out  so  as  to  show 
the  anterior,  middle  and  posterior  primary  lobes.  The  anterior  and 
posterior  lobea  are  stippled.  The  flssiu-es  and  parts  are  indicated  by  the 
terms  used  in  human  anatomy  in  order  that  the  peculiar  features  of  the 
human  cerebellum  may  be  made  evident.    (After  Elliot  Smith.) 

lobes  attain  a  very  great  size.  The  great  development  of  the  lateral  parts 
of  the  middle  primary  lobe  during  the  5th  and  6th  months,  leads  to  the  form- 
ation of  the  great  horizontal  fissure  (see  Figs.  90,  A  and  B). 

The  Superior  Medullary  Velum  is  part  of  the  roof  plate  of  the  4th  ventricle 
which  remains  between  the  superior  peduncles.  The  vestigial  laminae  which 
cover  it  form  the  lingula  (Fig.  89). 

Three  points  in  connection  with  the  development  and  comparative 
anatomy  of  the  cerebellum  are  especially  worthy  of  attention  : 

(1)  It  arises  from  the  alar  laminae,  which  are  directly  connected  with 
afferent  or  sensory  nerves  only  ;  further,  the  nuclei  in  the  mesencephalon, 
pons  and  medulla,  with  which  it  is  connected,  arise  from  the  alar  laminae. 

(2)  The  part  of  the  neural  tube  from  which  the  cerebellum  arises  is  the 
vestibular  neuromere — the  one  to  which  the  internal  ear  becomes  closely 
linked. 


THE  MID-  AND  HIND-BRAINS  95 

(3)  The  cerebellum  readies  its  greatest  development  in  primates  amongst 
mammals  ;  it  is  also  greatly  developed  in  swimming  vertebrates.  In 
primates,  as  in  swimming  mammals,  the  equilibrium  of  the  body  is  finely 
adjusted.  On  embryological  grounds  alone  we  would  infer  that  the  cere- 
bellum is  part  of  a  sensory  mechanism.  Clinical  and  experimental  observa- 
tions indicate  that  its  main  function  is  to  co-ordinate  the  various  muscles 
of  the  body  in  performing  definite  acts.  It  is  therefore  on  the  afferent  nerve 
system  arising  from  the  muscles,  joints  and  bones,  that  the  cerebellum  has 
been  developed,  but  its  position  was  determined  by  the  nuclei  of  the 
vestibular  nerves,  cells  of  which  invade  the  embryonic  cerebellar  plate. 


MID-BRAIN   OR  MESENCEPHALON. 

By  the  end  of  the  3rd  month  the  mid-brain  is  becoming  over- 
shadowed by  the  preponderating  growth  of  the  fore-  and  hind-brains, 
and  by  the  6th  month  is  reduced  to  the  peduncular  body  which  unites 
cerebrum  with  cerebellum,  its  ventricle  or  canal  becoming  reduced  to 
the  aqueduct  which  unites  the  4th  ventricle  to  the  3rd.  With  the  mid- 
brain we  reach  the  anterior  limit  of  the  primitive  neural  tube  ;  it  lies 
over  the  terminal  cephalic  part  of  the  notochord  (Fig.  80) ;  two  cranial 
nerves  (III  and  IV),  corresponding  to  the  anterior  roots  of  spinal 
nerves,  arise  from  it.  A  section  across  the  mid-brain  in  the  4th  week  of 
development,  reveals  the  same  divisions  as  in  the  cord — lateral  neural 
plates  made  up  of  basal  and  alar  laminae,  united  by  a  roof  plate  and  a  floor 
plate.  The  same  three  zones  arise — ependymal,  mantle  and  marginal. 
In  the  3rd  month  the  quadrigeminal  plate  develops  on  the  dorsal  part  of  its 
alar  laminae,  much  in  the  same  way  as  the  cerebellum  arises  within  the 
alar  laminae  of  the  hind-brain.  The  neuroblasts  invade  the  dorsal  marginal 
zone,  and  evolve  into  a  formation  which  may  be  described  as  a  cortex.  The 
quadrigeminal  plate  which  thus  arises  on  the  dorsum  of  the  mid-brain  may  be 
regarded  as  primary  receptive  centres  for  the  nerve  of  sight  (the  optic 
tracts),  and  in  birds  this  formation  assumes  great  size  and  importance. 
The  necessity  of  linking  the  receptive  nuclei  for  sight  with  those  for  hearing 
is  apparent ;  hence  we  find  the  cochlear  nuclei  connected  with  the  quadri- 
geminal formation  by  the  lateral  lemniscus.  In  the  development  of  the 
mid-brain  we  see  the  quadrigeminal  plate  become  divided  into  the  inferior 
colliculus — in  which  the  cochlear  tract  ends — and  the  superior  collicuhcs, 
which  receives  fibres  from  the  retina.  Thus,  in  the  main  the  mid-brain 
is  connected  with  sight ;  in  the  basal  laminae  arise  the  nuclei  for  the  Ilird 
and  IVth  nerves — the  chief  source  of  motor  supply  for  the  muscles  of  the 
eye-ball  (Fig.  93).  From  the  mid-brain  arise  also  sensory  fibres  of  the  Vth 
nerve  which  go  to  the  orbit.  They  differ  from  all  other  sensory  fibres  in 
having  their  cell  bodies  implanted  in  the  wall  of  the  neural  tube.  From 
the  3rd  month  of  development  onwards  the  mid-brain  becomes  the  highway 
of  developing  efferent  nerve  paths  which  unite  the  basal  masses  and  cortex 
of  the  fore-brain  with  the  nuclei  in  the  pons,  medulla  and  spinal  cord,  and  of 
afferent  or  sensory  paths  which  connect  the  nuclei  of  the  cord,  medulla 
and  cerebellum  with  the  basal  masses  of  the  fore-brain.     The  cerebral 


96 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


cortical  paths  develop  in  tlie  marginal  zone  of  the  basal  plates  and  form 
the  crura  cerebri,  while  the  afferent  paths — the  median  lemniscus; — 
develops  in  the  mantle  zone — the  tegmentum.  In  this  zone,  too,  appears 
the  red  nucleus,  but  as  yet  its  neuroblasts  have  not  been  traced  to  their 
source. 

The  Three  Neural  Flexures  (see  Figs.  80,  84,  85).— The  pontine  flexure, 

a  convexity  forwards  of  the  pons,  has  already  been  mentioned  ;  it  is 
the  result  of  the  elongation  of  the  neural  plates  of  the  hind-brain  due  to 
the  proliferation  of  the  neuroblasts  and  the  production  of  the  cerebellar 
plates.  The  nuchal  flexure  is  concave  forwards,  and  occurs  between  the 
medulla  and  cord.  The  latter  is  compensatory  and  of  but  small  import ; 
on  the  other  hand,  the  anterior  flexure,  whereby,  in  the  third  week  of 


PINEAU  GLAND 


CHOR   PLEX 
SUP-  COM 
PINEAL  RECESS       „.,„..^.. 

Jim 

POST:  COMMISSURE 

COMMISSURAL  ORQAN 


Fig.  91. — Section  of  the  anterior  part  of  the  Roof  of  the  Mid-Brain  of  a  Cat,  to  sliow 
the  subcommissural  organ.     (Dendy  and  Nicholls.) 

foetal  life,  the  fore-brain  appears  as  a  downward  and  forward  development 
until  it  comes  to  lie  on  the  ventral  aspect  of  the  cephalic  end  of  the  noto- 
chord,  leads  to  a  great  alteration  in  the  form  and  relationships  of  the  fore- 
and  mid-brains,  and  is  of  great  importance  (Fig.  85).  Even  in  the  embryos 
of  the  lowest  vertebrate  types  the  expansion  and  bending  of  the  anterior 
end  of  the  neural  tube  is  apparent.  The  mid-brain,  by  this  flexure, 
comes  to  be,  for  a  short  time,  the  most  anterior  part  of  the  neural  canal ; 
the  fore-brain  is  doubled  back  under  the  notochord.  Round  the  pro- 
jecting end  of  the  notochord — projecting  between  the  mid-  and  fore-brains 
—are  developed  the  posterior  clinoid  processes  and  dorsum  sellae.  The 
dorsum  sellae  marks  the  position  of  the  anterior  flexure  in  the  adult  brain. 
The  tentorium  cerebelli  is  developed  between  the  mid-brain  and  fore-brain, 
and  lies  at  first  at  right  angles  to  the  axis  of  the  mid-brain,  but  the  sub- 
sequent great  growth  of  the  cerebrum  forces  it  backwards  arid  downwards 
until  it  becomes  a  horizontal  partition  between  the  cerebellar  and  cerebral 
chambers  of  the  skull. 


THE  MID-  AND  HIND-BRAINS 


97 


Subcommissural  Organ. — For  some  time  it  has  been  known  that  the 
ependyma  on  the  roof  of  the  mid-brain  of  lower  vertebrates,  immediately 
behind  the  posterior  commissure  (see  Fig.  91),  is  modified  to  form  a  peculiar 
area  of  high  columnar  cells.  The  cells  are  related  to  a  certain  very  large 
fibre  (Reissner's  fibre),  which  descends  ventral  to  the  central  canal  of  the 
spinal  cord  in  fishes  and  amphibians.  Recently  Dendy  and  NichoUs  have 
shown  that  this  ependymal  structure,  to  which  they  have  given  the  name 
of  subcommissural  organ,  occurs  in  all  vertebrates,  including  man.  It 
is  quite  apparent  in  the  human  foetal  brain,  but  is  soon  reduced  to  a  vestige. 
The  fibres  are  not  nervous  in  nature.  The  function  and  significance  of  the 
structure  are  unknown. ^ 

Constitution  of  the  Mid-  and  Hind-Brain. — We  have  traced  the 
development  of  the  neural  tube  in  a  forward  direction,  and  have  reached 


ECTODERM  — 


NEURAL   PLATE 
NEURAL    CANAL> 


EUt?AL  CREST 


DORSAL  GANQLION 


BRANCHIOMERE 


Fig.  92. — Diagrammatic  Section  across  tlie  posterior  region  of  the  Head  of  Ammoe- 
cetes — tlie  immature  form  of  the  Lamprey — to  show  a  Branchiomere  and  the 
ganglia  derived  from  the  Neural  Crest  of  the  Hind-Brain.     (After  Froriep.) 

the  point  where  the  mid-brain  passes  into  the  fore-brain.  On  the  roof 
the  point  of  transition  is  marked  by  the  posterior  commissure  (Fig.  91)  ; 
below  the  floor  the  notochord  ends  (Fig.  80).  We  have  reached  the  end  of 
the  neural  tube  proper  ;  the  part  in  front — ^the  fore-brain — appears  to  have 
arisen  in  connection  with  two  great  organs  of  sense — ^the  nose  and  eye. 
We  find  that  the  neural  tube,  when  it  enters  the  region  of  the  head,  becomes 
greatly  altered  in  its  constitution.  This  is  due,  not  only  to  the  development 
of  special  parts  such  as  the  pons,  the  cerebellum,  quadrigeminal  plate  and 
special  nerve  tracts  which  unite  the  cerebral  and  spinal  centres,  but  especi- 
ally to  the  fact  that  the  structure  of  the  head  is  older  and  more  complex 
than  that  of  the  body.  In  the  head  region  another  element  appears — a 
ventral  mesodermic  somite  or  branchiomere — in  addition  to  the  dorsal 
mesodermic  somite  seen  in  the  trunk  region  (Fig.  92).  The  branchiomeres 
give  rise  to  the  gill  arches,  which  are  so  apparent  in  the  human  embryo  at 
the  end  of  the  first  month.     In  the  mid-  and  hind-brain  special  centres  and 

1  See  Nicholls,  Quart.  Journ.  Mic.  Sc.  1912,  vol.  58,  p.  1. 

G 


98      HUMAN  EMBRYOLOaY  AND  MORPHOLOGY 

nerves  are  developed  in  connection  witli  the  gill  arches.  In  the  spinal  cord 
there  were  two  columns  of  motor  nerves  in  the  basal  lamina,  one  for  the 
somatic  or  voluntary  muscles  of  the  body,  another  for  the  visceral  muscula- 
ture— ^the  splanchnic — but  here  a  third  or  intermediate  column  is  added — ■ 
the  motor  cells  for  the  muscles  connected  with  the  gills  (Fig.  82).  The 
branchial^or  lateral  somatic  nerves  are  represented  in  the  mid-  and  hind- 
brain  by  the  motor  or  ventral  root  of  the  Vth  nerve,  by  the  motor  part  of 
the  Vllth,  by  the  parts  of  the  IXth,  Xth,  Xlth,  which  supply  striated 
muscles.  The  presence  of  branchial  arches  in  the  head  region  gives  rise  to  a 
more  complex  arrangement  of  the  nerve  ganglia  (Fig.  93).  In  the  trunk 
region  the  neural  crest  gave  origin  to  posterior  root  ganglia,  the  ganglia 
of  the  sympathetic  chain  (prevertebral),  and  other  ganglia  stationed  in 
front  of  the  spine.  In  the  regions  of  the  mid-  and  hind-brain  the  neural 
crest  also  is  developed,  but  besides  giving  rise  to  ganglia  (see  Figs.  92,  93) 
representing  the  posterior  root  ganglion  and  sympathetic  ganglia  found 
in  the  region  of  the  trunk,  it  also  gives  origin  to  a  lateral  mass  of  nerve  cells, 
from  which  the  sensory  fibres  to  the  gills  are  produced.  Associated  with 
this  lateral  mass  are  also  cellular  formations  representing  two  rows  of 
sense  organs  ^ — an  upper,  the  organs  of  the  lateral  line  ;  a  lower,  the 
epibranchial  sense  organs.  In  man  only  vestiges  of  these  sense  organs 
appear.  The  ultimate  fate  of  the  epibranchial  rudiments  is  not  known  for 
certain,  but  it  is  probable  that  some  of  their  cells  are  included  in  the  ganglia 
at  the  trunks  of  the  Vllth,  IXth  and  Xth  nerves. 

Segmental  Arrangement  of  Cranial  Nerves. — We  have  seen  that  nine 
neuromeres  can  be  recognized  in  the  hind-brain  at  the  4th  week  of  develop- 
ment (Fig.  80),  and  we  may  assign  a  double  segmental  origin  to  the  mid- 
brain. But  when  we  look  at  the  ganglia  and  nerves  of  an  embryo  in  the 
6th  week  of  development  (Fig.  93)  it  will  be  realized  that  it  is  impossible  to 
assign  a  cranial  or  head  segment  to  each  of  these.  In  the  human  embryo 
it  is  easy  to  see  that  the  Vllth  nerve  enters  the  second  or  hyoid  arch  and 
may  be  regarded  as  the  nerve  of  the  hyoid  segment — which  may  be  reckoned 
the  3rd  segment  of  the  head,  but  the  nerve  of  the  segment  arises  from  the 
4:th  neuromere  of  the  hind-brain,  while  the  nucleus  of  the  Vlth  apparently 
arises  from  the  Vth.  Meanwhile  we  regard  both  of  these  neuromeres  as 
belonging  to  the  third  cranial  segment.  In  this  segment  of  the  head,  then, 
we  have  an  approach  to  the  full  complement  of  nerve  elements  found  in  a 
typical  cranial  segment.  The  somatic  motor  fibres  are  represented  by  the 
Vlth  nerve  (to  the  external  rectus) ;  the  lateral  somatic  motor  or  branchial, 
by  the  motor  fibres  of  the  Vllth  or  facial ;  the  splanchnic  efferent  or 
motor  by  the  secretory  fibres  of  the  chorda-tympani  of  the  Vllth  ;  the 
afferent  or  splanchnic  sensory  by  the  gustatory  fibres  of  the  Vllth  (chorda 
tympani  and  great  superficial  petrosal)  ;  the  somatic  sensory  fibres  by  the 
Vlllth  or  auditory  nerve.  The  cochlear  and  vestibular  ganglia  represent  a 
posterior  root  ganglion  ;  the  submaxillary  ganglion — a  vagrant  sym- 
pathetic ganglion.  Thus  the  4th  neural  segment  has  become  associated 
with  the  hyoid  (2nd  visceral)  arch,  the  eye  and  the  ear. 

^  The  formation  here  named  lateral  line  "  organ  "  is  better  termed  the  dorsolateral 
placode,  and  the  epibranchial  "  organ  "  epibranchial  placode. 


THE  MID-  AND  HIND-BRAINS 


99 


In  the  other  segments  there  have  been  great  changes  and  reductions. 
As  regards  the  nerves  of  the  first  cranial  segment,  only  its  somatic  motor 
nerve — the  Ilird  nerve — remains  ;  its  posterior  root  and  ganglion  are 
represented  by  the  ophthalmic  division  of  the  Vth  nerve.  The  ciliary 
ganglion  represents  the  sympathetic  ganglion  of  this  segment ;  the  fibres 
from  the  Ilird  to  this  ganglion,  the  efEerent  or  motor  splanchnic  fibres. 
In  the  ciliary  ganglion  there  may  also  be  motor  splanchnic  cells,  carried 
out  on  the  fibres  of  the  Ilird  nerve.  The  nerves  of  the  second  segment  are 
represented  by  the  I  Vth  or  trochlear  nerve  (somatic  motor),  the  nerves  to  the 
muscles  of  mastication  (lateral  somatic  or  branchial  root),  the  somatic 
sensory  by  the  maxillary  and  mandibular  divisions  of  the  Vth  nerve.  The 
sensory  root  of  the  Vth  nerve  has  spread  its  dominion  until  it  now  forms 


Fig.  93. — The  Nerves  and  Ganglia  of  the  Mid-  and  Hind-Brain  of  an  Embrs'o  at  the 
end  of  the  6th  weels  of  development.     (After  Streeter.) 

connections  with  all  the  segments  of  the  mid-  and  hind-brains,  and  even 
reaches  the  upper  part  of  the  spinal  cord.  There  are  no  sensory  somatic 
fibres  in  the  nerves  of  the  4th,  5th,  6th  and  7th  cranial  segments  with  the 
exception  of  the  auricular  branch  of  the  vagus.  The  IXth  or  glosso- 
pharyngeal is  the  nerve  of  the  4th  cranial  segment  and  contains  lateral 
somatic,  efferent  and  afferent  splanchnic  fibres.  The  vagus  and  bulbar 
roots  of  the  spinal  accessory  represent  the  splanchnic  efferent  and  aff'erent 
nerves  of  the  5th,  6th  and  7th  segments — the  most  important  segments  in 
the  neural  tube,  for  they  contain  the  nerve  centres  which  dominate  the 
heart,  the  lungs  and  the  greater  part  of  the  alimentary  canal.  The  somatic 
motor  roots  of  the  5th,  6th  and  7th  cranial  segments  are  represented  by  the 
fasciculi  of  origin  of  the  Xllth  nerve — the  motor  nerve  of  the  tongue  ;  they 
arise  from  the  8th  and  9th  neuromeres..  It  will  be  thus  seen  that  embryo- 
logy and  comparative  anatomy  supply  a  clue  to  the  manner  in  which  the 
cranial  nerves  are  arranged.  The  basis  of  that  arrangement  is  strictly 
a  j)hysiological  one,  but  the  specialization  in  certain  segments,  which  has 
occurred  in  the  course  of  evolution,  has  destroyed  the  original  simplicity 


100     HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

of  their  arrangement.^    Further  mention  of  the  cranial  nerves  will  be  made 
in  dealing  with  the  nose,  eye,  ear,  face  and  visceral  arches. 

In  the  human  embryo  vestiges  of  posterior  roots  and  ganglia  may  appear 
with  the  hinder  hypoglossal  fasciculi  (Froriep's  ganglion)  ;  we  may  infer 
that  at  one  time  the  occipital  segments  had  nerves  with  anterior  and 
posterior  somatic  roots.  Streeter  also  observed  that  the  spinal  rootlets 
of  the  Xlth  nerve  have  vestigial  ganglia  (visceral  sensory)  on  them  when 
first  formed  (Fig.  92). 

■  ^  For  segmentation  of  hind-brain  see  F.  P.  Johnson,  Anat.  Record,  1915,  vol.  10, 
p.  209  ;  J.  C.  Watt,  Contrib.  to  Embryology,  1915,  vol.  2,  p.  1  ;  H.  L.  Barniville 
reference,  p.  47  ;  Prof.  D.  Waterston,  Journ.  Anat.  1915,  vol.  49,  p.  90. 


CHAPTER  IX. 
THE  FORE-BKAIN  OR  PROSENCEPHALON. 

The  Origin  of  the  Cerebrum. — It  is  in  connection  with  tlie  fore-brain 
that  the  most  distinctive  and  most  complex  of  all  human  structures  arises 
■ — the  cerebrum.  If  we  confine  our  attention  purely  to  the  developmental 
changes  which  occur  in  the  fore-brain  of  the  human  embryo,  we  shaU 
understand  very  imperfectly  the  origin  and  nature  of  the  human  brain. 
It  is  true  that  on  developmental  evidence  alone  we  may  infer  that  the 

..™.       ./^"^ 


pined  body       x<^^^,,,^^0m2^  r.oH'^irrrxw/ 


•  K^^ 
S 


S^.-        3rd  uentricle 


\^. 


w^ 


ant:  com. 

tic.  'chiasma 

Fig.  94. — Longitudinal  Section  of  the  Brain  of  a  Larval  Fish,  to  show  the  primary 
form  and  relations  of  the  fore-brain.  (Kupffer.)  Note  especially  that  the  whole 
roof  is  formed  by  a  choroidal  velum. 

fore-brain,  although  situated  at  the  anterior  extremity  of  the  neural  tube, 
does  not  rejiresent  a  prolongation  of  all  the  elements  of  the  tube,  but 
only  of  its  alar  or  dorsal  laminae,  which  we  know  to  be  sensory  in  their 
nature.  We  may  infer  that  the  fore-brain  belongs  to  the  sensory  part  of 
the  nervous  system — not  to  its  motor  or  basal  lamina.  To  obtain  a  proper 
appreciation  of  the  fore-brain,  however,  one  must  study  this  structure 
in  the  lowest  of  vertebrates — the  Lamprey.  In  Fig.  95  the  brain  of  this 
primitive  fish  is  represented.  The  fore-brain  is  made  up  of  two  parts — 
a  posterior — the  thalamencephalon  or  diencephalon,  with  which  the  retinae 
and  optic  tracts  are  connected,  and  an  anterior  or  telencephalon,  in  which 

101 


102 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


the  olfactory  nerves  terminate.  The  two  parts  of  the  fore-brain  have 
thus  arisen  in  connection  with  the  sense  of  sight  and  the  sense  of  smell ; 
secondary  nerve  masses  have  arisen  in  these  two  parts  of  the  fore-brain — - 
the  optic  thalamus  in  the  posterior,  and  the  corpus  striatum  in  the  anterior  ; 
but  the  optic  thalamus  receives  not  only  nerve  tracts  connected  with  the 
sense  of  sight,  but  other  sensory  tracts  connecting  it  with  all  the  systems 
of  the  body — skin,  muscles,  joints,  ear,  etc.,  and  thus  becomes  a  higher 
centre  for  the  control  of  lower  centres.  The  corpus  striatum — ^the  secondary 
mass  in  the  anterior  or  olfactory  part — the  telencephalon — also  receives 
tracts  from  the  gustatory,  and  other  lower  centres  besides  those  from  the 
olfactory  tracts.  In  the  brain  of  the  lamprey  the  mid-brain  and  the  two 
parts  of  the  fore-brain  form  a  "  federation  of  centres."  ^  In  mammals  the 
telencephalon  becomes  the  dominant  part ;  the  cerebral  hemispheres 
arise  from  it.     Thus  our  cerebral  hemispheres  have  arisen  in  connection 


OLFACT : N 
OLFACT: BULB 

-  OPTIC    NERVE 
CEREBRAL . H: 
PINEAL    BODY 
THALAMENCEPHALON 
MID      BRAIN 
•■V^!'  NERVE 


HIND    BRAIN 


Fig.  95. — The  Brain  of  the  Lamprey  from  above.    (After  R.  H.  Burne.) 

with  parts  which  have  become  insignificant — the  olfactory  nerve  centres. 
The  telencephalon  has  received  and  formed  communications  with  all  parts 
of  the  central  nervous  system,  and  become  the  central  exchange  of  all 
sensory  impulses  and  also  the  seat  of  consciousness. 

The  Fore-Brain  of  the  Human  Embryo. — In  the  4th  week  of  de- 
velopment there  is  a  resemblance  between  the  human  fore-brain  and  that 
of  a  fish  ;  both  are  of  a  simple  vesicular  form  (compare  Figs.  94  and  96). 
In  some  respects  the  fish's  brain  is  the  more  instructive,  because  its  parts 
are  clearly  differentiated.  In  the  fish  the  roof  of  the  3rd  ventricle — the 
name  given  to  the  central  canal  of  the  thalamencephalon — contains  no 
nerve  tissue  ;  it  is  membranous,  and  forms  a  choroid  plexus.  The  pineal 
body  arises  from  the  posterior  part  of  the  roof,  immediately  in  front  of  the 
posterior  commissure  (Fig.  94).  The  representatives  of  those  parts  are 
seen  in  the  roof  of  the  3rd  ventricle  of  the  human  embryo  (see  Figs.  96,  97, 
98).  On  the  narrow  floor  of  the  3rd  ventricle  are  seen  the  infundibular 
part  of  the  pituitary  body  and  the  optic  chiasma — or  the  plate  in  which 

1  The  phrase  is  Professor  Elliot  Smith's,  whose  researches  on  the  evolution  of  the 
brain  form  the  basis  of  the  account  given  here. 


THE  FORE-BRAIN  OR  PROSENCEPHALON 


103 


the  chiasma  will  be  formed.  In  both  the  fish  and  the;  human  embryo 
the  anterior  wall  of  the  3rd  ventricle  is  formed  by  a  plate  of  neural  tissue 
— the  lamina  terminalis. 

Parts  Developed  in  the  Wall  of  the  Fore-Brain.— When  a  model 
of  the  fore-brain  of  an  embryo  in  the  4th  week  of  development  is  laid  open, 
as  in  Fig.  96,  it  is  possible  to  identify  its  two  main  divisions — a  posterior 
or  tlialamencepiialon,  its  central  cavity  becoming  the  3rd  ventricle  (Fig. 
96,  B),  and  an  anterior  or  telencephalon  which  will  enclose  the  lateral 
ventricle.  At  the  junction  of  these  divisions,  but  yet  lying  distinctly  in 
the  wall  of  the  thalamencephalon,  is  seen  the  wide  evagination  (Fig.  96,  A) 
which  gives  rise  to  the  optic  vesicle — the  basis  in  which  the  retina  and  optic 
tracts  will  develop.  A  section  across  the  thalamic  region  of  the  fore-brain 
at  this  stage  shows  a  right  and  left  lateral  plate,  their  basal  margins  being 
united  by  a  trough-like  floor  plate,  while  their  dorsal  margins  are  joined  by 


PALUAL  AREA 
OLFACT.    AREA    /  -THALAMIC    AREA 


MID  BRAIN 


OLFACT.  AREA 

PALLIAL  AREA 

THALAMIC  AREA 


NEUROPORE 


HYPOTHAL.AREA 
OPTIC    RECESS 
STRIATE    AREA 


Mid   BRAIN    - 

educt-J 


HYPOTHAL    AREA 
INFUNDie     AREA 


OPTIC     RECESS 
STRIATE    AREA 


Fig.  96. — Sections  of  the  Fore-Brain  at  the  beginning  {A)  and  near  the  end  (B)  of  the 
4th  weelc  of  development. 

a  roof  plate — which,  late  in  the  2nd  month,  becomes  converted  into  the 
choroid  plexus  of  the  3rd  ventricle — just  as  was  the  case  with  the  roof 
plate  of  the  4th  ventricle.  The  lateral  plates  show  the  usual  three  zones 
during  the  4th  week — an  inner  ej)endymal,  in  which  cellular  proliferation 
is  active,  a  middle  or  mantle  zone  and  an  outer  or  marginal  which,  in  the  3rd 
month,  becomes  invaded  by  the  great  trackways  leading  to  and  from  the 
fore-brain.  At  this  early  stage,  too,  a  groove  can  be  seen  running  obliquely 
on  the  lateral  wall  of  the  3rd  ventricle,  from  the  floor  of  the  mid-brain  to 
the  optic  recess  (Fig.  96,  B),  indicating  a  division  of  the  lateral  plate  into 
an  upper  or  thalamic  and  a  lower  or  hypothalamic  region.  In  the  upper 
region  will  be  differentiated  the  optic  thalamus,  the  epithalamus  (the 
pineal  body  with  its  ganglia  and  commissures)  and  the  metathalamus 
or  geniculate  bodies,  while  in  the  lower  region  and  in  the  floor  plate  are 
differentiated  the  infundibular  stalk  of  the  pituitary  body,  the  tuber 
cinereum,  the  mammillary  bodies  and  the  posterior  perforated  space.  Here 
we  are  chiefly  concerned  with  the  walls  of  the  3rd  ventricle,  but  it  may  be 
noted  in  Figs.  96,  A  and  B,  that  the  three  areas  of  the  telencephalon  can 


104      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

also  be  identified — the  cortical  or  pallial  area  of  tlie  cerebral  evagination, 
the  striate  area — forming  a  junctional  zone  between  the  thalamic  region 
of  the  3rd  ventricle  and  the  pallial  area  of  the  lateral  ventricle  (Fig.  96,  B) 
and  an  olfactory  area.  At  the  beginning  of  the  4th  week  (Fig.  96,  A)  the 
neuropore  is  still  open,  and  the  olfactory  areas  which  will  appear  at 
each  side  of  the  closed  opening  can  hardly  be  said  at  this  time  to  be 
differentiated. 

By  the  end  of  the  6th  week  certain  notable  changes  have  occurred  in 
the  fore-brain  (Fig.  97)  ;  the  cerebral  vesicle  is  now  rapidly  expanding, 
its  hinder  or  occipital  end  beginning  to  expand  over  and  cover  the  roof 
and  lateral  walls  of  the  thalamencephalon.  The  opening  of  the  lateral 
ventricle  has  become  relatively  smaller,  owing  to  the  upgrowth  and  more 
intimate  fusion  of  the  corpus  striatum  with  the  optic  thalamus.  Tn  the 
hypothalamic  region  we  can  now  see  a  recess  behind  the  optic  chiasma, 
indicating  the  outgrowth  of  the  infundibular  process  of  the  pituitary  body 

CEREBRAL.     VCSIC. 


FOR.  OF  MONRO 

1                  _, 

ROOF  OF  PLATE. 

A  ^ 

THALAMUS 

1  ^,,„se0mm2s^ 

y^^^  metathauamus 

£^ —  SITE  OF  Pineal 

Ss:     ly"^  '  ^ 

^%.      MIDBRAIN 

OLFACTORY  LOBE 
CORP.  ST  HI  AT, 
LAM.  TERM 

OPTIC  REceas 

OPTIC    CHIAS. 


HYPOTHALAMUS 
INFUNOIB .  RECESS 


PITUITARY 

Fig.  97. — The  Thalamenceplialon  towards  the  end  of  the  6th  week  of  development. 

(hypophysis)  and  growing  towards  it  an  ingrowth  of  ectoderm  from  the 
embryonic  mouth  or  stomodaeum.  The  roof  plate  is  now  beginning  to 
be  converted  into  a  secretory  structure — ^the  choroid  plexus  of  the  3rd 
ventricle.  The  roof  plate  can  be  seen  to  extend  (Fig.  97)  from  a  slight  dip 
or  fold  over  the  foramen  of  Monro  to  the  region  of  the  pineal  body  at  the 
anterior  border  of  the  mid-brain. 

In  Fig.  98,  which  represents  in  a  diagrammatic  manner  the  simple  fore- 
brain  of  an  embryo  at  the  end  of  the  first  month  of  development,  there 
have  been  represented — following  a  scheme  devised  by  Professor  Elliot 
Smith-^the  great  sensory  pathways  which  terminate  in  the  thalamence- 
phalon and  make  it  into  the  great  court  of  sensory  appeal.  These  fibre 
tracts,  which  do  not  begin  to  make  their  way  through  the  mid-brain  from 
the  medulla  and  cord  until  the  end  of  the  3rd  month  of  development, 
are  depicted  by  simple  arrows — the  medial  lemniscus  and  auditory  tract 
ending  in  the  optic  thalamus,  while  the  gustatory  tract  ends  in  the  hypo- 
thalamic region.  Relays  of  fibres  commencing  in  the  thalamencephalon 
carry  optic,  auditory,  gustatory  and  common  sensory  impulses  to  the 
telencephalon — the  highest  court  of  sensory  appeal.  It  is  when  this  broad 
conception  of  the  relationship  of  the  fore-brain  to  the  sensory  tracts  is 


THE  FORE-BRAIN  OR  PROSENCEPHALON  105 

grasped  that  we  begin  to  understand  the  reason  for  the  transformation 
of  the  simple  fore-brain  of  the  embryo  into  the  elaborate  cerebrum  of  the 
adult. 

The  Lamina  terminalis  forms  the  anterior  or  terminal  wall  of  Ijie  simple 
fore-brain  of  the  4th  week  embryo  (Fig.  96,  B)  ;  it  is  completed  by  the 
closure  of  the  neuropore  (Fig.  96,  A).  When  the  cerebral  vesicles  grow  out 
it  becomes  demarcated  as  a  plate  stretching  from  the  foramen  of  Monro 
above  to  the  optic  chiasma  below  (Fig.  97),  and  joining  together  the 
olfactory  areas  of  the  cerebral  vesicles.  This  simple  plate,  which  comes  to 
form  the  anterior  wall  of  the  3rd  ventricle,  begins  to  assume  great  import- 
ance in  the  second  month,  because  it  serves  as  a  bridge  for  the  crossing  of 
nerve  tracts  between  the  right  and  left  halves  of  the  telencephalon.  The 
development  of  these  commissural  tracts  will  be  mentioned  later  ;  in 
the  meantime  it  may  be  pointed  out  that  part  of  it  retains  almost  its 

TELENCEPHALON 

HMENCEPHALON 

METATHALAylUS 

MID    BRAIN 


OPTIC    TRACT  GUSTATOPY 


SENSORY     PATHS 
(MELD-  LCLMNISCUSj 


Fig.  98. — Diagram  of  the  EmTDryonic  Fore-Brain,  to  show  how  its  various  parts  become 
linked  to  sensory  tracts.     (Elliot  Smith.) 

embryonic  state  in  the  adult  and  forms  the  lamina  cinerea  which  closes 
the  anterior  wall  of  the  3rd  ventricle  between  the  oj)tic  chiasma  below 
and  the  corpus  callosum  above. 

Glands  arising  from  the  Walls  of  the  3rd  Ventricle. — We  have  seen  that 
the  roof  plate  of  the  3rd  ventricle  is  converted  into  a  secretory  structure 
— the  choroid  velum.  We  now  proceed  to  note  the  manner  in  which  two 
remarkable  glandular  bodies  arise  in  connection  with  the  3rd  ventricle — ■ 
the  pituitary  in  relationship  to  the  anterior  part  of  its  floor  and  the  pineal 
from  the  hindmost  part  of  its  roof.  We  must  suppose  that  their  functions 
are  closely  related  to  the  nutrition  and  welfare  of  the  nerve  system. 
The  position  and  connections  of  these  two  bodies  will  be  seen  in  Figs.  99, 
100.  A  sagittal  section  of  the  pituitary  body  of  a  foetus  of  the  5th  month 
is  drawn  in  Fig.  99,  showing  the  neural  part  derived  from  the  floor  of 
the  3rd  ventricle  and  the  buccal,  derived  from  the  ectoderm  of  the  primitive 
mouth.  In  the  buccal  part  the  central  cavity  divides  the  glandular  mass 
into  a  part  applied  to  the  neural  lobe  into  a  paraneural  or  intermediate  and 
a  larger  anterior  part  or  lobe.  Besides  these  tioo  parts  there  is  a  third, 
the  lateral  or  tuberal  part,  which  is  seen  on  section  in  Fig.  99,  applied  as  a 
plate  to  the  neck  of  the  infundibulum.  The  pineal  body  of  a  newly  born 
child  is  represented  in  Fig.  100  ;    it  is  about  the  size  of  a  wheat   grain, 


106 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


resting  on  the  roof  of  the  mid-brain,  between  the  superior  corpora  quadri- 
gemina.     On  each  side  are  seen  the  upper  surfaces  of  the  optic  thalami. 

Pituitary  Body. — As  is  so  often  the  case  in  the  development  of  the 
human  body,  procedures  which  take  place  obscurely  in  man,  present 
themselves  with  almost  diagrammatic  sharpness  in  low  vertebrates — 
particularly  in  selachians,  of  which  the  dog-fish  may  be  taken  as  a  type. 


lam.  term, 
optic,  cliiasma 
tuber  ciiier. 
infiindi'o. 


t~~neural  part. 


buccal  part. 


striae  term. 


[ent 
"horn. 

hab. 

i^uadrig. 


Fig.  99. — Section  of  the  Pituitary  Body  of  a  Human  Foetus  in  the  5th  month. 

(Edinger.) 
Fig.  100. — Showing  the  position  of  the  Pineal  Body  and  its  commissure  and  gangUon. 

The  original  saccular  form  of  the  pituitary  body  and  its  division  into  three 
parts  or  lobes  are  well  seen  in  the  pup  dog-fish  (Fig.  101).  The  original 
stalk  is  indicated,  and  the  three  parts  into  which  the  sac  becomes  divided 
by  the  growth  and  proliferation  of  the  epithelial  cells  in  its  walls  are  shown. 
The  lateral  or  tuberal  parts  arise  as  right  and  left  diverticula  near  the  root 
of  the  stalk.     The  tuberal  part,  as  a  distinct  element  of  the  pituitary 


INTER/MEDIATE 


ANTERIOR    PART 


Fig.  101.— Sagittal   Section   of  the   Pituitary   Body   of  a   Pup   Dog-Fish. 
(After  Baumgartner.) 

complex,  was  first  recognized  by  Dr.  Tilney  in  1913,^  but  since  then  its 
presence  has  been  noted  in  all  vertebrates,  including  man.  The  lateral 
or  tuberal  parts  as  they  expand  become  applied  to  the  infundibular  region 
of  the  floor  of  the  3rd  ventricle,  their  cells  invading  the  arachnoid  and 
occupying  its  meshes. 

1  For  recent  literature  relating  to  the  development  and  morphology  of  pituitary, 
see  Miss  K.  M.  Parker's  excellent  paper,  Journ.  Anat.  1917,  vol.  51,  p.  181  (pituitary 
of  Marsupials) ;  Prof.  J.  E.  S.  Frazer,  Lancet,  1916,  vol.  2,  p.  45  ;  Dr.  E.  Rudel, 
Anat.  Hefte,  1917,  vol.  55,  p.  187  (pituitary  of  Man) ;  E.  A.  Baumgartner,  Journ. 
Morph.  1915,  vol.  26,  p.  391  ;  vol.  28,  p.  209  ;  W.  J.  Atwell,  Anat.  Eec.  1918,  vol.  15, 
p.  73  ;  Prof.  P.  T.  Herring,  Journ.  Exper.  Physiol.  1908,  vol.  1,  p.  121. 


THE  FORE-BRAIN  OR  PROSENCEPHALON 


107 


By  the  end  of  the  4th  week  the  basis  of  the  buccal  part  of  the  pituitary 
can  be  recognized  in  the  roof  of  the  primitive  mouth  or  stomodaeuni 
(Fig.  102,  A),  just  in  front  of  the  oral  plate,  which  at  this  time  closes  the 
anterior  end  of  the  fore-gut.  The  stomodaeum  is  lined  with  ectoderm, 
and  it  is  therefore  an  ectodermal  evagination,  known  as  Rathke's  pocket, 
which  goes  to  form  the  buccal  part  of  the  pituitary.  It  will  be  noted 
that  the  ectodermal  element  is  closely  applied  to  the  floor  of  the  fore-brain 
from  the  start ;  in  the  5th  week  the  adjacent  part  of  the  neural  floor 
begins  to  grow  out,  and  becomes  the  infundibular  process.  One  other 
point  should  be  noted  ;  just  behind  the  upper  attachment  of  the  oral 
plate  the  entoderm  of  the  fore-gut  forms  a  slight  pocket.  Seessel  found 
that  in  some  animals  (birds)  this  pocket  also  took  part  in  the  formation 


STOMODAEUM 


LAM   TEflM 
SPHCNOIO 


STALK 
NASO-PHAft 


ORAL  PLATE. 
roREGUT 


SECSeUS  POCKB.T 

NOTOCHQRO 


3"*  VENT 

INFUNOIBULUM 


NEURAL  PART 
BUCCAL    PART 


BASAL   PLATE 


CA)   4'^^Week.  3-5  mm. 


(B)  7^^  Week,   /em  m. 


Fig.  102. — Development  of  the  Pituitary.    A,  its  condition  in  a  Human  Embryo 
4  weeks  old  ;   B,  in  an  Embryo  in  the  7th  week  of  development.     (Rudel.) 

of  the  pituitary,  and  hence  is  called  Seessel's  pocket— it,  however,  does  not 
share  in  the  production  of  the  human  pituitary. 

By  the  7th  week  marked  changes  have  occurred  (Fig.  102,  B).  The  in- 
fundibular process  (the  neural  part)  is  now  quite  evident ;  its  cavity  is 
still  open,  becoming  filled  up  in  the  9th  week.  The  buccal  evagination  has 
assumed  a  pocket  form — pressing  against  the  neural  process,  its  neck  having 
become  drawn  out  into  a  long  stalk,  because  the  base  of  the  skull  is  being 
laid  down  between  the  roof  of  the  pharynx  and  the  floor  of  the  fore-brain. 
The  nasal  and  buccal  cavities  are  being  developed,  the  buccal  end  of  the 
stalk  coming  ultimately  to  lie  at  the  posterior  border  of  the  nasal  septum. 
Usually  some  fragments  of  the  pituitary  stalk  persist  in  the  mucous  mem- 
brane on  the  roof  of  the  nasopharynx ;  cases  occur  in  which,  owing  to  a 
malformation  of  the  base  of  the  skull,  the  whole  pituitary  body  lies  in  the 
posterior  part  of  the  nasal  septum.  By  the  9th  week  the  stalk  has  dis- 
appeared, but  occasionally  a  canal  in  the  body  of  the  sphenoid  bone  of 
the  adult — the  cranio-pharyngeal  canal — marks  the  site  of  the  embryonic 
stalk. 

During  the  3rd  month  the  epithelial  lining  of  the  pituitary  sac  grows 
rapidly,  particularly  in  the  anterior  part  where  glandular  masses  encroach 


108      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

upon  the  lumen  (Fig.  103),  ultimately  obliterating  all  but  the  central  space 
between  the  anterior  and  intermediate  parts.  The  gland  encapsules  itself 
in  the  tissues  of  the  dura  mater,  branches  of  the  internal  carotids  and 
mesodermal  tissue  entering  the  glandular  masses  as  they  begin  to  pro- 
liferate into  the  central  cavity  (Fig.  103). 

Many  theories  have  been  framed  to  account  for  the  position  and  formation 
of  the  pituitary  in  the  floor  of  the  3rd  ventricle,  a  favourite  one  being  that 
it  had  been  formed  round  the  opening  or  mouth  of  the  central  canal  of  the 
nervous  system  when  that  canal  was  alimentary  in  nature.  It  seems  more 
probable,  judging  from  recent  observations  of  Gushing,  that  the  pituitary 
is  so  placed,  because  it  discharges  a  secretion  into  the  3rd  ventricle,  which 
circulates  in  the  cerebro-spinal  fluid.  Gaskell,  who  regarded  the  neural 
or  cerebro-spinal  canal  as  the  homologue  of  the  invertebrate  alimentary 
canal,  homologized  the  pituitary  evagination  of  the  buccal  ectoderm 
with  the  invertebrate  mouth  and  gullet,  and  the  pituitary  body  itself  with 


PIA-ARACH  : 
NEURAL  PAPT 
PABA  NEURAL 

OURAL  CAPSULE 
CENTRAL  CAVITY 
SUBCAP.   SPACE 


INTRACAPSULAR    SPACE 


Fig.  103. — Coronal  Section  of  the  Pituitary  Body  of  a  Human  Foetus  at  tlie  beginning 
of  the  4th  month  of  development.     The  section  is  across  the  anterior  lobe. 

the  coxal  glands  of  crustaceans.  The  pituitary  body  exercises  a  curious 
influence  on  the  growth  of  certain  parts,  especially  on  the  face  and  limbs. 
Disease  of  the  pituitary  body  may  lead  to  overgrowth  of  the  limbs,  as  in 
giants,  or  of  the  face,  as  is  seen  in  cases  of  acromegaly. 

Pineal  Body  or  Epiphysis.^ — In  recent  years  it  has  been  shown  that  both 
pituitary  and  pineal  bodies  secrete  substances  which  have  a  powerful 
influence  on  the  development  and  growth  of  tissues,  that  of  the  pineal 
being  more  especially  on  those  parts  which  are  correlated  with  sexual 
maturity.  The  situation  of  the  pineal  body  at  the  hinder  end  of  the  roof  of 
the  3rd  ventricle  is  shown  in  Fig.  100,  but  its  connections — especially  with 
the  posterior  commissure,  habenular  commissure  and  choroid  j)lexus — are 
better  seen  in  Fig.  91.  Originally  the  Pineal  organ  was  a  complex  structure, 
consisting,  as  is  shown  in  Fig.  104,  of  a  parietal  organ  or  eye,  the  organ 
being  socketed  in  the  sagittal  suture,  and  an  adjacent  glandular  structure 
opening  on  the  roof  of  the  3rd  ventricle.  In  mammals,  as  in  man,  only 
the  3rd  or  glandular  part,  nerve  nuclei  and  commissures  are  developed. 
In  fossil  reptiles  and  in  some  forms  still  living  it  forms  a  median  eye  which 

^  Papers  on  development  and  nature  of  the  pineal  body  are  :  Dr.  Knud  Krabbe, 
Anat.  Hefte,  1916,  vol.  54,  p.  191  ;  A.  Dendy,  Phil.  Trans.  1911,  Ser.  B,  vol,  201, 
p.  227  ;  J.  Warren,  Amer.  Journ,  Anat,  1911,  vol.  11,  p.  313. 


THE  FORE-BRAIN  OR  PROSENCEPHALON 


109 


perforates,  and  appears  on,  the  dorsum  of  the  head,  between  the  parietal 
bones.     It  differs  from  the  lateral  eyes  which  grow  from  the  third  ventricle 


PARIETAL     ORGAN 


EPIPHYSIS 


POST  :  comm: 


COM 
HABENL/UA 


Fig.  104. — The  Pineal  Gland  and  Sense  Organ  in  a  Lizard.    (Gaupp.) 

as  the  optic  vesicles  in  this,  that  it  produces  the  lens  as  well  as  the  retina 
and  optic  stalk.  The  retina  is  inverted — i.e.  the  apices  of  the  rods  and 
cones  point  towards  the  vitreous  chamber.     The  ganglion  of  the  habenula, 


ANT    LOBE 


Pin    body 


HABEN .  COMM  IS 


HABEN.  COMMIS. 


(A)    Z'^ MONTH  . 


(B)   61^^ month. 


'Em.  105. — Showing  stages  of  development  of  the  Pineal  Bodj^  in  the  roof  of  the 
Fore-Brain  :   A,  in  the  3rd  month  ;   B,  in  the  Cth  month.    (After  Krabbe.) 

situated  on  the  dorsal  and  inner  aspect  of  the  optic  thalamus,  appears  to 
represent  its  terminal  ganglion,  but  it  must  also  be  remembered  that  this 
ganglion  receives  the  striae  pinealis  which  arise  from  part  of  the  rhinen- 


no      HUMAN  EMBRYOLOaY  AND  MORPHOLOGY 

cephalon.     The  two  ganglia  become  connected  across  the  roof  plate  by  a 
commissure  (the  superior  or  habenular  commissure)  (Fig.  100). 

The  manner  in  which  the  pineal  body  arises  in  man  is  shown  in  Fig.  105. 
At  the  posterior  end  of  the  roof  of  the  fore-brain  the  ependymal  lining 
grows  out  as  a  pocket  in  the  6th  week  of  development.  The  evaginated 
cells  form  a  zone  for  cellular  production  (Fig.  105,  A),  as  we  have  seen  is 
the  case  everywhere  in  the  neural  tube,  but  in  this  instance  the  cells  pro- 
duced are  mainly  glandular  in  nature,  there  being,  however,  as  Dr.  Krabbe 
has  observed,  also  some  neuroglial  and  neuroblastic  elements.  From 
the  anterior  wall  of  the  pocket  a  mass  of  cells  separates  early  to  form  an 
anterior  lobe  (Fig.  105).  In  the  sixth  month  (Fig.  105,  B)  the  body  is 
assuming  its  final  form  ;  part  of  the  recess  or  pocket  has  become  closed  off 
in  the  distal  part  of  the  gland.  The  glandular  masses  are  invaded  by 
vascular  and  mesenchymal  tissue,  and  the  same  formation  of  interlacing 
columns  is  produced  as  is  seen  in  the  buccal  part  of  the  pituitary  or  cortical 
part  of  the  adrenals.  How  this  body  has  become  associated  with  the 
development  of  the  sexual  system  is  an  enigma. 


CHAPTER  X. 
THE  FORE-BRAIN  OR  PROSENCEPHALON  {continued). 

CEREBRAL  VESICLES. 

We  are  now  to  follow  the  development  of  the  organ  whicli  has  given  men 
the  domination  of  the  world — the  cerebrum  proper,  comprising  the  right  and 
left  cerebral  hemispheres.  Nothing  could  be  simpler  than  the  cerebral 
vesicles  at  the  end  of  the  1st  month  of  development ;  they  are  merely 
button-like  bulges  on  the  right  and  left  walls  of  the  fore-brain  (Fig.  96). 
Each  button-like  vesicle  may  be  demarcated  into  three  areas — a  relatively 
small  olfactory  area  in  front,  which  will  be  evaginated  to  form  the  olfactory 
vesicle,  afterwards  converted  into  the  olfactory  bulb  and  tract ;  a  striate 
area  in  which  that  great  basal  mass  of  nerve  nuclei,  known  as  the  corpus 
striatum,  will  be  developed ;  and  a  pallial  or  mantle  area  in  which  the 
cortical  centres,  which  make  up  the  great  mass  of  the  cerebral  hemispheres, 
are  produced.  In  each  vesicle  there  is  also  a  fourth  or  secretory  area, 
which,  however,  does  not  become  defined  until  the  middle  of  the 
2nd  month,  when  it  is  folded  within  the  cavity  of  the  vesicle  to  form  the 
glandular  covering  of  the  choroid  plexus  of  the  lateral  ventricle. 

It  is  important  to  note  the  manner  in  which  the  cerebral  vesicles  are 
connected  to  the  walls  of  the  3rd  ventricle  and  to  each  other.  At  wha;t 
may  be  called  its  posterior  border  each  vesicle  is  continuous  with  the  optic 
thalamus  (Figs.  96,  97)  ;  at  its  lower  border,  with  the  hypothalamic 
region.  At  both  of  these  borders  it  is  the  striate  area  which  joins  with 
the  thalamic  regions  ;  nerve  tracts  which  arise  in  the  nuclei  of  these  regions 
and  pass  to  the  mantle  areas  must  traverse  the  striate  zone  ;  there  is  no 
other  route.  Hence  the  corpus  striatum  becomes  the  bond  which  links 
each  cerebral  vesicle  to  the  thalamencephalon  ;  it  becomes  the  highway 
for  the  internal  capsule,  the  name  given  to  the  great  afferent  and  efferent 
nerve-tracts  which  link  the  lower  nerve  centres  to  the  cortex  and  the 
cortex  to  the  lower  centres. 

Having  thus  examined  the  connections  of  the  cerebral  vesicles  along 
their  posterior  and  inferior  borders,  we  now  turn  to  the  remaining  two 
— the  anterior  and  superior  borders  (Figs.  96,  97).  Along  the  anterior 
border  one  cerebral  vesicle  is  united  to  the  other  by  the  lamina  terminalis, 
which,  we  shall  see,  becomes  enormously  distorted  by  the  development 
within  it  of,  (1)  the  anterior  commissure,  (2)  the  hippocampal  commissure 
and  (3)  the  corpus  callosum  (see  Figs.  116,  117,  118).     Along  the  superior 

111 


112     HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

border  the  vesicles  are  united  by  a  roof  plate  ;  posteriorly  the  vesicular 
roof  plate  is  continuous  with  the  roof  plate  of  the  3rd  ventricle,  which  at 
the  6th  week  becomes  transformed  into  choroidal  ependyma  (Fig.  108). 
A  similar  change  affects  the  vesicular  roof  plate  and  a  neighbouring  area  of 
the  vesicular  wall  to  form  the  velum  interpositum.  The  vesicular  roof  plate 
lies  over  the  widely  gaping  orifice  of  the  cavity  of  the  cerebral  vesicle 
(Fig.  96).  By  the  end  of  the  6th  week  (Fig.  97)  the  expansion  of  the 
cerebral  vesicles  has  commenced ;  we  can  then  name  the  cavity  of  each 
vesicle — lateral  ventricle,  and  its  constricted  communication  with  the 
3rd  ventricle,  the  interventricular  opening  or  foramen  of  Monro. 

Expansion  of  the  Cerebral  Vesicles.— By  the  middle  of  the  second 
month  the  rapid  expansion  of  the  cerebral  vesicles  has  commenced,  as  may 
be  seen  in  Fig.  106.  Already  the  posterior  or  temporal  region  is  passing 
backwards  and  downwards  on  the  side  and  roof  of  the  thalamencephalon 
(Fig.  107)  ;   the  frontal  region  is  bulging  forwards  over  the  olfactory  bulb, 

C£flea«AL  v£S.       oeaip.PEaioN 

CUtUOATE  NUCLEUS 
FmONTAL.  REGION        \  I        ^^    TEMP.  REGION 

\^  \      ~_  iii/ij   li         .  .  " ^KOOr  PI.ATE 

FOR  ■  OF  MONRO      ^x^^g^^^^^^V^^^^^^  ^■^^^^ PINEAL. 

CORP    STRIAT 

OLF    VESICLE 


OP.   THAL. 
HY POT  HAL 


Fig.  106. — The  expansion  of  the  right  Cerebral  Vesicle  and  the  formation  of  the  Corpus 
Striatum  on  its  floor,  during  tlie  6th  week  of  development. 

while  the  roof  of  the  vesicle,  which  is  cut  away  in  Fig.  106  to  expose  the 
corpus  striatum  in  the  floor,  is  rising  up  so  that  between  the  right  and 
left  vesicles  there  now  exists  a  fissure — the  commencement  of  the  longi- 
tudinal fissure,  which  will  become  deeper  and  longer  as  the  vesicles  expand. 
We  have  already  seen  that  the  striate  area  of  each  vesicle  is  continuous 
with  the  thalamic  areas  of  the  fore-brain,  and  thus  as  the  corpus  striatum 
becomes  differentiated  it  is  continuous  with  the  optic  thalamus  (Fig.  106), 
and  hence  this  striate-thalamic  junction  may  be  looked  on  as  the  stalk 
or  hilum  from  which  the  cerebral  expansion  takes  place.  The  corpus 
striatum  occupies  the  floor  of  the  vesicle,  so  that  in  the  fully  formed  brain 
we  find  the  caudate  nucleus  stretching  along  the  lateral  ventricle  from  the 
foramen  of  Monro  to  the  end  of  the  descending  horn  which  represents  the 
posterior  or  caudal  pole  of  the  embryonic  brain. 

It  is  also  instructive  to  note  the  expansion  of  the  cerebral  vesicle  as  seen 
on  its  lateral  aspect  (Fig.  107).  At  the  6th  week  the  bean-shaped  vesicle 
still  leaves  the  greater  part  of  the  thalamencephalon  exposed  (Fig.  107)  ; 
it  then  shows  only  a  frontal  and  temporal  pole,  but  by  the  end  of  the  3rd 
month,  the  expansion  has  reached  the  mesencephalon,  and  now  there  has 
appeared  a  third  or  occipital  pole  (Fig.  107,  B)  ;  by  the  end  of  the  5th  month 
the  occipital  region  overlaps  and  covers  the  hind-brain.  On  embryo- 
logical  grounds  alone  one  could  infer  that  the  dominance  of  the  cerebrum 


THE  FORE-BRAIN 


113 


is  one  of  the  more  recent  products  of  evolution.  In  the  lateral  aspect  we 
again  see  how  the  corpus  striatum  forms  the  basis  or  fixed  area  from  which 
the  cerebral  expansion  is  produced.  The  three  primary  constituents  of  the 
cerebral  vesicle  are  indicated  in  Fig.  107,  A,  the  small  olfactory  area, 
the  large  mantle  formation  and  the  position  of  the  striate  element  at  the 
junction  of  these  two.  The  position  of  the  corpus  striatum  determines 
the  non-expansion  of  the  overlying  cortex — ^which  later  becomes  differ- 
entiated to  form  the  Island  of  Reil.  The  position  and  relationships  of  the 
islandic  region  towards  the  end  of  the  3rd  month  are  shown  in  Fig.  107,  B. 

The  Velum  Interpositum. — It  is  during  the  growth  backwards  of  the 
cerebral  hemispheres  over  the  thalamencephalon  that  the  basis  of  that 
complex  structure — the  velum  interpositum — is  formed.  The  basis  of  this 
structure  is  really  that  area  of  the  pia  mater — ^the  mesodermic  and  vascular 


CEREB    VES. 

POSI-riOH    OF    CORP    STRIAT. 

PIMLAL 


FOR.  OF  MONRO 

JNT     CAPSULE 

CHOROIDAL.   FIS 


OCCIP 
POLE 


OLFACr.  BULB 


OPTIC    STALK 


MESENCEPHALON 


FRONT  LCBE 

OLF.  BULB 


CORP.  STRIAT 
ISLANOIC     AREA 


MESENCEPH. 


(A)  e^^Week     J3-G  m.m 


f^RIFORM    AREA 

(B)  71'"'' Wee ^     53  m.m 


Fig.  107. — The  Expansion  of  the  left  Cerebral  Vesicle  as  seen  on  its  lateral  aspect. 
A,  at  the  6th  week  ;  B,  at  the  11th  week  ;  in  J5,  a  window  has  been  cut  to  expose 
the  lateral  ventricle,  the  corpus  striatum  and  the  choroidal  gap.     (After  His.) 


capsule  of  the  brain— which  is  enclosed  between  the  thalamencephalon  and 
expanding  cerebral  vesicles  (Fig.  108).  The  essential  parts  of  the  velum 
are  its  lateral  edges,  which  project  within  the  lateral  ventricles  and  its 
lower  surface  lying  over  the  third  ventricle — parts  which  are  covered  by 
reflections  of  those  areas  of  the  neural  tube  which  have  been  converted 
into  a  glandular  or  secretory  epithelium.  These  parts  form  the  choroidal 
villi — or  plexuses — covered  by  the  ependymal  epithelium,  which  secrete 
the  cerebro-spinal  fluid. 

We  have  seen  that  in  the  anterior  j)art  of  the  roof  plate  of  the  -Ith  ventricle 
the  cerebellum  is  developed,  while  its  posterior  half  becomes  the  inferior 
medullary  velum — a  secretory  membrane  (Fig.  86,  p.  91).  The  roof  plate 
of  the  third  ventricle,  from  the  foramina  of  Monro  backwards  to  the  stalk 
of  the  pineal  body,  becomes  modified  in  a  similar  manner  (Fig.  108).  It 
merely  forms  the  ependymal  covering  of  the  lower  surface  of  the  velum 
interpositum,  thus  clothing  the  choroid  plexus  on  the  roof  of  the  3rd  ventricle 
(Fig.  109).  The  anterior  part  of  the  roof  plate  is  produced  into  the  cerebral 
vesicles  at  the  foramina  of  Monro,  and  covers  the  apex  of  the  velum 


114 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


interpositum  (Fig.  108).  The  mesial  wall  of  eacli  cerebral  vesicle  from  the 
foramen"  of  Monro  back  to  the  posterior  extremity  of  the  vesicle  (Figs. 
108,  109),  which  becomes  the  tip  of  the  descending  horn,  is  also  inflected 
and  forms  a  secretory  ependyma,  covering  the  velmn  interpositum  and 
choroid  plexus  within  the  lateral  ventricles.  Into  this  inflection — the 
choroidal  fissure  of  the  embryonic  neural  wall — spreads  the  mesoderm, 
carrying  vessels  with  it.  The  velum  interpositum  is  thus  composed  of  a 
basis  of  mesoderm,  while  its  intraventricular  parts  have  an  ependymal 

.ant  horn 

olfact  depression 
Jor.  of  Monro 

caud.  nucleus 
choroid  plexus, 
choroidal  fissure 
velum  interposit. 

desc.  horn 

roof  of  3rd  vent, 
optic  thalamus 
thalamen  cephalon 


inflect  of 
mes.  wall. 


mid  brain 


Fig.  108. — A  dorsal  view  of  the  Fore-  and  Mid-Brain  at  the  6th  week  of  development 
to  show  the  formation  of  the  Velum  Interpositum.  The  Cerebral  Vesicles  are 
laid  open  and  the  inflection  of  the  roof  of  the  Fore-Brain  shown  on  the  ingrowing 
Velum.  The  Roof  Plate  of  the  3rd  Ventricle  is  also  exposed.  (Modified  from 
His.) 

covering  derived  from  the  neural  wall.  When  the  velum  interpositum  is 
withdrawn  from  the  foetal  brain  (Fig.  107,  B)  a  linear  opening  is  seen  extend- 
ing from  the  foramen  of  Monro  to  the  temporal  end  of  the  cerebral  vesicle. 

The  ependymal  covering  of  the  entire  velum  is  derived  from  : 

(1)  The  roof  j)late  of  the  3rd  ventricle  (lower  surface)  ; 

(2)^  The  roof  plate  of  the  foramina  of  Monro  ; 

(3)  An  inflection  of  the  mesial  wall  of  the  cerebral  vesicle. 

The  choroid  plexus,  which  merely  fringes  the  velum  in  the  adult,  com- 
pletely fills  the  cavities  of  the  embryonic  lateral  ventricles.     These  for  the 


THE  FORE-BRAIN 


115 


first  three  months  are  relatively  very  large  and  their  containing  walls  thin. 
The  velum  and  choroid  plexus  must  play  an  important  part  in  the  develop- 
ment of  the  cerebral  vesicle  in  the  early  period  of  growth.  The  spread 
of  the  vesicles  backwards  and  downwards  over  the  optic  thalami  obscures 
the  original  simple  relationship  of  the  velum  to  the  brain  ;  but,  when  with- 
drawn from  the  transverse  fissure,  the  velum  is  seen  to  rest  on  the  optic 
thalami  and  project  within  the  ventricle  from  the  foramen  of  Monro  to 
the  tip  of  the  descending  horn.  This  stretch  marks  the  line  at  which 
the  choroidal  inflection  took  place  (Fig.  109).  The  taenia  semicircularis, 
in  the  groove  between  the  optic  thalamus  and  caudate  nucleus  (Fig.  100), 
marks  the  line  at  which  the  mesial  wall  of  the  cerebral  vesicle  was  primarily 
attached. 

The  fibrous  substance  of  the  velum  interpositum  is  continuous  with  the 
pial  covering  of  the  brain,  and  also  with  the  edge  of  the  tentorium  cerebelli, 


long.  fis. 
lot.  uent 


Corp.  callosum 


Island  of 
Reil. 


claustrum 


uelum  interpositum 

mudate  nuc. 
int.  cap. 
/choroid  plex\ 
\^f .3rd  uent.) 

optic  thai. 

3rd  uent. 
■optic  chiasma 


Fia.  109.- 


-Diagrammatic  Section  across  tlie  3rd  and  lateral  Ventricles  of  tlie  Adult 
to  show  tlie  Structures  formed  in  their  Walls. 


for  as  the  cerebral  vesicles  expand  not  only  do  they  evaginate  their  proper 
mesodermal  covering,  the  pia-arachnoid,  but  also  the  inner  or  dural 
stratum  of  the  primitive  cranial  capsule.  The  corpus  callosum  and 
cerebral  vesicles,  as  they  develop,  grow  backwards  and  enclose,  between 
the  optic  thalami  below  and  the  pillars  of  the  fornix  above,  the  fibrous 
basis  of  the  velum  interpositum  (Fig.  109).  The  veins  of  Galen  are  de- 
veloped in  the  velum  and  join  the  straight  sinus  in  the  tentorium. 

Corpus  Striatum. — When  a  coronal  section  is  made  of  the  adult  brain 
(Fig.  109)  to  exjDose  the  connections  of  the  velum  interpositum,  it  is  clear 
that  a  mere  overgrowth  of  the  cerebral  vesicles  will  not  account  for  all  the 
relationships  shown.  We  shall  see  that  the  development  of  the  com- 
missures— particularly  of  the  fornix  and  corpus  callosum — introduces 
elements  not  seen  in  the  simple  brain  of  the  embryo  ;  but,  besides  the 
commissural,  another  change  has  come  about  in  the  relationship  of  the 
corpus  striatum  to  the  optic  thalamus.     So  intimate  and  extensive  has 


116 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


the  union  between  them  become  that  the  corpus  striatum  now  forms  a 
cap  upon  the  lateral  aspect  of  the  optic  thalamus  (Fig.  109).  Although 
developed  in  the  wall  of  the  cerebral  vesicle  the  lenticular  nucleus  of  the 
corpus  striatum,  and  the  islandic  cortex  are  now  constituents  of  the  lateral 
wall  of  the  3rd  ventricle — the  cavity  of  the  original  thalamencephalon 
(Fig.  109).  The  intimate  union  of  the  corpus  striatum  with  the  optic 
thalamus  we  must  regard  as  the  result  of  two  developmental  processes  : 
the  formation  of  the  nerve  tracts  of  the  internal  capsule — ^which  begin  to 
appear  in  the  3rd  month — and  the  neurobiotactic  attraction  which  exists 
between  the  neuroblastic  centres  of  the  two  great  basal  ganglia. 

Two  figures  (Figs.  110,  A,  B)  taken  from  a  recent  paper  by  Professor 
Elliot  Smith  ^  throw  a  new  light  on  the  nature  and  origin  of  that  complex 


CAT  VENT. 


CAUDATE  NUC. 


LESITIC.HUC. 


SITE   or   NEOPALLIUM 


PYRirORM  LOSE 
CLAUSTRUM 
LAT   STRIATE  ART. 
INT    CAPSULE. 


Pig.  110,  A, — Sagittal  Section  of  Turtle's  Cerebral  Vesicle  made  along  the  mesial 
plane  so  as  to  expose  the  cavity  of  the  vesicle — the  lateral  ventricle. 
(Elliot  Smith.) 

riG.  110,  B. — Coronal  Section  of  a  Primitive  Mammalian  Cerebrum,  made  across  its 
anterior  part,  in  front  of  the  3rd  ventricle,  to  show  the  origin  of  the 
constituents  of  the  Corpus  Striatum.     (Elliot  Smith.) 

product  of  the  cerebral  vesicle— the  corpus  striatum.  When  the  cerebral 
hemisphere  of  a  turtle's  brain  is  laid  open,  as  in  Fig.  110,  A,  we  see  the 
same  three  elementary  parts  as  in  the  cerebral  vesicle  of  the  human  em- 
bryo :  (1)  the  hollow  olfactory  bulb,  containing  an  extension  of  the  ventri- 
cular cavity,  (2)  a  mass  occupying  the  floor  and  lateral  wall  representing 
the  corpus  striatum,  (3)  the  vesicular  wall,  mantle  or  pallium.  We  see  that 
the  basal  mass  is  made  up  of  three  projections,  (1)  the  hypopallium 
representing  for  the  greater  part  the  caudate  nucleus  ;  (2)  at  its  hinder  or 
temporal  end  the  amygdaloid  nucleus  ;  (3)  the  oldest  part  of  all  lying  over 
the  olfactory  bulb  (Fig.  110,  A),  and  continuous  with  the  hypothalamic 
region — the  palaeostriate  body.  From  Fig.  110,  A,  it  is  clear  that  the 
corpus  striatum  has  a  close  connection  with  the  olfactory  region  of^the 
cerebral  vesicle.     The  coronal  section  in  Fig.  110,  B  shows  the  relationship 

1  Journ.  of  Anat.  1919,  vol.  53,  p.  271. 


THE  FORE-BRAIN  117 

of  the  corpus  striatum  to  the  remaining  parts  of  the  wall  of  the  cerebral 
vesicle  of  a  primitive  mammal — the  type  of  organ  from  which  the  human 
cerebrum  has  been  evolved.  An  artery — ^the  lateral  striate  ^ — one  of  the 
perforating  branches  of  the  anterior  cerebral,  is  seen  to  enter  the  wall  of  the 
brain  between  the  pyriform  lobe  above  and  the  olfactory  tubercle  below 
and  end  in  the  corpus  striatum.  At  the  point  where  the  artery  enters,  the 
cortex  of  the  pyriform  lobe  is  also  inflected,  although  no  fissure  is  present, 
and  forms  a  stratum — the  claustrum — on  the  outer  aspect  of  the  basal 
mass  (Fig.  109).  The  cortical  stratum,  after  forming  the  claustrum,  bends 
inwards  to  become  continuous  with  the  nuclei  of  the  corpus  striatum. 
On  the  mesial  wall  of  the  vesicle  (Fig.  110,  B)  the  hippocampus  arises  in  a 
somewhat  similar  way — by  an  inward  growth  of  a  cortical  stratum— with- 
out the  production  of  an  open  fissure.  Elliot  Smith,  therefore,  regards 
the  corpus  striatum  as  a  cortical  derivative  ;  growth  has  taken  place 
towards  the  cavity  of  the  ventricle  tending  to  fill  up  the  cavity  with  a 
cortical  product.  In  the  bird's  brain  the  cortical  growth  is  chiefly  intra- 
ventricular. 

The  Mantle  or  Pallium. — We  have  followed  the  fate  of  the  striate  area 
of  the  embryonic  cerebral  vesicle  ;  the  differentiation  of  the  olfactory 
area  will  be  dealt  with  when  we  come  to  consider  the  nose  and  sense  of 
smell ;  there  remains  for  consideration  the  third  or  paUial  area  of  the 
cerebral  vesicle.  Even  up  to  the  end  of  the  3rd  month  the  pallial  wall  of 
the  vesicle  remains  thin  ;  it  then  measures  only  about  one  millimetre  in 
thickness.  Originally  the  pallial  wall  shows  the  same  three  strata  or 
zones  as  were  seen  in  other  parts  of  the  neural  tube — namely  an  inner  or 
ependymal  zone,  in  which  neuroblasts  are  produced  ;  a  middle  or  mantle 
zone  in  which  they  are  differentiated  and  an  outer  or  marginal  zone  (Fig. 
111).     In  the  spinal  cord  the  masses  of  neuroblasts  were  differentiated 


NEUROBLASTS 


EPENDYMAU  MIDDU£        OR 

■ZONE.  MANTLE  ZONE 

Fig.  111. — Diagram  to  show  the  differentiation  of  the  Pallial  Wall  of  the  Cerebral 
Vesicle.    (After  His.) 

within  the  middle  zone,  where  they  remained,  but  in  the  cerebellum — and 
the  same  is  true  of  the  pallial  wall — they  invade  the  marginal  zone.  It 
is  within  the  neuroglial  scaffolding  of  the  marginal  zone  that  the  grey 
cortical  matter  of  the  cerebral  hemispheres  is  formed.  In  the  2nd  month 
the  migration  of  the  neuroblasts  to  form  a  cortical  layer  has  already  com- 
menced ;  the  process  is  particularly  active  in  the  3rd  and  4th  months. 
Not  only  is  there  a  migration  from  the  ependymal  to  the  marginal  layer,  but 
the  production  is  particularly  abundant  where  the  mantle  joins  the  corpus 

1  See  paper  by  Col,  J.  L,  Sbellshear,  Journ.  of  Anat.  1921,  vol.  55,  p,  27, 


118      HUMAN  EMBRYOLOaY  AND  MORPHOLOaY 

striatum.  The  middle  zone,  wMch  contains  grey  matter  in  the  spinal 
cord,  is  here  the  highway  for  the  fibres  developed  from  the  cortical  cells ; 
it  forms  the  white  medullary  mass  of  the  cerebral  hemispheres. 

Evolution  of  the  Neopallium.^ — Nothing  could  be  more  humble  than 
the  origin  of  man's  master  organ  ;  it  was  evolved  in  connection  with  the 
sense  of  smell.  The  cerebral  hemispheres,  as  we  know  them  in  the  lowest 
vertebrates,  are  for  the  reception  and  interpretation  of  impulses  from  the 
olfactory  end  organs.  Connections  are  established  between  the  olfactory 
brain  and  the  motor  centres  in  the  cord  and  in  the  hind-  and  mid-brain ; 
olfactory  impressions  can  thus  lead  to  action.  Further,  it  became  advan- 
tageous that  there  should  be  a  nervous  mechanism  for  the  blending  of 
impressions  from  the  nose  with  impulses  derived  from  sight,  hearing  and 
touch,  and  hence  there  arose  connecting  tracts  by  which  stimuli  streaming 
in  from  the  various  senses  could  be  combined  and  their  reactions  co- 
ordinated with  those  streaming  in  from  the  nose.  In  the  stem  of  verte- 
brates which  became  mammalian  the  supreme  co-ordinating  mechanism 

neopallium- — ^...: .^.     ,.  . 

lat.  vent-    ..4^1111  H'"".*";-  f°T^"" 

mesial  wall 


later,  wall 
corp,  stn'at: 


Yparaterm  body. 


Fig.  112. — Section  across  the  Left  Hemisphere  of  the  Brain  of  a  primitive  vertebrate 
brain  anterior  to  the  Lamina  Terminalis,  to  show  the  small  extent  of  the 
Neopallium  and  tlie  relatively  great  development  of  the  Corpus  Striatum  and 
Bhinencephalon.     (After  Elliot  Smith.) 

was  evolved  in  that  part  of  the  neural  system  connected  with  smell — 
the  telencephalon. 

In  Fig.  112  is  represented  a  diagrammatic  section  across  the  anterior 
part  of  the  cerebral  vesicle  of  one  of  the  lower  vertebrate  types — such  a 
one  as  we  may  suppose  preceded  the  modern  mammalian  form  of  cerebral 
hemisphere.  There  is  a  cavity  within  it — the  lateral  ventricle.  The 
inner  or  mesial  wall  is  formed  of  two  parts  :  (1)  the  hippocampus  or  hippo- 
campal  formation — true  cerebral  cortex  or  mantle  ;  (2)  below  the  hippo- 
campus, the  paraterminal  body — a  nuclear  mass  connected  with  the 
hippocampal  formation  by  nerve  tracts.  The  lateral  or  outer  wall  of  the 
primitive  hemisphere  is  made  up  of  two  parts — the  corpus  striatum — a 
nuclear  mass  partly  covered  by  the  cortex  of  the  pyriform  lobe  (see  Fig. 
110,  B).  The  pyriform  lobe  receives  fibres  from  the  outer  root  of  the 
olfactory  tract.  These  four  parts — hippocampus,  paraterminal  body, 
pyriform  lobe,  corpus  striatum — are  connected  with  smell,  and  form  the 
primitive  mantle  (archipallium)  of  the  brain.  In  the  roof  of  the  ventricle 
an  expansion  of  the  mantle  appears  between  the  hippocampal  formation 
on  the  inner  side  and  the  pyriform  lobe  on  the  outer  side  (Fig.  117)  ;   to 

1  See  Prof.  Elliot  Smith's  "Arris  and  Gale  Lectures,"  Lancet,  1910,  Jan.  1st,  15th, 
22nd. 


THE  FORE-BRAIN 


119 


this  expansion  Elliot  Smith,  whose  account  is  followed  here,  gave  the  name 
of  neopallium.  It  is  this  new  mantle  which  becomes  the  basis  for  the 
higher  combination  of  the  sensory  impressions  coming  in  from  all  the  organs 
of  sense.  It  becomes  the  seat  of  consciousness  and  memory,  and  in  man 
assumes  enormous  proportions  ;  hence  the  great  and  rapid  expansion  of  the 
cerebral  vesicles  in  the  human  foetus. 

As  may  be  seen  from  Fig.  117  the  primitive  mantle— all  the  cortical 
formation  directly  connected  with  the  sense  of  smell — is  arranged  around 
the  peduncular  attachment,  which  may  be  described  as  the  cerebral 
hilum.  In  Fig.  118  is  shown  how  greatly  the  distribution  of  the  primitive 
mantle  is  altered  when  the  great  commissures  become  developed. 

Projection  Fibres  to  the  Neopallium. — A  transverse  section  of  a 
mammalian  brain  of  a  primitive  type— made  further  back  and  in  a  more 
advanced  stage  of  development  than  that  represented  in  Fig.  112 — is  shown 


U^T:   VELNTRlCLE 
HIPPOCAMPUS 


OPTIC    NUCLEUS 


AUDITORY     NUCLEUS 


OPTIC    THALAMUS 


RHINAL  FISSURE.  - 
LENTICULAR  NUCLE-US 
PYRIFQRM     LOBE. 


Fig.  113.— Coronal  section  of  the  right  half  of  the  Cerebral  Vesicle  of  a  Primitive  Type 
of  Mammal,  showing  the  termination  of  projection  fibres  arising  in  the  optic 
thalamus,  in  the  neopallium.    (Elliot  Smith.) 

in  Fig.  113.  The  section  illustrates  the  manner  in  which  projection  fibres 
arise  from  two  of  the  sensory  nuclei  in  the  optic  thalamus— those  con- 
nected with  the  nerves  of  sight  and  of  hearing — and  spread  outwards  into 
the  neopallium — each  set  streaming  into  the  area  which  lies  nearest  to  it. 
In  this  way  the  mantle  of  the  telencephalon  becomes  a  higher  sensorium  for 
the  reception,  blending  and  storing  of  all  sensory  impressions.  Other 
illustrations  of  the  cortical  afferent  tracts  are  given  in  Figs.  98  and  110,  B. 

Localization  of  Function  in  the  Neopallium. — In  Fig.  114  the  brain 
of  a  primitive  mammal  is  represented  on  its  lateral  aspect.  The  major 
part  is  seen  to  be  made  up  of  pyriform  lobe,  olfactory  bulb  and  tubercle, 
all  of  them  parts  of  the  rhinencephalon.  The  rhinal  fissure  marks  the 
junction  of  the  neopallium  with  the  older  parts  of  the  mantle  on  the  outer 
or  lateral  aspect  of  the  hemisphere.  The  areas  adjacent  to  the  various 
nuclei  of  the  optic  thalamus  receive  projection  fibres  from  these  nuclei. 
Thus  it  comes  about  that  the  lower  and  most  posterior  part  of  the  neo- 
pallium, which  forms  the  basis  of  the  temporal  lobe,  receives  fibres  from 


120 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


the  auditory  centre  ;  in  tlie  upper  posterior  part  the  fibres  from  the  optic 
nuclei  end  ;  this  area  becomes  the  main  part  of  the  occipital  lobe.  An- 
terior to  these  two  areas  terminate  the  projection  fibres  connected  with 
the  sensory  nuclei  of  the  Vth  nerve  and  with  the  nuclei  of  common  sensation 
— receiving  impulses  from  the  leg,  trunk,   arm  and  head.     Hence  the 


CONTROL 
ARE.A 


OLFACT;  BULB 

OLFACT:  TRACT 


AUDITORY 
HINAL  FISSURE 


Pyriform    lobe 


OLFACT. TUBERCLE 

Fig.  114. — Lateral  Aspect  of  the  Cerebrum  of  a  Primitive  Mammal  to  show  the  Ehinal 
Fissure  which  separates  the  Neopallium  above,  from  the  older  parts  of  the  mantle 
below,  represented  by  the  Pyriform  Lobe.  The  areas  of  the  Neopallium  in  which 
the  projection  tracts  from  the  optic  thalami  terminate  are  also  sho'mi.  (Elliot 
Smith.) 

surface  areas  of  the  body  are  represented  in  the  neopallium.  Naturally  it 
is  in  connection  with  this  area — the  area  of  common  sensation — that  the 
cortical  fibres  which  control  the  lower  somatic  motor  nuclei  arise.  Anterior 
to  the  motor  areas — occupying  the  region  of  the  frontal  pole — is  an  area 
connected  with  the  control  of  the  higher  centres.     These  are  the  primary 

corpus  callosum 

fiippoc.  com.  , 

^  ,    \^L. supmcal.  our. 

cailosO'marg.  /s  ^    ^-"^        ^  ^ 

parieto-occip.fis. 
refro-ca'car.  Jis. 

/'—^ — calcar.  fis. 
/y^collaf.  fis. 
-fimbria  (fornix) 

uncus...  /x^     W^^^  clentatus 
liippoc.  fissure 
crus  cerebri 

Fig.  115. — The  Anterior,  Hippocampal  and  Callosal  Commissures,  with  the  primary 
fissures  on  the  Mesial  Aspect  of  a  typical  Mammalian  Cerebrum.    (Elliot  Smith.) 

areas  of  the  neopalhuni.  In  the  course  of  evolution,  secondary  or 
associated  zones  have  appeared  round  the  primary  areas,  separating 
them  widely  and  giving  rise  to  the  great  mass  of  the  human  cerebrum. 

Development  of  Cerebral  Commissures.— In  order  to  secure  a  co- 
ordinated action  of  the  whole  brain,  it  is  necessary  not  only  that  the 
cerebral  centres  of  each  hemisphere  should  be  linked  up  by  association 
p,nd  projection  fibres,  but  that  the  centres  of  one  hemisphere  should  b§ 


paraterm.  body 
(sept  luc. 
subcaL  gijr.) 


ant.  com 


lam.  term. 


THE  FORE-BRAIN  121 

united  by  transverse  or  commissural  tracts  with  the  corresponding  centres 
of  the  other  hemisphere.  The  lamina  terminalis  (see  Figs.  97,  116,  117) 
affords  a  natural  bridge  for  the  formation  and  passage  of  the  commissures. 
In  the  most  primitive  vertebrates,  in  all  of  which  the  cerebral  hemispheres 
are  chiefly  olfactory  in  nature,  the  anterior  commissure  is  already  present. 
The  next  to  appear  is  a  dorsal  or  hippocampal  commissure  which  unites  the 
hippocampal  areas  on  the  mesial  surfaces  of  the  cerebrum  (Figs.  115,  117). 
The  last  and  greatest  to  be  formed  is  the  corpus  callosum  ;  it  appears  in 
the  true  mammals— not  in  the  monotr ernes  and  marsupials.  Its  de- 
velopment is  commensurate  with  the  size  of  the  neopallium ;  hence  it  is 
largest  in  man. 

The  cerebral  hemispheres  are  thus  connected  by  fibres  which  cross  in 
the  lamina  terminalis,  and  form  three  commissures.     (1)  The  anterior  or 

hipppc.com. 

johor.  fis. 

^hab.  com. 
Corp.  caf.. 

ant  com..-^ST     ^^rn^iw^^  Y^^post  com. 
lam.  term. v— .^^  ^mrn^       ^^^^  ,yf,j^, 

olf.bulb ;7^/^    ^^cerebe/lum 

0  en.    / 

8rd  uent 


Fig.  116. — Mesial  Aspect  of  the  Brain  of  a  Human  Foetus  in  4th  month  of  develop- 
ment, shomng  the  Lamina  Terminalis  and  positions  at  which  Commissures  are 
formed.    (After  Goldstein.) 

ventral  commissure,  which  connects  the  olfactory  tracts,  and  afterwards 
parts  of  the  temporal  lobes  ;  (2)  the  dorsal  or  hippocampal  commissure 
also  formed  in  the  lamina  terminalis  ;  in  man  this  commissure  becomes 
the  fornix  ;  (3)  the  corpus  callosum,  which  unites  the  neopallium  of  one 
side  with  that  of  the  other.  It  is  formed  in  the  lamina  terminalis  above 
the  dorsal  or  hippocampal  commissure  (Figs.  114,  115,  116).  The  middle 
or  grey  commissure  (Fig.  100)  is  merely  an  adhesion  between  the  ependymal 
coverings  of  the  optic  thalami ;  the  optic  chiasma  (p.  207),  the  habenular 
or  superior  commissures  (p.  110)  need  only  be  again  mentioned.  The 
posterior  commissure  is  formed  in  the  roof  plate  at  the  junction  of  the  mid- 
and  fore-brains  (Figs.  94,  100). 

(1)  The  Anterior  Commissure  (Figs.  116,  117)  is  developed  in  the  lamina 
terminalis — the  primitive  anterior  wall  of  the  fore-brain.  The  com- 
missure crosses  in  the  lamina  terminalis  below  and  rather  anterior  to  the 
foramen  of  Monro. 

(2)  Hippocampal  Commissure. — Four  parts  are  recognized  in  the 
fprnix  of  the  human  brain  (Fig.  118) :  (1)  the  body,  adherent  to  the  undep 


122 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


surface  of  the  corpus  callosum  ;  (2)  the  posterior  pillars,  which  are  con- 
tinuous with  (3)  the  fimbriae  and  fibres  of  the  alveus,  covering  the  ventricular 
aspect  of  the  hippocampus  ;  (4)  the  anterior  pillars  which  end  in  the 
corpora  mammilaria  and  optic  thalami.  The  fornix  contains  two  systems 
of  fibres  :   (1)  those  which  cross  in  the  body  and  connect  the  hippocampal 


HIPPO.  COM. 
FOeiNIX 

ANT  COM. 
PARATERM   AREA 
OLFACT.    BULB 


F£OUNC.  ATTACH. 
PYRIFORM  AREA 


Fig.  117. — Mesial  aspect  of  the  Cerebral  Vesicle  of  a  Foetus  about  3  months  old,  show- 
ing the  commissures  developing  in  the  lamina  terminalis  and  the  distribution 
of  the  cortical  areas  which  belong  to  the  rhinencephalon.     (After  Streeter.) 

cortex  of  one  side  with  that  of  the  other,  and  form  the  true  dorsal  or  hippo- 
campal commissure,  (2)  fibres  which  connect  together  the  various  parts  of 
the  rhinencephalon  of  the  same  side,  and  with  the  corpora  mammilaria 
and  optic  thalami. 

To  understand  the  development  of  this  system  it  is  necessary  to  obtain 
a  clear  conception  of  the  relationships  of  the  lamina  terminalis  to  the 


^ippOCAMP^RUOi^e^/-, 


CORP.  CALL 

FOR    or  MONRO 
ANT.   COM 
PARATERM    AREA 


FORNIX 

.      HIPPOCAMP.  COM 


LAT    ROOT 


Choroidal  gap 

pedunc  attach, 
hippoc.  line 

fascia  dentata 
Fimbria 


A.MYQ    NUCLEUS 


RHINAL    FIS. 


PYRirORNI   AREA 


Fig.  118. — Diagram  to  show  the  structures  formed  in  the  Lamina  Terminalis  and 
Primitive  Callosal  Gyrus.     (After  Elliot  Smith.) 


various  parts  which  have  been  distinguished  in  the  rhinencephalon  (Figs. 
115,  117).  On  each  side  the  lamina  terminalis  is  continuous  with  the  para- 
terminal  body — that  part  of  the  rhinencephalon  which  lies  immediately 
in  front  of  the  lamina  terminalis.  The  paraterminal  body  becomes  the 
subcallosal  gyrus  and  septum  lucidum  in  the  mature  brain  (Fig.  118). 
The  hippocampal  formation,  which  includes  the  hippocampus  and  fascia 
dentata,   bounds  the  choroidal  fissure   above  (Figs.    117,    118).     Fibres 


THE  FORE-BRAIN  123 

developed  in  the  hippocampal  formation  cross  to  the  opposite  side  in  the 
lamina  terminalis  above  the  anterior  or  ventral  commissure,  thus  forming 
the  dorsal  commissure  (Fig.  116).  It  becomes  included  in  the  body  of  the 
fornix.  The  posterior  pillar  is  developed  in  the  hippocampal  cortex, 
which  forms  the  lip  of  the  choroidal  fissure.  The  anterior  pillar  lies  in 
the  paraterminal  body  and  lamina  terminalis. 

(3)  Corpus  Callosum. — The  corpus  callosum  is  the  commissure  of  the 
neopallium,  and  hence  in  man,  in  whom  the  neopallium  forms  by  far  the 
greatest  part  of  the  cerebrum,  this  commissure  attains  an  enormous 
development.  The  commissural  fibres  begin  to  form  towards  the  end  of 
the  third  month,  crossing  in  the  lamina  terminalis  with  the  fibres  of  the 
hippocampal  commissure,  but  situated  on  their  upper  or  dorsal  aspect 
(Figs.  116,  117).  As  the  corpus  callosum  rapidly  increases  within  the 
lamina  terminalis,  it  presses  backwards  on  the  hippocampal  formation, 
and  forwards  on  the  paraterminal  body.  The  hippocampal  commissure 
is  stretched,  and  forms  the  body  and  anterior  pillars  of  the  fornix.  The 
hippocampal  formation  becomes  (1)  the  supra-callosal  gyrus,  the  hippo- 
campus and  fascia  dentata  (compare  Figs.  117  and  118).  The  velum 
interpositum  is  overwhelmed  and  buried  during  the  growth  backwards 
of  the  corpus  callosum  and  fornix.  The  paraterminal  body  is  stretched 
to  form  the  septum  lucidum  and  subcallosal  gyrus  (Fig.  118).  Thus  by 
the  development  of  the  corpus  callosum  those  two  parts  of  the  rhinen- 
cephalon — the  paraterminal  body  and  hippocampal  formation — originally 
in  close  union,  become  widely  separated.  The  supra-callosal  and  sub- 
callosal gyri  are  vestiges  of  their  former  union.  The  corpus  callosum 
may  not  be  developed — a  rare  occurrence  ;  it  is  remarkable  that  this 
condition  cannot  be  detected  during  the  life  of  the  individual.^ 

Formation  of  Fissures. — Until  the  5th  month  the  surface  of  the  cerebral 
vesicle  is  comparatively  smooth.  Up  till  then  the  three  strata  of  the 
cerebral  vesicle,  the  ependymal  layer  within,  the  cortical  or  nerve-cell 
layer  on  the  surface  and  the  medullary  or  nerve-fibre  layer  between,  have 
increased  at  an  equal  rate.  In  the  6th  and  7th  months  certain  areas  of 
the  cortex  increase  rapidly,  the  increase  affecting  the  superficial  area  to  a 
very  much  greater  extent  than  the  deep,  and  affecting  the  cortex  much 
more  than  the  medulla,  with  the  result  that  the  surface  of  the  cerebrum 
becomes  raised  into  certain  definite  eminences  or  gyri,  separated  by  de- 
pressions or  fissures.  The  chief  fissures  are  already  well  differentiated  in 
the  foetus  of  the  7th  month  ;  during  the  last  two  months  of  intra-uterine 
development  the  secondary  and  tertiary  sulci  appear.  The  process  of 
fissuration  and  convolution-formation  are  thus  practically  finished  at 
birth.  In  the  spinal  cord  the  tracts  of  nerve  fibres  are  formed  outside  the 
masses  of  grey  matter  ;  in  the  cerebral  vesicle  the  tracts  are  formed  beneath 
the  grey  matter — between  the  grey  matter  and  the  ependyma  (see  p.  80). 
The  neuroblasts  in  the  cortex  have  reached  nearly  their  full  number  by  the 
7th  month  ;  after  then  it  is  their  dendrites  and  collateral  fibres  that  con- 
tinue to  develop  (His). 

1  See  cases  described  by  Elliot  Smith  and  by  Cameron,  J.  Anat.  and  Physiol.  1907, 
vol.  41,  pp.  234,  293. 


124     HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

Development  o£  the  Cortex. — The  mantle  or  wall  of  the  cerebral 
vesicle  of  the  brain  becomes  differentiated  into  a  thin  outer  grey  layer  or 
cortex,  containing  the  nerve  cells,  and  an  inner  deep  stratum — the  medulla 
■ — of  great  thickness  and  made  up  of  nerve  fibres  and  tracts  associated  with 
the  nerve  cells  of  the  cortex.  The  cortex  is  the  substratum  of  conscious- 
ness, memory  and  mind.  We  naturally  expect  the  great  mental  develop- 
ment which  takes  place  in  the  earlier  years  of  life  to  be  accompanied  by  a 
corresponding  change  in  the  microscopic  structure  of  the  cortex.  There 
is  such  a  change,  but  it  is  difficult  to  measure  for  two  reasons  :  (1)  every 
area  of  cortex  has  its  own  peculiar  structure  and  thickness  ;  Elliot  Smith  ^ 
has  distinguished  28  areas  in  the  cortex,  each  having  its  own  peculiar 
structure  ;  (2)  Dr.  Joseph  Bolton  ^  observed  that  in  some  cases  a  newly 
born  child  may  show  a  more  mature  development  than  a  child  of  3  months, 
there  being  as  much  variation  in  structure  of  cortex  as  in  degree  of  ability. 
The  latter  observer  noted  that  the  cortex  began  to  laminate  or  divide 
into  three  strata  of  nerve  cells  at  the  beginning  of  the  6th  month,  when  the 
fissures  and  convolutions  are  in  process  of  formation.  He  also  made  the 
important  observation  that  the  outer  or  pyramidal  stratum  was  the  latest 
in  growth,  and  that  the  great  development  of  this  layer  is  the  characteristic 
of  the  human  cortex. 

The  Principal  Fissures.^ — The  principal  fissures  of  the  brain  are  :  (1) 
those  connected  with  the  rhinencephalon — the  rhinal  fissure  (Figs.  114, 
118)  and  the  pseudo-hippocampal  fissure — a  mere  linear  depression  (Elliot 
Smith)  ;  (2)  those  connected  with  the  isolation  of  the  Island  of  Reil — 
the  fissure  of  Sylvius,  the  superior,  inferior  and  anterior  limiting  fissures  ; 
(3)  those  in  the  occipital  cortex  connected  with  the  sense  of  sight — the 
calcarine,  retro-calcarine,  lunate  sulcus  (Affenspalte)  parieto-occipital  and 
collateral,  (4)  the  callo  so -marginal  of  uncertain  import,  (5)  the  fissure  of 
Rolando,  which  is  formed  between  motor  and  sensory  areas  of  the  cortex, 
(6)  the  orbital,  (7)  the  sulcus  rectus,  (8)  the  intra-parietal,  (9)  the  1st  tem- 
poral or  parallel,  which  partially  demarcates  the  auditory  cortex.  In  the 
7th  month  the  fissures  on  the  human  brain  have  a  remarkable  correspond- 
ence to  those  on  the  cerebrum  of  an  ape  (Figs.  121, 123).  We  have  already 
seen  that  the  so-called  choroidal  fissure  is  formed  by  an  inflection  of  the 
vesicular  wall  to  form  the  choroidal  villi  of  the  lateral  ventricles  (Fig.  108). 

Significance  o£  Convolutions. — There  is  some  circumstance  which 
limits  the  thickness  of  the  cortex.  If  the  cortical  cells  increase  in  number, 
accommodation  is  obtained,  not  by  adding  to  the  thickness  of  the  cortex, 
but  by  enlarging  the  superficial  area  of  the  cerebrum.  The  cortex  is 
correlated  in  its  extent  with  the  bulk  of  the  body  and  with  the  area  of  the 
integumentary  covering.  Hence  large  animals  such  as  whales  and  ele- 
phants have  much  convoluted  brains.  The  rich  convolutions  of  man's 
brain  may  be  in  some  degree  related  to  the  nude  and  sensitive  skin  of  his 
body  (Elliot  Smith).  The  most  satisfactory  explanation  of  the  number 
and  arrangement  of  the  convolutions  of  the  human  brain  is  to  be  found  in 

1  Prof.  Elliot  Smith,  Journ.  of  Anat.  and  Physiol  1907,  vol.  41,  p.  237. 

2  Pr,  Joseph  S.  Bolton,  Brain,  1910,  vol.  32,  p.  26. 


THE  FORE-BRAIN  125 

a  study  of  the  evolution  of  its  various  functional  areas.  The  cortex  was 
originally  composed  of  primary  sensory  areas — connected  with  sight, 
touch,  hearing,  smell,  etc.  When  secondary  and  higher  zones  were  pro- 
duced in  connection  with  the  primary  areas,  the  surface  of  the  brain  was 
necessarily  thrown  into  folds  and  fissures  to  provide  the  increase  of  surface 
required.  Hence  we  find  that  the  principal  fissures  are  distinctly  related 
to  certain  cortical  areas.  Elliot  Smith  distinguishes  three  kinds  of  fissures  : 
(1)  those  like  the  calcarine  and  central  fissures  which  separate  one  cortical 
area  from  another  (being  limiting  fissures  or  sulci)  ;  (2)  those  like  the 
lunate  (Fig.  126),  where  the  line  of  cortical  demarcation  lies,  not  at  the 
bottom  of  the  fissure,  as  in  the  last,  but  at  the  brink  of  the  fissure.  These 
are  named  operculate,  because  the  convolution  or  operculum,  which 
causes  the  fissure  or  depression,  arises  at  the  junction  of  two  areas  ;  (3) 
a  developing  area  may  fold  inwards,  thus  giving  rise  to  a  depression  in  the 
centre  of  an  area,  like  the  retro-calcarine  in  the  midst  of  the  visuo-sensory 
area  (Fig.  127).     The  hippocampal  linear  depression  and  Sylvian  fossa, 

occi'p.  lobe 

cereb.  uesic/ef-  ,  ,^J^^opticthai. 

Sylvian  dep. 

.      ,         pituitary 
olf.  lobe 

optic  nerve 

Fig.  119. — Lateral  Aspect  of  the  Cerebral  Hemisphere  at  the  end  of  the  2nd  month, 

as  we  have  already  seen  (pp.  113,  117),  are  peculiar  in  their  formation. 
Two  fissures — the  retro-calcarine  and  the  collateral — actually  cause  an 
infolding  of  the  whole  thickness  of  the  mantle,  and  give  rise  to  two  eleva- 
tions in  the  posterior  and  descending  horns  of  the  lateral  ventricle. 

Formation  oJ  the  Island  of  Reil  and  Fissure  of  Sylvius.— When 
the  lateral  wall  of  the  cerebral  vesicle  is  examined  at  the  5th  month  (Fig. 
120)  an  area  of  cortex  is  seen  to  be  rapidly  becoming  submerged  by  the 
overgrowth  of  the  surrounding  cortex.  The  submerged  area  is  the  Island 
of  Reil ;  it  covers  that  part  of  the  wall  of  the  cerebral  vesicle  which  is 
thickened  by  the  corpus  striatum  (Figs.  107,  B,  109).  The  submerged 
area  becomes  triangular  in  shape,  the  apex  being  directed  backwards  ; 
it  is  bounded  by  three  limiting  sulci — an  anterior,  superior  and  inferior. 
The  rising  lips  of  cortex,  which  bound  the  limiting  sulci,  form  the  temporal, 
fronto-parietal  and  orbital  opercula,  and  ultimately  meet  over  the  sub- 
merged area  (Fig.  122).  The  fissure  of  Sylvius  separates  the  opercula.  It 
will  be  readily  grasped  that  the  development  of  the  corpus  striatum  pre- 
vents the  expansion  of  the  insular  part  of  the  vesicle,  whereas  the  thin- 
walled  mantle,  out  of  which  the  other  lobes  of  the  brain  are  developed, 
expands  readily  and  overwhelms  the  thickened  area.  The  corpus  striatum 
begins  to  form  during  the  2nd  month,  hence  as  early  as  that  date  the 


126 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


insular  depression  is  visible  on  the  lateral  wall  of  tlie  hemisphere  (Fig.  119). 
This  mechanical  explanation  of  the  origin  of  the  fissure  of  Sylvius  is  probably- 
only  partially  true  ;  the  relatively  great  growth  of  the  cortex  which  forms 
the  opercula  is  due  in  the  main  to  its  great  functional  importance.  By 
comparing  Figs.  119  and  121  it  will  be  realized  that  the  formation  of  the 


fronto-par.op. 


oooip. 

_  cerebellum 

"^-  'P-   I         \sland.  are^ 
olfact.  temp:  op. 

Fig.  120. — The  same  Aspect  during  the  5th  month. 

Sylvian  fossa  is  connected  with  the  expansion  and  downward  growth  of 
the  temporal  lobe.  The  growth  of  the  temporal  lobe  and  the  differentiation 
of  the  occipital  pole  (see  Fig.  120)  give  the  impression  that  there  has  been  an 
actual  rotation  downwards  of  the  cerebral  vesicle  on  the  Island  of  Reil. 
The  lower  end  of  the  stem  of  the  Sylvian  fissure  also  indents  the  rhinen- 


sup.  precentral 


upper  Rolandic 

limb.  fis.  of.  Sylu. 


fronto-par.  op. 


1st  temp.  fis. 

temp.  op. 
orbit  op.        islandic  area 

Fig.  121. — The  same  Aspect  during  the  7th  month. 

cephalon,  separating  the  uncinate  gyrus  from  the  anterior  parts  of  the 
rhinencephalon  (Figs.  118,  120). 

The  student  is  already  familiar  with  the  fact  that  the  Island  of  Reil 
forms  a  cortical  cap  to  the  corpus  striatum.  The  structures  between 
the  islandic  cortex  and  the  foramen  of  Monro  rejDresent  a  section  of  the 
thickened  wall  of  the  cerebral  vesicle  (Fig.  109).  Convolutions  appear 
on  it  at  the  7th  month,  when  the  rest  of  the  cortex  also  becomes  convoluted. 


THE  FORE-BRAIN  127 

Further,  the  larger  the  area  of  cerebral  cortex  in  any  primate,  the  larger  is 
the  Island  of  Reil ;  the  more  convoluted  the  cortex,  the  more  convoluted 
the  Island.  Flechsig  has  shown  that  the  cortex  of  the  Island  is  joined 
to  all  the  cortical  areas  of  the  mantle  by  bands  of  association  fibres.  Hence 
the  Island  must  be  regarded  as  playing  a  highly  important  part  in  co- 
ordinating the  functions  of  the  brain. 

The  Opercula. — Three  opercula  grow  up  and  cover  the  Island  of  Reil 
(see  Figs.  121  and  122)  :  (1)  the  temporal,  (2)  the  fronto-parietal,  (3)  the 
orbital.     The  late  Professor  D.  J.  Cunningham  found  that  during  the 

anter.  limb       ^^\ 

post  horiz.  limb. 
^/  ^ fronto-par.  operc. 

'^^^if^-^ stem  of  fissure 

/ 

y/^  vallecula  Suluii 

p.  triang.  ant.  asc.  limb r--'      \ 

ant.  horiz.  j"^  K  "  "      "     ^ 

limb.   1   U    m^'^'^'^^^-^^-posf.  horiz.  limb 


A. 


U 


pars  orbit-^ 
/- ' 


'^ual/ecula  Syluii 


Fia.  122. — Diagram  of  the  OpercuJa  and  Fissure  of  Sylvius. 
In  A  the  orbital  operculum  is  undivided ;   in  iJ  it  is  subdivided.    (After  Cunningham.) 

later  months  (7-9)  of  foetal  life  an  opercular  part,  known  as  the  pars  tri- 
angularis (Fig.  122,  B)  appeared  in  quite  50  %  of  brains  and  was  more 
frequently  present  on  the  left  side  than  on  the  right,  probably  owing  to  the 
dominant  centre  for  speech  being  situated  on  the  left  side.  The  jDars 
triangularis  is  the  anterior  part  of  the  upper  or  dorsal  operculum  (labelled 
fronto-parietal  in  Figs.  120,  121),  the  horizontal  limb  of  the  fissure  of 
Sylvius  being  the  anterior  continuation  of  the  ujDper  limiting  sulcus  of  the 
Island  of  Reil  (Elliot  Smith).  The  pars  triangularis  is  cut  off  from  the 
dorsal  operculum  by  the  formation  of  the  ascending  limb  of  the  fissure  of 
Sylvius  (Fig.  122,  B).  The  temporal  operculum  rises  first  (5tli  month), 
the  others  a  month  later.  The  opercula  which  bound  the  posterior  hori- 
zontal limb  of  the  fissure  of  Sylvius  are  the  first  to  come  in  contact.  By 
the  end  of  the  first  year  after  birth  all  three  opercula  meet  over  the  Island 


128 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


and  completely  Mde  it.  At  birth  there  is  still  a  part  of  the  Island  exposed 
behind  the  orbital  operculum  and  in  lower  human  races  this  is  frequently 
the  condition  throughout  life.  The  anterior  opercula  (pars  triangularis 
and  pars  orbitalis)  become  part  of  the  centre  of  speech  and  represent 
later  additions  to  the  human  brain.  If  the  pars  triangularis  be  not  sepa- 
rated from  the  dorsal  operculum,  which  is  commonly  the  condition  on  the 
right  hemisphere,  then  the  anterior  limb  of  the  fissure  of  Sylvius  is  not 
subdivided  into  anterior  horizontal  and  ascending  parts  (Fig.  122,  A 
and  B).  The  posterior  limb  of  the  fissure  of  Sylvius  is  a  limiting  fissure  ; 
it  separates  the  audito-sensory  area,  situated  in  the  first  temporal  gyrus 
— especially  in  the  annectant  convolutions  of  this  gyrus  buried  in  the 
posterior  part  of  the  Sylvian  fissure,  from  the  sensory-motor  areas  above 
the  fissure. 

Comparative  Anatomy  of  the  Opercula  and  Island. — The  Island 
of  Reil  and  its  opercula  are  only  well  developed  in  the  higher  primates. 
In  the  typical  mammalian  brain  the  upper  limiting  sulcus  of  the  Island 
of  Reil  is  represented  by  the  supra-Sylvian  fissure  (Fig.  125),  the  inferior 
limiting  sulcus  by  the  pseudo-Sylvian  fissure,  the  anterior  limiting  by  the 
fronto-orbital  fissure  (Elliot  Smith).  There  are  no  opercula — ^the  cortex 
corres]3onding  to  the  Island  of  Reil  forms  part  of  the  surface  of  the  brain. 
Figs.  123  and  124,  A,  B,  represent  stages  in  the  evolution  of  the  Island 
and  opercula  in  the  primates.     In  Fig.  123  the  condition  in  dog-like  apes  is 

sup./ront      infra-parieta/  ^ar-qcc.  fis. 

affenspalte 


(  island  of  Reil 
ant  lim.  sulcus  (fronto-orbital) 


temp.  fis. 


occip.  fis. 


Fig.  123. — The  Island  of  Reil  and  Fissures  on  the  Lateral  Aspect  of  the  Brain  of  a 

dog-like  Ape. 

represented.  Only  the  upper  and  lower  limiting  sulci  of  the  Island  are  hid 
by  opercula,  the  anterior  limiting  sulcus  (fronto-orbital)  being  still  freely 
exposed.  The  Island,  which  is  small,  is  continuous  anteriorly  with  the 
frontal  lobe.  In  anthropoids  (the  gorilla,  chimpanzee,  orang  and  gibbon) 
the  Island  is  larger  ;  the  upper  and  lower  limiting  sulci  are  buried  ;  an 
imperfect  anterior  limiting  sulcus  (fronto-orbital  fissure)  partially  separates 
the  Island  from  the  orbital  surface  of  the  frontal  lobe.  In  man  all  three 
limiting  sulci  are  covered  by' opercula  and  completely  isolate  the  Island, 


THE  FORE-BRAIN  129 

and  occasionally  this  is  the  condition  (Fig.  124,  B)  in  the  higher  anthro- 
poids, but  it  is  in  man  only  that  the  orbital  operculum  grows  up  and  meets 
with  the  other  opercula.  This  can  be  the  more  easily  understood  when  it 
is  remembered  that  the  orbital  part  of  the  3rd  frontal  convolution  is 
connected  with  sj^eech. 

Hippoeampal  and  Ectorhinal  Fissures. — The  hippocampal  linear 
depression,  which  demarcates  the  hippocampal  cortex  (Figs.  117,  118) 
from  the  neopallial,  we  have  already  seen  to  be  a  mere  superficial  indication 
of  a  cortical  ingrowth  (p.  113).  The  incisura  temporalis  (Figs.  125  and 
118),  all  that  remains  of  the  ectorhinal  fissure  of  the  typical  mammalian 
brain,  separates  the  uncus — part  of  the  rhinencephalon — from  the  cortex 

fronto-par.  open 

post  limb.  fis.  syl. 

temp.  op. 
island 


temp,  open 
island 


Fig.  12-t,  A. — The  more  common  condition  of  the  Island  of  Reil  in  Anthropoids. 
B. — The  complete  isolation  of  the  Island  of  E-eil,  the  condition  seen  con- 
stantly in  the  Human  Brain  and  occasionally  in  the  Anthropoid. 

or  neopallium  of  the  temporal  lobe.  The  ectorhinal,  or  rhinal  fissure,  as 
it  is  usually  named,  is  thus  a  limiting  fissure  between  olfactory  and  temporal 
cortex. 

The  Calloso-Marginal  Fissure. — This  fissure  on  the  mesial  aspect  of  the 
brain  arises  from  the  fusion  of  the  genual  and  intercalary  fissures  of  the 
typical  mammalian  brain  (Fig.  115).  Its  origin  is  probably  the  result  of  a 
pressure  due  to  the  growth  of  the  cortex  surrounding  the  corpus  callosum, 
for  if  that  structure  be  absent,  the  usual  form  of  this  fissure  is  completely 
altered.  It  separates  one  set  of  cortical  areas  from  another  (Elliot 
Smith). 

The  •  Calcarine  and  Correlated  Fissures.^ — In  the  typical  mammalian 
brain  the  calcarine  fissure  forms  part  of  the  same  arcuate  system  as  the 
genual  and  intercalary  (Fig.  115).  The  part  of  the  cerebral  wall  in  which 
it  is  formed  projects  within  the  posterior  horn  of  the  lateral  ventricle  (Fig. 

^See  Prof.  Elliot  Smith,  Journ.  A)iat.  and  Physiol.  1907,  vol.  41,  p.  198. 

I 


130     HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

226,  B,  p.  220).  The  cortex  on  the  lower  or  posterior  lip  of  the  fissure 
shows  the  stria  of  Gennari  which  characterizes  the  cortex  in  which  the 
optic  radiations  end.  The  calcarine  fissure  is  thus  a  limiting  fissure  formed 
between  striate  and  non-striate  cortex.  The  retro-calcarine  fissure  or 
depression,  which  continues  the  calcarine  sulcus  backwards  to  the  occipital 
pole,  is  formed  by  the  growth  and  involution  of  the  striate  cortex  (Fig. 
127).  In  the  ape's  brain  the  striate  cortex  on  the  lateral  aspect  of  the 
occipital  pole  increases  rapidly  and  rises  up  as  a  lip  or  operculum  over 
the  cortex  of  the  parietal  lobe.  The  depression  in  front  of  the  operculum 
is  known  as  the  simian  fissure  (Afienspalte)  or  sulcus  lunatus  (Fig.  123). 
In  the  human  brain  the  great  increase  of  the  parietal  cortex,  a  seat  of 
association  centres,  has  pushed  the  sulcus  lunatus  almost  to  the  occipital 
pole  (Fig.   126),  or  it  may,  especially  in   the  more    civilized   races,    be 

suprasylu.  (sup.  limit  sulc) 
Grucial  \   lateral  (intrapariet) 

coronal 
(inf.  &  asc.  fron 

occip.  pole 

■post-syluian  (1st  temp.) 
olf.  bulb  ■     '''^sfe^^j;^^  ^^^pseudo-sylu.  (inf.  limit  sub.) 

orbital.  -w-^iij-- 

,  ^,  , .  .  /    /     I      —         ectorfi.  fis.  (incis.  temp.) 

diagonal  (fronto-oroit).    ,  /   .  \ 

jlslana  \    pydform  lobe  (uncus.) 
olf  tub.    ant  perf.  sp. 

Fig.  125. — The  Fissures  on  the  Lateral  Aspect  of  a  typical  Mammalian  Brain.  (Elliot 
Smith.)  The  Fissures  to  which  these  correspond  in  the  Human  Brain  are  indicated 
within  brackets.  The  parts  of  the  Rhinencephalon  are  stippled ;  the  cortex 
corresponding  to  the  Island  of  Hail,  shaded. 

comjoletely  obliterated.  Further  the  Y-shaped  occipital  sulcus  (external 
calcarine)  on  the  lateral  aspect  of  the  occipital  pole  (Figs.  123,  126)  may 
join  the  retro-calcarine  sulcus.  More  recently  Elliot  Smith  has  distin- 
guished not  only  the  striate  area,  in  which  the  optic  radiations  end,  but  two 
surrounding  areas,  an  outer  zone — the  peristriate,  and  an  intermediate — 
between  the  outer  zone  and  the  striate  area.  Certain  sulci  have  arisen 
in  connection  with  the  evolution  of  these  two  association  or  visuo-psychic 
areas.  The  collateral  fissure  below  the  calcarine  (Fig.  127)  probably 
results  from  a  mechanical  pressure  exercised  by  the  growth  of  the  striate 
area.  The  parieto-occipital  fossa  or  dejDression  on  the  mesial  aspect  of  the 
brain  results  from  the  inflection  of  an  area  of  cortex  between  the  calcarine 
areas  of  growth  behind,  and  the  area  of  association  centres  on  the  mesial 
aspect  of  the  parietal  lobe  in  front  (Fig.  127).  The  production  of  the 
parieto-occipital  fossa,  with  its  complex  of  buried  convolutions  and  sulci, 
is  also  related  to  the  growth  backwards  of  the  corpus  callosum.     In  human 


THE  FORE-BRAIN 


131 


brains  where  this  structure  is  absent  the  buried  convolutions  and  sulci 
are  superficial. 

Orbital  Fissure. — This  fissure  is  present  in  most  mammalian  brains, 
but  its  significance  is  still  doubtful. 

Fissure  of  Rolando  or  Central  Fissure,  appears  during  the  sixth 
month  as  an  upper  and  lower  linear  depression,  which  join  together  in 
the  course  of  development  (Fig.  121).  The  fissure  appears  between  the 
motor  areas  of  cortex  in  front  of  it,  and  the  sensory  areas  behind  it,  and 
is  therefore  a  limiting  fissure.  The  upper  part  does  not  quite  correspond 
to  the  crucial  sulcus  of  the  brain  of  the  cat  and  dog,  for  in  them  that 
sulcus  forms  the  anterior  limit  of  the  motor  areas  (Elliot  Smith)  (Fig.  125)  ; 
the  lower  part  may  represent  part  of  the  coronal  fissure.  The  fissure  of 
Rolando  reaches  its  fullest  development  in  man  ;  it  is  found  only  in  the 
higher  primates  (monkeys  and  anthropoids). 


oce.  mcis. 


par.  oco.  incis. 

arc  intercun. 
nterpar.     par.-occipf. 


retro.-calc.  f.-f')^  /  , 


-y  shaped  fis. 


\S.  lunatus  (affensp. )       ^^'^^^-  f'^' 


calcar.fis. 


Fig.  126.- 


-The  Lateral  Aspect  of  the  Occipital  Lobe  of  a  Human  Brain,  showing 
the  Sulcus  Lunatus  (Affenspalte).     (Elliot  Smith.) 

Fig.  127. — The  Mesial  Aspect  of  the  Occipital  Lobe  of  a  Human  Brain,  showing  the 
complex  nature  of  the  Parieto-Occipital  Fissure.    (Elliot  Smith.) 

Sulcus  Rectus. — The  sulcus  rectus,  or  straight  fissure,  appears  before 
that  of  Rolando,  and  is  found  in  primate  brains  in  which  the  Rolandic 
fissure  is  absent  (Figs.  123,  125).  It  forms  in  the  adult  brain  (1)  part  of 
the  inferior  frontal  fissure,  (2)  the  lower  part  of  the  precental  fissure  (as- 
cending frontal).  It  lies  between  two  areas  of  frontal  cortex  which  are 
of  different  structure,  and  corresponds  to  the  coronal  fissure  of  the  cat's 
brain  (Fig.  125). 

Intra-parietal  Fissure. — The  intra-parietal  fissure  appears  between 
three  areas  of  growth  :  (1)  the  cortex  of  the  inferior  parietal  lobule  below, 
chiefly  consisting  of  association  areas  related  to  the  visual  and  auditory 
and  perhaps  also  to  the  areas  of  common  sensation  ;  (2)  the  occipital  cortex 
posteriorly  ;  (3)  the  cortex  behind  the  up^jer  end  of  the  fissure  of  Rolando 
above  and  in  front.  It  corresponds  to  the  lateral  fissure  of  the  cat's 
brain  (Fig.  125),  while  the  whole  of  the  intra-parietal  fissure  of  the  ape's 
brain  (Fig.  123)  may  be  regarded  as  equivalent  to  the  ascending  rami 
in  the  human  brain  (Jefferson).     The  ascending,  horizontal  and  occipital 


132      HUMAN  EMBEYOLOGY  AND  MORPHOLOaY 

limbs  of  this  fissure  arise  independently  in  connection  with  separate  areas. 
They  may  or  may  not  become  conjoined.  All  the  parts  of  the  fissure  are 
limiting  in  nature.^ 

Parallel  or  First  Temporal  Fissure. — The  first  temporal  fissure  sepa- 
rates the  first  temporal  gyrus,  in  which  the  auditory  centres  are  situated, 
from  the  neighbouring  cortex  (Figs.  123,  125).  As  the  first  temporal  gyrus 
rises  to  form  the  inferior  operculum  of  the  island  of  Reil,  part  of  it,  in  the 
form  of  a  number  of  gyri  which  connect  it  with  the  island,  are  buried  in 
the  fissure  of  Sylvius.  In  these  gyri  Campbell  has  located  the  terminations 
of  the  auditory  tracts,  the  superficial  part  of  the  first  temporal  convolution 
forming  association  areas  for  the  auditory  centre  (Fig.  246).  The  first 
temporal  fissure  corresponds  to  the  post-Sylvian  fissure  of  the  typical 
mammalian  brain  (Fig.  125). 

Secondary  Sulci  and  Gyri. — During  the  eighth  and  ninth  months 
the  remaining  sulci  and  convolutions  of  the  brain  are  formed.  For  the 
greater  part  these  are  peculiar  to  the  human  brain. 

OTIC    vesiCLE 

HINDBftA'N  ^ 

SP'NAL   CORD 


SUBNEURAL 
ANAST 


—  ,^  .^,^       ^  X  ^  DORSAL  AORTA 

FOREBRAIN  yC       ^        /  \  \^ 

/V/^/'  \  ^AORTIC  STEM 

OPTIC     VES  /  I  N. 

C^RFRRAL     ART  »  EXT      CAROTID 

t.^HEBRAL    ART.  , ^^   CAROTID 

Fig.  128. — Diagrammatic  Representation  of  the  Arteries  of  the  Brain  at  the  end  of 
the  first  month  of  development.     (After  Evans.) 

Vessels  of  the  Brain. — The  embryonic  arteries  from  which  the  cerebral 
and  vertebral  arteries  become  evolved,  are  shown  in  Fig.  128.  The  dorsal 
aorta,  in  which  the  aortic  arches  end,  is  continued  forward  to  the  fore-brain, 
where  at  the  root  of  the  optic  vesicle  and  near  the  site  of  the  future  vallecula 
of  Sylvius  it  divides  into  anterior  and  posterior  branches ;  the  anterior 
branch  will  become  the  stem  of  the  middle  and  anterior  cerebral  arteries 
as  the  cerebral  vesicles  begin  to  expand,  while  the  posterior  branch  becomes 
continuous  with  the  subneural  anastomotic  vessel,  from  which  the  posterior 
communicating  and  basilar  arteries  will  become  differentiated  and  from 
which  the  posterior  cerebral  will  arise.  The  subneural  anastomotic  chain 
is  fed  by  segmental  vertebral  branches  of  the  dorsal  aorta  (Fig.  128).  From 
this  segmental  network  is  formed  the  vertebral  arteries.  The  right  and 
left  anastomotic  vessels  fuse  under  the  hind-brain  during  the  6th  week  to 
form  the. basilar  artery.^ 

^  As  to  nature  of  the  intra-parietal  complex  see  Geoffrey  Jefferson,  Journ.  Anat. 
1913,  vol.  47,  p.  365. 

^  For  further  details  see  Bertha  de  Vriese,  Archiv  de  Biol.  1904,  vol.  21,  p.  357  ;  F. 
P.  Mall,  Amer.  Journ.  Anat.  1905,  vol.  4,  p.  1  ;  H.  M.  Evans,  Keibel  and  MalVs  Manual 


THE  FORE-BRAIN  133 

The  embryological  basis  out  of  which  the  venous  sinuses  and  cerebral 
veins  are  developed,  is  shown  in  Fig.  129.  At  the  middle  of  the  2nd  month 
the  veins  of  the  fore-  and  mid-brains  unite  behind  the  stalk  of  the  optic 
vesicle  to  form  the  primitive  vein  of  the  head,  which  passing  backwards 
internal  to  the  Gasserian  ganglion  leaves  the  interior  of  the  cranial  cavity 
just  in  front  of  the  internal  ear,  passes  through  in  the  region  of  the  middle 
ear  to  become  the  jugular  or  anterior  cardinal  vein.  Before  leaving  the 
interior  of  the  skull  it  receives  a  cerebellar  venous  trunk  (Fig.  129)  and 
after  its  exit  a  medullary  trunk — which  escapes  by  the  jugular  foramen. 
With  the  expansion  backwards  of  the  cerebral  vesicles  during  the  3rd, 
4th  and  5th  months  the  system  of  the  longitudinal  and  transverse  sinus 
becomes  evolved  by  the  union  of  the  venous  network  included  in  the  longi- 
tudinal fissure  between  the  cerebral  vesicles  and  between  the  cerebral 
vesicles  and  cerebellum.     The  main  changes  are  indicated  in  Fig.  129  ; 

TRAN3V-  SINUS 
MID.  PLEXUS  I  SIGMOID    SINUS 

\                          j                            OTIC    VESICLE 
ANT  PLEXUS    ^S^\r:f^  A  ^""^     [  POST.PLEy.US 


PfflM.VEIN  OF  HEAD 

N   TRIGEMINUS 


Fig.  129. — The  Primitive  Vein  of  the  Head  and  its  tributaries  in  the  6th  week  of  de- 
velopment, with  indications  of  the  new  anastomotic  channels  opened  up  during 
the  3rd  and  4th  months.     (After  Streeter.) 

only  part  of  the  primitive  vein  persists — the  part  lying  internal  to  the 
Gasserian  ganglion  which  becomes  the  cavernous  sinus ;  the  extra-cranial 
part  disappears  towards  the  end  of  the  2nd  month,  but  it  occasionally 
persists  as  an  emissary  vein  opening  near  the  root  of  the  zygomatic  process. 
Two  important  anastomotic  channels  open  up  :  (1)  the  precerebellar 
which  drains  the  tributaries  of  the  fore-  and  mid-brain  into  the  primitive 
cerebellar  trunk  ;  (2)  the  post-cerebellar  which  unites  the  cerebellar  trunk 
with  the  veins  of  the  hind-brain  ;  the  hind-brain  trunk  escapes  by  the 
jugular  foramen.  Thus,  as  the  cerebral  vesicles  grow  back  their  veins  are 
transferred  first  from  the  primitive  vein  to  the  cerebellar  and  then  to  the 
venous  system  of  the  hind-brain.  From  the  anastomic  channel  thus  opened 
up  are  fashioned  the  transverse  and  sigmoid  sinuses  ^  (Fig.  129). 

Membranes  of  the  Brain. — Even  before  the  cephalic  part  of  the  neural 
tube  is  enclosed,  mesodermal  cells  spread  in  between  it  and  the  surrounding 

of  Human  Embryology,  1912,  vol.  2,  p.  570  ;  G.  L.  Streeter,  Contributions  to  Embryology, 
1918,  vol.  8,  p.  5. 

1  See  G.  L.  Streeter,  Amer.  Journ.  Anat.  1915,  vol.  18,  p.  145  ;    1916,  vol.  19,  p.  67. 


134      HUMAN  EMBRYOLOaY  AND  MORPHOLOaY 

ectoderm  to  form  a  primitive  covering.  Out  of  the  covering  become 
differentiated  the  capsule  proper  of  the  brain — the  pia-arachnoid  with  its 
vessels,  the  membranous  cranial  capsule,  from  which  are  differentiated  the 
dura  mater,  enclosing  bones  and  pericranium,  and  the  connective  tissues 
of  the  scalp.  The  differentiation  of  the  membranes  of  the  brain  and 
spinal  cord  is  closely  related  to  the  establishment  of  a  cerebrospinal  fluid 
system.  We  have  seen  that  the  choroid  plexuses  of  the  ventricles  of  the 
brain  become  developed  in  the  7th  week  when  the  human  embryo  is  about 
15  mm.  long.  Dr.  L.  H.  Weed  found  that  at  this  stage  of  development 
in  the  pig,  the  cerebro-spinal  fluid  formed  in  the  4th  ventricle  began  to 
escape  through  a  localized  area  in  the  inferior  medullary  velum  and  to 
collect  in  the  overlying  mesodermal  tissue.^  At  the  site  of  escape  an 
opening  is  formed  in  the  medullary  velum,  the  foramen  of  Magendie, 
arising  in  this  way.  The  foramen  formed  in  each  lateral  recess  of  the  4th 
ventricle  are  produced  in  a  similar  manner.  The  subarachnoid  spaces  thus 
commence  over  the  4th  ventricle  and  round  the  medulla  oblongata  and 
from  the  region  of  the  hind-brain  the  system  extends  proximally  and 
distally  until,  by  the  middle  of  the  3rd  month  of  development,  the  entire 
neural  tube  is  enclosed  by  the  arachnoid.  The  mesodermal  condensation 
which  bounds  the  subarachnoid  system  becomes  the  arachnoid ;  the 
pia  mater  represents  the  subarachnoid  tissue.  At  the  same  time  as  the 
subarachnoid  spaces  are  being  formed  another  plane  of  cleavage  sets  in 
external  to  the  arachnoid,  the  arachnoid  being  thus  separated  from  the 
dural  layer  of  the  cranial  capsule  and  a  potential  space  produced — ^the 
subdural.  These  spaces,  particularly  the  subarachnoid,  do  not  represent 
parts  of  the  lymphatic  system ;  lymphatic  vessels,  we  shall  see,  arise  like 
blood  vessels  ;  nor  does  the  cerebro-spinal  fluid  represent  a  species  of  lymph. 

^  In  connection  with  the  development  of  the  cerebro-spinal  fluid  system  consult : 
Dr.  L.  H.  Weed,  Anat.  Rec.  1916,  vol.  10,  p.  475  ;  Contributions  to  Embryology,  1917, 
vol.  5,  p.  3 ;  1920,  vol.  9,  p.  425  (production  of  Hydrocephalus) ;  Percival  Bailey, 
Journ.  Comp.  Neur.  1916,  vol.  26,  p.  79. 


CHAPTER  XI. 
THE  CRANIUM. 

Natural  Divisions  of  the  Skull. — The  human  skull  is  the  product  of 
many  long  epochs,  during  which  it  has  undergone  great  changes,  but  we 
have  every  reason  for  supposing  that  its  general  functions  have  remained 
much  the  same  since  the  vertebrate  form  of  animal  was  evolved.  In  the 
first  place  it  has  to  form  a  brain-case — a  neuro-cranium.  Man's  brain 
has  reached  a  degree  of  development  which  rendered  great  changes  neces- 
sary in  this  part  of  the  skull.  In  the  second  place,  the  skull  has  to  shelter 
and  protect  the  special  organs  of  sense — the  ear  (temporal  bone),  the  eyes 
(orbits),  the  olfactory  area  (nasal  region),  and  taste  (bucco-pharyngeal 
region).  In  the  third  place,  the  skull  forms  an  essential  part  of  the  struc- 
tures concerned  in  mastication  ;  the  facial  part  of  the  skull  is  in  reality  a 
scaffolding  for  the  palate  and  teeth.  In  the  main  the  facial  part  of  the 
skull  is  visceral  or  splanchnic  in  function,  and  hence  is  sometimes  spoken 
of  as  the  splanchno-cranium.  The  outstanding  characters  of  the  human 
skull  are  the  great  size  of  the  neuro-cranium  and  the  smaU  size  of  the 
splanchno-cranium. 

Certain  Phases  in  the  Evolution  of  the  SkuU.^The  skull  has  also 
been  closely  related  to  the  function  of  respiration.  In  fishes  the  visceral 
skeleton  of  the  skull  forms  the  arches  which  carry  the  gills.  We  have  seen 
that  the  representatives  of  these  arches  make  a  temporary  appearance  in 
the  head  of  the  human  embryo.  When  a  pulmonary  replaced  a  branchial 
system  a  nasal  airway  was  separated  from  the  mouth  by  the  formation  of  a 
primitive  palate,  such  a  palate  as  is  seen  in  amphibians,  reptiles  and  birds. 
With  the  evolution  of  chewing  teeth  in  the  mammalian  stock  the  complete 
palate  was  formed,  thus  allowing  the  mammalian  young  to  suck,  and  the 
adult  to  chew  and  breathe  freely  at  the  same  time.  We  see  all  of  these 
stages  manifested  in  the  development  of  the  human  skuU.^ 

^  For  recent  research  on  development  of  skull  see :  Ed.  Fawcett  (Chondrocranium 
of  water-rat),  Journ.  Anat.  1917,  vol.  51,  p.  309  ;  (Chondrocranium  of  hedgehog), 
Journ.  Anat.  1918,  vol.  52,  p.  211  ;  (Chondrocranium  of  seal),  Journ.  Anat.  1918,  vol. 
52,  p.  412  ;  (Skull  of  Miniopterus),  Journ.  Anat.  1919,  vol.  53,  p.  315  :  C.  R.  Bardeen, 
Keibel  and  Mall's  Textbook  of  Embryology,  vol.  1,  1910  ;  Warren  H.  Lewis,  Contrib. 
Embryology,  1920,  vol.  9,  p.  299  ;  John  Kernan  (Chondrocranium  of  20  mm.  embryo), 
Amer.  Journ.  Anat.  1916,  vol.  17,  p.  605 ;  Chas.  C.  Macklin  (Cranium  of  40  mm.  foetus), 
Amer.  Journ.  Anat.  1914,  vol.  16,  p.  317  ;  R.  J.  Terry  (Chondrocranium  of  cat),  Journ. 
Morph.  1917,  vol.  29,  p.  281  ;    Eliz.  A.  Fraser  (Trichosurus),  Proc.  Zool.  Soc.  Lond. 

1915,  p.  299  ;    Phihppa  C.  Esdaile  (Chondrocranium  of  perameles),  Phil.  Trans.   (B) 

1916,  vol.  207,  p.  439  ;    D.  M.  S.  Watson  (Duckbill),  Phil.  Trans.  (B)  1916,  vol.  207, 

135 


136      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

Cartilaginous  SkulL — In  trying  to  interpret  the  meaning  of  many  of 
the  developmental  processes  which  we  see  taking  place  in  the  human 
embryo,  it  is  often  profitable  to  seek  light  from  comparative  anatomy,  and 
no  group  of  the  lower  vertebrates  can  help  us  in  this  respect  so  well  as  the 
selachians — the  group  of  cartilaginous  fishes  to  which  sharks,  rays  and 
dog-fish  belong.  This  is  particularly  true  of  the  chondrocranium — the 
cartilaginous  skull,  seen  in  the  human  foetus  during  the  2nd  month  of 
development.  In  Fig.  130  is  represented  the  cartilaginous  cranial  wall 
which  encloses  the  brain  of  a  shark  ;  we  see  at  once  that  the  base  is  made 
up  of  two  parts — a  chordal  in  which  the  remains  of  the  anterior  part  of  the 
notochord  are  embedded  and  a  prechordal  lying  in  front  of  the  notochordal 
part.  The  fossa  for  the  pituitary  body  occupies  the  posterior  part  of  the 
prechordal  base.  The  chordal  part  represents  a  continuation  forwards 
of  the  vertebral  column,  only  the  cartilage  never  becomes  segmented  but 
remains  as  a  continuous  plate,  and  thus  gives  solidity  to  the  part.  Signs 
of  segmentation  are  seen  in  the  series  of  foramina  by  which  the  roots  of  the 

CRANIOPORB 
OLFACT  CAPSULE 


NOTOCHORO 
PRECHORDAL  BASE  \  CHORDAL    BASE 

PITUITARY  CM. 

Fig.  130. — The  Chondrocranium  of  a  Shark  laid  open  by  a  mesial  sagittal  section. 
(After  Gegenbaur.) 

hypoglossal  nerve  escape.  The  sujDra-chordal  part  of  the  cranial  cavity  is 
occupied  by  the  hind-  and  mid-brains,  which  also  show  traces  of  a  seg- 
mental origin.  In  the  lateral  wall  of  this  part  of  the  skull  is  also  placed 
the  otic  vesicle — the  vestibular  or  balancing  apparatus.  It  will  be  re- 
membered that  it  was  the  attachment  of  this  organ  to  the  hind-brain 
which  occasioned  the  development  of  the  cerebellum  ;  it  also  gives  rise 
to  a  disturbance  of  the  skull,  for  the  cartilaginous  capsule  which  is  developed 
round  the  otic  vesicle  is  thrust  into  the  cranial  wall  and  pushes  backwards 
the  representatives  of  the  neural  arches  of  the  chordal  cranium  (Fig.  130). 
The  prechordal  part  of  the  skull  serves  as  the  capsule  of  the  fore-brain. 
At  no  time  is  there  a  segmentation  of  the  fore-brain  or  of  its  cranial  capsule  ; 
we  are  here  dealing  with  a  part  of  the  skull  which  lies  in  front  of  the  ancient 
vertebral  region,  and  has  arisen,  as  has  the  fore-brain  itself,  in  connection 
with  two  organs  of  sense — the  nose  and  eye.  The  olfactory  organ  is 
enclosed  in  a  capsule  of  cartilage  which  is  placed  like  the  watchman  of  a 
ship,  on  the  prow  of  the  primitive  skull  of  all  aquatic  vertebrates.  The 
capsule  of  the  optic  vesicle  never  forms  part  of  the  cranial  wall,  but 
becomes  differentiated  to  form  the  sheath  of  the  optic  nerve  and  of 
the  eyeball. 

p.  311  ;  (Amphibia),  Phil.  Trans.  (B)  1919,  vol.  209,  p.  1  ;  E.  S.  Goodrich  (Cranium 
of  dog-fish),  Quart.  Journ.  Mic.  Sc.  1918,  vol.  63,  p.  1  ;  Graham  Kerr,  Textbook  of 
Embryology,  vol.  2,  1919. 


THE  CRANIUM  137 

If  we  examine  the  chondrocranium  of  a  human  foetus  in  the  8th  week 
of  development  (Fig.  131)  we  note  the  same  divisions  as  are  shown  in  Fig. 
130.  The  base  shows  chordal  and  prechordal  parts.  In  the  chordal  part 
we  note  the  cartilaginous  otic  capsule  thrusting  backwards  the  combined 
occipital  elements  in  the  lateral  wall ;  we  see  the  prechordal  part  passing 
forwards  as  a  rostral  beam  to  support  the  nasal  or  ethmoidal  capsule. 
But  of  the  cartilaginous  roof  only  mere  remnants  are  present.  There  is  : 
(1)  the  tectal  plate,  or  parietal  plate  as  it  is  sometimes  named  ;  it  is  attached, 
along  the  lower  border,  to  the  auditory  capsule  and  occipital  element ;  (2) 
there  are  two  small  processes  springing  from  the  sides  of  the  prechordal 
base  (Figs.  131,  132) — the  ala  temporalis — the  fundament  from  which  the 
great  wing  of  the  sphenoid  will  be  developed  and  the  orhito- sphenoid — 
the  j)late  from  which  the  small  wing  will  be  fashioned.     These  three  carti- 


TECTUM 

MAST .  roR 

O    ENDOLYh\ 

OOP    SELLA. 


oneiTo  SPN 

OLFACT    CAP 


ALI  SPHCN ■ 
^PftECHORDAL     BASE 


Fig.  131. — Sagittal  Mesial  Section  of  the  Chondrocranium  of  a  Human  Foetus  20  mm. 
long  and  in  the  8th  week  of  development.    (Warren  Lewis.) 

laginous  plates  are  all  that  appear  in  the  human  embryo  to  represent  the 
cartilaginous  roof  of  the  primitive  skull. 

If  we  turn  to  Fig.  132  we  can  see  why  the  primitive  cartilaginous  skull 
of  the  human  embryo  has  become  so  profoundly  modified  and  reduced.  It 
is  a  result  of  the  large  mass  attained  by  the  mammalian  central  nervous 
system  at  an  early  state  of  development.  When  a  builder  is  to  erect  a 
great  edifice  he  does  not  begin  by  repeating  the  evolutionary  history  of 
house  building,  but  marks  out  from  the  beginning  the  extent  of  his  founda- 
tions. It  is  so  in  laying  down  the  human  brain  ;  it  is  laid  down  on  big 
lines  almost  from  the  first ;  the  ancient  roof  has  become  altogether  inade- 
quate ;  we  see  the  tectal  plate  growing  up  and  covering  the  roof  of  the  4th 
ventricle  ;  it  meets  with  its  fellow  of  the  opposite  side  and  forma  that  part 
of  the  occipital  bone  which  completes  the  posterior  fossa  of  the  skull  and 
encloses  the  hind-brain.  But  all  the  rest  of  the  roof  is  formed  by  a  mem- 
branous capsule  in  which  cartilage  never  develops.  A  glance  at  Fig.  132 
will  show  why  the  roof  must  be  fashioned  from  plastic  material,  for  during 
the  3rd,  4th  and  5th  months  the  cerebral  vesicles,  lying  over  the  prechordal 
region  of  the  base,  expand  upwards  and  backwards  until  their  occipital 
poles  reach  the  tectal  plates. 


138 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


There  are  certain  other  features  seen  in  the  lateral  aspect  of  the  foetal 
chondrocranimn  which  call  for  comment  here.  The  auditory  capsule,  the 
auditory  ossicles  and  the  region  of  the  tympanum,  save  for  their  covering  of 
soft  parts,  lie  exposed  on  the  surface  of  the  skull.     If  we  turn  to  the  lateral 


CEREBELLUM- 
TECTUM 

OTIC    CAP. 
f ORAM  EN 

MED.  OBLON, 


IS^CERV. 

PAROCCIP. 
MASTOID 


CRISTA 

Olpact  cap 


ORBITO-SPHENOID 


HYOID 
INCUS 
MECKEL^  CATKT. 
AUI-SPHEhl. 
PIT  U  IT. 


Fig.  132. — Chondrocranium  of  a  Human  Embryo  in  the  8th  week  of  development, 
seen  from  the  side.     (Warren  Lewis.) 

aspect  of  the  cartilaginous  skull  of  a  shark  we  obtain  an  evolutionary 
explanation  of  this  arrangement.  At  the  anterior  end  (Fig.  133)  is  seen 
the  nasal  or  ethmoidal  region  ;  the  hind  end  is  formed  by  the  occipital  area 
— compounded  from  occipital  vertebrae.  Between  these  two  extreme 
areas  lies  a  large  intermediate  part  which  is  definitely  demarcated  into 


ORBIT   REGION 

POST  ORBIT.  P/fOC. 
OTIC    REaiOH 

POST  OTIC  PROC. 
OCCIP.  REGION 


MANDIBLE. 

QUADRATE 


TYMPANO  HYAL 
ARTICULARE 


PTER  YGOID 

Fia.  133. — Lateral  Aspect  of  Skull  of  Shark.     (After  Gegenbaur.) 

two  regions — orbital  and  otic  (Fig.  133).  Lying  on  the  otic  area  and 
attached  to  it  are  the  primitive  maxillary  apparatus — the  tympano-hyal 
(Fig.  133)  which  corresponds  to  the  stapes,  the  quadrate  part  of  the  palato- 
quadrate^which  has  been  shaped  in  mammals  to  form  the  incus,  and  the 
upper  end  of  the  primitive  mandible  which  gives  rise  to  the  malleus — all 


THE  CRANIUM  139 

lying  exposed  just  as  in  the  human  embryo.  The  cartilaginous  promin- 
ence— named  post-orbital  in  Fig.  133,  because  it  bounds  posteriorly  the 
orbital  region  of  the  primitive  skull — is  worthy  of  note  because  it  represents 
the  point  at  which  a  new  mandibular  joint — the  temporo-mandibular — 
becomes  evolved  in  mammals,  and  thus  sets  free  the  old  maxillary  parts 
for  the  service  of  the  ear.  The  post-orbital  process  of  the  primitive  skull 
becomes  the  site  of  the  articular  eminence  in  the  mammalian  skull,  while 
the  pre-orbital  is  represented  by  the  internal  angular  process  of  the  mam- 
malian orbit.  Thus,  out  of  the  primitive  orbital  region  is  fashioned,  not 
only  the  orbit,  but  the  whole  floor  of  the  temporal  fossa,  the  malar  bone  and 
zygomatic  arch  being  later  formations  evolved  out  of  membranous  skeletal 
elements.  Similarly  in  the  skull  of  the  human  embryo,  as  in  that  of  the 
shark,  there  are  no  cartilaginous  representatives  of  the  maxilla  or  premaxilla. 

Growth  of  the  Cranial  Cavity. — The  neuro-cranium  is  framed  by 
the  disposition  of  its  bones  and  sutures,  so  as  to  allow  a  free  and  easy  ex- 
pansion of  the  brain.  By  a  mechanism  we  do  not  fully  understand  the 
bones  entering  into  the  formation  of  the  cranial  cavity  grow  as  demand  is 
made  on  them  by  the  brain  ;  at  least,  this  is  so  in  early  life.  AVTien  the 
cranial  bones  begin  to  form  in  the  latter  part  of  the  second  month,  the 
brain  (cerebral  vesicles)  is  only  half  an  inch  long — from  frontal  to  occipital 
pole  ;  in  the  adult  the  length  is  fourteen  times  as  much  and  its  volume 
fifteen  hundred  times  larger.  As  the  cerebral  vesicles  expand  the  develop- 
ing bones  alter  in  shape.  By  the  7th  month  of  the  foetal  life  the  relative 
proportions  become  approximately  fixed.  During  the  first  four  years  of 
life,  brain  and  cranial  growth  go  on  rapidly.  At  birth  the  brain  has  at- 
tained from  20  to  22  per  cent,  of  its  size  ;  by  the  4th  year  over  80  per  cent. 
of  the  volume  is  already  present.  There  is  a  steady  increase  until  the  18th 
or  20th  year,  when  the  maximum  is  obtained  (about  1500  cubic  centimetres 
in  Englishmen) ;  after  then  there  is  a  decline  in  the  capacity  of  the  cranium. 
The  changes  in  the  cranial  walls  are  secondary  to  those  in  the  brain. 

From  Fig.  134  it  will  be  apparent  that  the  walls  of  the  cranium  are  made 
up  of  two  very  different  parts — basilar  and  capsular.  The  basilar  part 
is  thick  and  developed  in  a  cartilaginous  basis.  Growth  proceeds  as  in  a 
long  bone  ;  the  lines  between  the  basi-occipital  and  basi-sphenoid,  the  basi- 
and  pre-sphenoid,  and  between  the  pre-sphenoid  and  ethmoid  are  growth 
or  epiphyseal  lines.  The  growth  of  the  base  of  the  skull  is  determined 
as  much  by  the  needs  of  the  splanchno-cranium  as  by  those  of  the  neuro- 
cranium.  The  capsular  part — occipital,  parietal,  frontal  and  temporal 
bones — on  the  other  hand,  respond  easily  to  the  expansion  of  the  brain. 
They  grow  at  their  edges  ;  the  sutures  are  growth  lines.  Growth  at  the 
coronal  and  lambdoid  sutures  adds  to  the  calvarial  length  ;  gro^vth  at 
the  sagittal  and  squamous  sutures  increases  the  calvarial  breadth.  At  the 
same  time  there  is  also  a  constant  deposition  or  growth  on  the  outer  table 
and  an  absorption  on  the  inner.  In  this  manner  the  bones  are  modelled, 
and  growth  of  cranial  cavity  and  brain  are  co-ordinated.  Only  those 
bones  which  enter  into  the  formation  of  the  cranial  cavity  and  help  to  form 
the  brain  chamber  are  dealt  with  in  this  chapter.  These  bones  are  the 
frontal,  parietal,  occipital,  temporal,  ethmoid  and  sphenoid. 


140 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


Is  the  Skull  made  up  of  Segments? — We  have  just  seen  that  the 
body  is  made  up  of  33  or  more  segments.  Is  the  skull  made  up  of  a  series 
of  segments  ?  The  theory  supported  by  Owen  and  many  others  that  the 
cranium  is  really  composed  of  four  modified  vertebrae  is  now  no  longer 
tenable.  On  the  other  hand  the  arrangement  of  the  nerves  and  muscles,  the 
evidence  of  development  and  comparative  anatomy,  indicate  that  it  is  com- 
posed of  a  number  of  segments,  probably  nine  in  number.  The  four 
posterior,  which  form  the  occipital  region  of  the  skull,  are  recognizable  at 


BREGMATfC    FONTANELLE 


LAMBOOID 
FONTANELLE. 


SUPRA  OCCIP: 

membrane..^ 


SUPRA    OCCIPITAL 

(cartilage.) 


Fig.  134.- — Median  Sagittal  Section  of  the  Skull  of  a  Foetus  of  the  ninth  month. 

an  early  stage  of  development,  but  at  no  period  in  the  development  of  the 
embryo  has  cranial  segmentation  been  seen  anterior  to  the  otic  vesicle. 

The  Primitive  Membranous  Skull. — The  brain  is  developed  in  the 
same  manner  as  the  spinal  cord  from  the  medullary  plates  of  the  neural 
groove.  In  the  same  manner  the  mesoderm  grows  under  and  over  the 
cephalic  part  of  the  neural  canal,  and  forms  for  it  a  mesenchymal  or  mem- 
branous covering.  The  covering  of  mesoderm  thus  formed  is  the  primitive 
Anlage  of  the  skull  in  the  embryo. 

Membrane  and  Cartilage  Bones. — Only  the  base  of  the  human  skull  is 
developed  in  cartilage,  the  rest  is  developed  in  membrane.  How  has  such 
a  condition  arisen  ?  The  brain  of  amphioxus,  if  it  can  be  said  to  possess 
one,  is  wrapped  in  a  membranous  covering.  In  fishes  with  cartilaginous 
skeletons  this  embryonic  mesodermal  capsule  becomes  chondrified — 
plates  of  cartilage  develop  in  it.  As  in  the  spinal  column,  the  process  of 
chondrification  begins  at  the  base  and  spreads  slowly  round  to  the  crown 
or  dorsum  of  the  head.     The  cartilaginous  cranium  is  an  advance  on  the 


THE  CRANIUM 


141 


membranous  stage.  In  many  fishes  a  further  most  important  element 
is  added.  The  dermal  bony  plates,  to  which  the  placoid  scales  are  fixed, 
are  applied  to  the  cartilage  over  the  sides  and  dorsum  of  the  skull.  Thus 
to  the  cartilaginous  element  of  the  skull  is  added  a  third  element — bone 
formed  in  membrane.  Now,  in  the  mammalian  skull,  and  especially  in 
that  of  man,  the  cerebral  vesicles  grow  so  quickly  that  long  before  the 
process  of  chondrification  has  had  time  to  spread  in  the  membranous 
capsule  from  the  base  to  the  crown,  the  dermal  bones  have  formed,  and  thus 
supplant  the  cartilage  on  the  calvarium.  Hence,  in  the  human  skull, 
while  the  process  of  chondrification  occurs  in  the  base,  and  afterwards  under- 
goes ossification,  the  roof  and  sides  (calvaria)  of  the  skull  are  formed  by 
bones  which,  historically,  are  dermal  bones,  and  hence  are  formed  directly 


bregma 


ant.  font 


Sag.  font 
parietaL 

lambda 
asten'on. 

oocip.  (memb.) 
occip.  (cartilag 


frontal 


pterion 


wing  of  Sphen. 

squamosal 
tympanic  ring 


petro-mast 


Fig.  135.- 


-The  Centres  of  Ossification  for  the  Dermal  Bones  of  tlie  Skull, 
which  are  formed  in  cartilage  are  stippled. 


The  bones 


in  membrane.  The  dermal  bones  of  the  human  skull  are  :  (1)  the  frontal, 
(2)  the  parietal,  (3)  the  inter-parietal  part  of  the  occipital  (the  part  above  the 
superior  curved  lines),  (4)  the  squamous  part  of  the  temporal. 

Thus  the  calvarial  part  of  the  skull  passes  directly  from  the  membranous 
to  the  bony  stage,  while  the  base  of  the  skull,  like  the  spinal  column, 
passes  through  three  stages  :  (1)  membranous,  (2)  cartilaginous,  (3)  bony. 
It  will  be  thus  seen  that  the  base  of  the  skull,  developed  in  cartilage,  is  the 
most  ancient  part,  while  the  dermal  bones,  which  form  the  calvaria, 
represent  a  later  addition. 

Development  of  the  Calvarial  (membranous  or  dermal  part)  of  the 
Skull. — In  the  8th  week  of  foetal  life — the  foetus  being  then  about  25  mm. 
(1  in.)  long — there  appear  on  each  side  of  the  membranous  cranial  capsule 
four  centres  of  ossification  : 

(1)  For  the  frontal  bone,  at  a  point  which  becomes  afterwards  the 
frontal  eminence  (Fig.  135)  ; 

(2)  For  the  parietal,  at  the  position  of  the  parietal  eminence  ;    this 


142 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


centre  is  double  or  even  trijDle  in  nature,  but  the  separate  points  are  placed 
closely  and  soon  fuse  together  ; 

(3)  For  the  squamosal,  at  the  base  of  the  zygoma  (Fig.  135)  ; 

(4)  For  the  membranous  part  of  supra-occipital  (part  above  superior 
curved  line).  Maggi  and  Hepburn  ^  have  shown  that  there  may  be  four 
centres  (two  on  each  side)  in  the  membranous  supra-occipital  (Fig.  137). 

The  two  or  four  occipital  centres  fuse  early  into  one  at  the  position  of 
the  external  occipital  protuberance,  but  occasionally  these  centres  may 
form  two,  three  or  four  separate  bones.  The  two  frontal  ossifications 
fuse  about  the  end  of  the  first  year  ;  the  metopic  suture,^  which  separates 
them,  disappearing  then.  This  suture  occasionally  persists.  One  or  both 
parietals  may  be  divided  by  a  suture  or  by  a  complex  of  sutures.^     The 


WORMIAN 


SUPRA  OCCIP: 
Cmembrane) 


EXOCCIPITAL 


BASI   OCCIPITAL 


SuPRA-OCCIp! 
(membrane) 


Fig.  136. — The  Occipital  Bone  at.  the  4th  month,  showing  pre-interparietal  Wormian 

Bones.     (After  Sappey.) 

Fig.  137. — The  Supra-occipital  from  a  Foetus  of  3  months,  showing  four  Centres 

of  Ossification  for  the  Membranous  Supra-occipital.     (After  Maggi.) 

centres  of  ossification  in  these  cases  have  not  fused.  The  parietal  bones 
ossify  together,  at  the  sagittal  suture,  late  in  life,  commonly  between  the 
35th  and  45th  year,  when  the  growth  of  the  skull  has  entered  a  retrograde 
phase.  The  squamosal  partly  covers  the  petro-mastoid  cartilaginous 
element  and  fuses  with  it  in  the  first  year,  the  temporal  bone  being  thus 
formed.  These  bones,  as  they  are  laid  down,  accurately  follow  the  contour 
of  the  brain.  That  organ  forms  a  relatively  small  sphere  when  ossification 
commences.  Hence  the  convexities  or  eminences  at  the  regions  of  earliest 
formation. 

The  Manner  in  which  these  Bones  are  Developed. — In  Fig.  138  a 
vertical  section  of  the  skull  of  a  foetus  5  months  old  is  represented.  The 
coverings  of  the  brain  are  seen  to  be  then  (1)  scalp,  (2)  a  stout  white  fibrous 
capsule,  (3)  a  fine  membrane  lining  it — the  inner  layer  of  the  dura  mater, 
(4)  the  arachnoid  covering  the  brain  (not  shown  in  figure).     Ossifying 

^  Professor  D.  Hepburn,  Journ.  Anat.  and  Physiol.  1907,  vol.  42,  p.  88. 

2  Professor  T.  H.  Bryce,  Journ.  Anat.  1917,  vol.  51,  p.  153  ;  Dr.  A.  H.  Schultz, 
Amer.  Journ.  Anat.  1918,  vol.  23,  p.  259. 

^  Professor  Patten,  Zeitschrift  fiir  Morph.  und  Anthrop.  1912,  vol.  14,  p.  527  ;  Pro- 
fessor R.  J.  A.  Berry,  Journ.  Anat.  and  Physiol.  1910,  vol.  44,  p.  73. 


THE  CRANIUM 


143 


fibres  whicli  form  the  parietal  are  seen  developing  within  the  capsule  and 
radiating  out  from  the  centre  of  ossification.  The  ossific  fibres,  as  they 
spread  outwards  from  a  common  centre,  unite  by  branches,  thus  forming  an 
irregular  network  with  osteoblasts  and  growing  vessels  within  its  meshes. 
Lower  down  are  seen  the  ossifying  fibres  of  the  squamosal.  The  base  of 
the  skull  is  formed  of  cartilage  which  is  covered,  or  ensheathed,  by  a  peri- 
chondrium continuous  with  the  membranous  capsule.  In  the  cartilage 
appear  the  centres  of  ossification  for  the  sphenoid. 

As  the  bony  fibres  of  the  parietal  spread  out,  they  divide  the  primitive 
cranial  capsule  into  an  outer  layer — the  pericranium — and  an  inner — the 
periosteal  layer  of  the  dura  mater.  At  the  periphery  of  the  bone  and  in 
the  sutures  the  continuity  of  these  two  layers  persists.  The  growth  of  the 
fibroblasts  and  osteoblasts  in  the  sutural  lines  between  the  bones  keeps 


sup.  long.  sin.        ^nt  font 


■fibrous  capsule 


spicules  of 
parietal 


squam. 


pencran. 

—parietal 
dura  mater 


4-temp.  fas. 

tenlp.  muso 
squam. 


basi-sphen.     /^,_ /,,,  ,,,^. 

Fig.  138. — A  Coronal  Section  of  the  Skull  of  a  Foetus,  5  months  old. 

time  with  the  growing  brain  which  expands  the  capsule,  but  there  is,  at 
each  corner  of  the  parietal  bone,  until  the  end  of  the  first  year,  a  part  of 
the  primitive  cranial  capsule  left  unossified.  These  imossified  parts  of  the 
membranous  capsule  are  the  fontanelles. 

The  Fontanelles. — There  are  five  fontanelles  connected  with  each 
parietal  bone,  one  at  each  of  its  rounded  angles,  and  one,  the  sagittal 
(Fig.  135),  which  occurs  between  the  radiating  fibres  of  the  parietal  near 
the  posterior  end  of  the  sagittal  suture.  The  parietal  foramen  marks  its 
position  in  the  adult.  In  about  15  %  of  children  this  fontanelle  is  un- 
closed at  birth  ;  a  large  parietal  foramen  may  permanently  mark  its 
situation.  The  posterior  inferior  fontanelle,  situated  at  the  asterion 
(Fig.  135),  the  anterior  inferior  at  the  pterion,  and  the  posterior  superior  at 
the  lambda,  close  before  or  about  the  time  of  birth.  Separate  ossifications, 
which  become  Wormian  bones,  are  often  developed  in  the  primitive  capsule 
of  the  skuU  at  those  three  fontanelles  and  thus  close  them.  The  anterior 
superior  fontanelle,  at  the  bregma,  cannot  be  distinctly  felt  during  life  after 
the  first  year  (Warner),  but  it  is  not  completely  closed  until  the  second 


144     HUMAN  EMBRYOLOGY  AND  MORPHOLOaY 

year  is  nearly  over.  This  fontanelle  is  lozenge-shaped,  being  bounded  by 
four  bones,  viz.  the  two  parietals  and  two  frontals.  The  bregmatic  or 
anterior  superior  and  lambdoid  or  posterior  superior  fontanelles  are  median 
and  common  to  both  parietals. 

The  membrane-formed  bones  consist  at  first  of  a  thin  lamella  of  osseous 
fibres  radiating  out  from  the  point  at  which  ossification  commenced.  The 
osteoblasts  beneath  the  pericranium  on  the  outer  surface  of  the  lamella 
and  the  dura  mater  on  the  inner  surface,  deposit  bone,  and  by  the  5th 
year  an  outer  and  an  inner  table,  with  diploic  tissue  between,  are  developed. 
Into  the  diploe  of  the  frontal  bone  protrude  the  growing  buds  of  the  two 
frontal  sinuses.  As  the  brain  expands  new  bone  is  formed  at  the  sutures 
to  increase  the  capacity  of  the  skull,  but  the  operation  of  craniotomy  to 
allow  the  expansion  of  a  confined  brain,  by  the  formation  of  a  new  suture, 
is  founded  on  the  assumption  that  the  arrest  of  brain-growth  in  micro- 
cephalic idiots  is  due  to  the  closure  of  the  sutures,  whereas  it  is  probably  due 
to  an  inherent  defect  in  the  growth  of  the  brain.  We  frequently  see  skulls 
where  one  or  more  sutures  have  been  prenaaturely  closed,  but  in  such 
cases  there  has  been  compensatory  growth  at  other  sutures,  giving  rise 
to  a  peculiarity  in  cranial  form.  Growth  of  the  cranial  cavity  could  take 
place  by  a  deposit  of  bone  on  the  outer  table  and  an  absorption  from  the 
inner  ;  for  this  manner  of  growth,  sutures  are  unnecessary.  The  syno- 
stosis of  the  sutures  does  not  necessarily  prevent  growth  ;  synostosis  of 
the  skull  bones  occurs  only  when  the  brain  has  ceased  to  expand.  If  the 
brain  of  the  infant  is  arrested  in  its  growth,  premature  ossification  of  the 
sutures  occurs,  the  condition  of  microcephaly  resulting  therefrom.  In 
hydrocephaly,  when  the  ventricles  become  enormously  dilated,  the  mem- 
branous capsule  of  the  cranium  expands  so  quickly  that  the  process  of 
ossification  cannot  keep  up  with  its  rapid  growth.  Hence  in  hydrocephaly 
the  fontanelles  are  enormous.  The  growing  points  of  ossific  fibres  are 
detached  and  form  Wormian  bones.  The  cartilaginous  part  of  the  skull  is 
scarcely  afiected  in  this  disease.  The  membrane-formed  part  of  the  skull 
is  liable  to  diseases  which  do  not  affect  the  cartilage-formed  part.  The 
dura  mater  is  very  adherent  to  the  bones  formed  in  cartilage. 

Development  of  Bones  formed  in  Cartilage. — (1)  The  Occipital  Bone. 

— The  occipital  bone  is  developed  from  the  parachordal  cartilages.  Two 
cartilaginous  bars,  although  appearing  separately  in  the  development  of 
fishes,  are  united  from  their  first  appearance  in  the  human  embryo,  forming 
a  basilar  plate  (Robinson).  The  plate  is  formed  in  the  mesenchymal  sheath 
of  the  notochord,  its  centre  of  chondrification — the  first  in  the  base  of  the 
skull- — appearing  at  the  end  of  the  1st  month  of  development.  The  basal 
plate  may  be  regarded  as  a  continuation  of  the  vertebral  bodies,  while 
the  lateral  processes  (Fig.  140)  which  are  perforated  at  their  bases  by  the 
foramen  or  foramina  for  the  hypoglossal  nerve  and  which  separate  the 
jugular  foramen  in  front  from  the  foramen  magnum  behind,  may  be  re- 
garded as  a  continuation  of  the  neural  arch  series.^     Fused  to  the  lateral 

^  For  the  variations  in  the  manifestation  of  partly  separated  occipital  vertebrae 
see  Gladstone  and  Powell,  Journ.  Anat.  1915,  vol.  49,  p.  190  ;  Elliot  Smith,  Brit.  Med. 
Journ.  1908,  II.  p.  594.     See  also  references  p.  56. 


THE  CRANIUM  145 

process  and  also  to  the  otic  capsule  is  the  roof  plate  already  mentioned — 
the  tectal  plate  (Fig.  132).  While  the  lateral  processes  never  reach  the 
posterior  or  dorsal  margin  of  the  foramen  magnum,  it  is  quite  otherwise 
with  the  right  and  left  tectal  plates ;  they  extend  round  the  hind-brain 
until  they  meet  and  unite,  thus  forming  the  posterior  margin  of  the  foramen 
magnum  and  the  supra-occipital  plate  of  cartilage.  Thus  the  cartilaginous 
basis  of  the  occipital  bone  is  formed  out  of  three  elements  on  each  side — - 
the  basal  j^late  representing  the  centre  and  hypochordal  arches  of  cervical 
vertebrae,  the  lateral  processes,  corresponding  to  the  neural  vertebral  arches 
and  an  extra  element — the  tectal  plate. 

In  Fig.  140  the  condition  of  the  occipital  region  is  shown  in  a  5th-month 
foetus.  Four  centres  of  ossification  appear  in  the  tectum  (Fig.  137),  and 
quickly  fuse  to  form  the  cartilaginous  j)art  of  the  supra-occipital.     A 


PROC.  ALAR/S 

ALA    TEMP. 


DORSUM  SELLAE 


BASILAR  PLATE 

INT.AUO.  MEAT. 


HI  AT.  VESTIB, 


Jug  .  FOR. 

CONDYL.  FOR. 


AUDIT-CAP. 
TECTUM 

LAST  OCC'^VERT 
ATLAS 
AXIS 

Fig.  139. — Cranial  Aspect  of  the  Basal  Plate  and  Occipital  Parts  of  the  Chondro- 
cranium  of  a  Human  Foetus  in  the  8th  week  of  development.     (Warren  Lewis.) 

suture  between  the  membranous  and  cartilaginous  parts  is  clearly  visible — 
especially  near  the  fontanelle  at  the  asterion.  The  membranous  and 
cartilaginous  parts  of  the  supra-occipital  become  completely  fused  soon 
after  birth.  It  will  be  observed  that  the  process  of  fusion  between  the 
lateral  parts  of  the  cartilaginous  supra-occipital  is  not  complete  in  the 
5th  month  (Fig.  140).  The  occipital  fontanelle  projects  upwards  between 
them  from  the  foramen  magnum.  This  fontanelle  is  filled  by  a  continuation 
of  the  posterior  atlanto-occipital  ligament,  and  becomes  closed  soon  after 
birth.  It  is  the  most  common  site  of  a  cerebral  meningocele — ^a  saccular 
protrusion  of  the  membranes  of  the  brain  which  contains  cerebro-spinal 
fluid,  and  usually  a  part  of  the  occipital  lobes  distended  by  a  dilatation  of 
the  posterior  horns  of  the  lateral  ventricles. 

Separate  centres  of  ossification  appear  in  the  occipital  cartilages  to  form 
(1)  the  basi-occipital,  (2)  the  two  exoccipitals,  and  (3)  the  supra-occipital.^ 
The  occipital  consists  of  four  pieces  until  the  fourth  year,  when  synostosis 
occurs.     The  occipital  condyles  are  formed  from  the  exoccipitals  and 

^  For  a  very  complete  account  of  the  dates  at  which  all  centres  of  ossification  appear 
in  the  skeleton  see  Mall,  A7ner.  Jouni.  of  Anat.  1906,  vol.  5,  p.  433. 

K 


146 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


font  at 
asten'on 


basi-occipital,  the  exoccipital  element  constituting  in  the  adult  by  far 
the  larger  part,  but  when  the  condyles  first  appear  they  are  con- 
tinuous at  the  anterior  border  of  the  foramen  magnum,  forming  a 
single  or  median  condyle  as  in  reptiles,  birds,  and  lower  mammals.  The 
foramen  for  the  hypoglossal  nerve,  which  may  be  subdivided  into  two 
or  even  three  compartments,  is  formed  between  the  two  elements  and 
thus  corresponds  to  the  inter-vertebral  series.  The  occipital  protuber- 
ance is  formed  by  both  membranous  and  cartilaginous  parts  of  the 
supra-occipital. 

(2)  The  Petro-mastoid  forms  part  of  the  base  of  the  skull.  We  shall 
see  that  the  petrous  bone  (p.  224)  is  primarily  developed  as  an  independent 
cartilaginous  capsule  round  the  inner  ear,  but  at  an  early  date  (6th  week) 
it  fuses  at  certain  points  with  the  parachordal  basis  of  the  occipital  bone, 

parietal 
lambda 

supra-occip. 
(memb.) 

fontanelle 
petro-mast. 
supra-occ.  (cartilj 
occip.font, 
atlas 
axis 

Fig.  140. — The  Occipital  Eegion,  in  a  Foetus  of  5  months. 

while  an  extension  from  the  mastoid  part  of  the  capsule  enters  into  the 
formation  of  the  tectum.  Even  as  late  as  the  thirtieth  year  remnants  of 
the  tectal  cartilage  may  be  found  between  the  petro-mastoid  and  occipital 
bones,  especially  between  the  jugular  process  of  the  occipital  and  the 
mastoid.  The  fibro- cartilage  in  the  foramen  lacerum  medium  and  perhaps 
Eustachian  cartilage,  which  is  continuous  with  it,  are  remnants  of  the 
periotic  cartilaginous  capsule. 

(3)  Trabeculae  Cranii.- — The  basilar  plate,  containing  the  notochord  and 
fashioned  out  of  the  parachordal  cartilages  terminates  in  the  dorsum 
sellae,  in  the  hind  wall  of  the  pituitary  fossa.  The  prechordal  part  of  the 
base  of  the  skull,  in  the  lowest  vertebrates,  appears  first  as  two  irregular 
plates  of  cartilage — the  trabeculae  cranii  (Fig.  141).  Even  in  the  mam- 
malian skull  the  trabeculae  can  still  be  traced  in  the  pituitary  region 
(Fawcett).  Their  posterior  extremities  fuse  round  the  anterior  termination 
of  the  notochord  with  the  basilar  plate.  The  buccal  part  of  the  pituitary 
grows  into  the  cranial  cavity  in  front  of  the  notochord  and  keeps  the  two 
cartilages  apart ;    but  in  front  of  the  pituitary  the  two  bars  fuse  in  the 


THE  CRANIUM 


147 


tmbeciila 


coma 


trabecula 


middle  line.     The  mesial  fused  parts  of  the  trabeculae  grow  into  the 
embryological  basis  of  the  nasal  septum  (Fig.  142).     The  posterior  part 
of  the  median  fused  bars  forms  the 
cartilaginous  basis  of  the  pre-sphenoid 
and  basi-sphenoid  (Fig.  142). 

Development  of  the  Sphenoid. — 

Recently  Professor  Fawcett  ^  has 
examined  the  manner  in  which  the 
cartilaginous  basis  of  the  sphenoid 
is  formed  in  the  human  embryo. 
The  mesodermal  or  mesenchymatous 
basis  of  the  sphenoid  becomes  chon- 
drified  during  the  second  month — 
right  and  left  centres  representing 
the  original  trabeculae.  While  the 
cartilage,  in  which  the  centres  for  ossi- 
fication of  the  basi-  and  pre-sphenoids 
appear,  is  formed  out  of  the  trab  ecular 
or  prechordal  plate,  the  great  and 
small  wings  have  a  separate  origin 

Fia. 


pituitary 


peri  otic 
\pefro-mast ). 


parach.  cart 


notoch. 


141. — Diagram  of  the  Trabeculae 
Cranii,  Parachordal  Cartilages,  and  Periotic 
Capsules. 


OPTIC    ^Off 


We  have  already  seen  (p.  137)  that  on 
each  side  of  the  prechordal  plate  there 
are  formed  two  plates  of  cartilage, 
rudiments  of  the  lateral  wall  and  roof  of  the  primitive  cartilaginous  cranium 
(Fig.  133).  The  anterior  of  these — the  orbito-sphenoids — form  the  cartila- 
ginous basis  of  the  lesser  wings.     In  the  8th  week  of  development  (Fig.  142) 

each  is  a  sickle-shaped  plate 
lying  over  the  stalk  of  the 
optic  vesicle,  sending  one 
process  under  the  optic 
nerve  to  join  the  cartila- 
ginous'prominence — the  pro- 
cessus hypochiasmata — from 
which  the  muscles  of  the 
eyeball  take  origin.  The 
other  process  of  the  orbito- 
sphenoid  fuses  with  its 
fellow  above  the  j^rechordal 
plate  and  thus  completes 
the  optic  foramina  (Fig. 
143).  The  great  wing  or  ali- 
sphenoid  arises  in  a  rather 
complicated    manner.       In 


BASISPHEN. 


TEMP-  WINd 

ALAR    PROC 


ORB.  yvif^G- 


PRESPHEN- 
HYPOCH  PROC. 


DOR    SEL. 

Fig.  142. — The  Prechordal  Base  of  the  Chondrocranium  in 
the  8th  week  of  development.    (Warren  Lewis.) 

manner 
the  8th  week  it  is  represented  by  two  small  nodular  masses  of  cartilage 
(Fig.  142),  the  alar  process  attached  to  the  prechordal  plate  and  the  tem- 
poral wing.  The  internal  carotid  artery  lies  on  the  mesial  side  of  the  alar 
process,  which  is  represented  merely  by  the  Lingular  process  of  the  fullv 
1  Journ.  Anat.  1910,  vol.  44,  p.  303.     See  also  references,  p.  135. 


148 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


developed  bone.  The  temporal  wing  lies  under  the  Gasserian  ganglion 
and  separates  the  2nd  from  the  3rd  division  of  the  nerve.  The  mesodermal 
tissues  round  the  temj^oral  wing  undergo  a  secondary  chondrification, 
and  it  is  from  this  new  formation  that  the  greater  part  of  the  ali-sphenoid 
is  formed  ;  ■  as  it  extends  it  encloses  the  2nd  and  3rd  divisions  of  the  fifth 
nerve,  the  round  and  oval  foramina  being  thus  formed.  A  gap  remains 
between  the  orbito-sphenoid  and  ali-sphenoid  to  form  the  sphenoidal 
fissure.  The  dorsum  sellae  may  have  a  separate  centre  of  chondrification. 
At  birth  the  sphenoid  bone  consists  of  three  parts,  the  great  wings 
being  separated  from  the  rest  of  the  bone.     The  sphenoidal  turbinate 


NASAL    CART 


ETHMOIDAL 


cribriform  pl: 
orbito-sph: 
optic   for: 

ala   tempor 

alar    proc 
Meckel's  cart: 


Auditory 


NASAL  CART  : 

FRONTAL 


:^^^—  E.THMOiDAL 


PARIETAL    PL 


TECTUM 


ORBITO-SPH: 

OPTIC  FOR: 

ALAR    PROC: 
ALA   TEMPOR: 

GASSERIAN  GANG: 
PARIETAL  PL: 
AUDITORY 


Fig.  143,  A. — Left  half  of  the  Cartilaginous  Basis  of  the  Skull   in  a  Foetus  of 
3J  months.     (After  KoUmann.) 
B. — Right  half  of  the  Cartilaginous  Basis  of  the  Skull  in  a  Foetus  of 
2i  months.     (After  Fawcett.) 

bones,  afterwards  inflated  by  the  development  of  the  sphenoidal  air  sinuses, 
are  then  nodules  of  bone,  surrounded  by  cartilage.  They  also  are  separate 
and  are  derived  from  the  lateral  ethmoidal  cartilaginous  plates  which 
represent  the  olfactory  capsule.  The  internal  pterygoid  plates  are  also 
separate  ossifications  laid  down  in  the  membrane  over  a  plate  of  cartilage, 
representing  part  of  the  palato-quadrate  bar  of  lower  vertebrates  (Fig. 
133).  Only  its  hamular  process  is  formed  in  cartilage  (Fawcett).  The 
internal  becomes  adherent  to  the  external  plate  during  the  fourth  month 
of  foetal  life.  The  external  plate  is  developed  as  a  membranous  outgrowth 
from  the  ali-sphenoids  or  great  wings.  The  pre-sphenoid  unites  with  the 
basi-sphenoid  in  the  8th  month ;  the  great  wings  unite  with  the  basi- 
sphenoid  soon  after  birth.  The  lingula  (alar  proc.  Fig.  143,  B)  which 
bounds  the  outer  side  of  carotid  groove  is  ossified  from  a  centre  which 
appears  during  the  4th  month  of  foetal  life. 


THE  CRANIUM  149 

Tlie  wings  of  the  splienoid  develop  in  the  orbital  region  of  the  primitive 
skull  (Fig.  133).  The  enormous  expansion  of  the  cerebral  vesicles  and  the 
evolution  of  a  new  system  of  mastication  have  worked  a  revolution  in 
the  primitive  orbital  region  ;  the  temporal  lobes,  as  it  were,  have  burst  the 
ancient  cartilaginous  wall.  The  ala-temporalis  appears  first  in  the  embryo 
as  a  process  from  which  the  muscles  of  mastication  take  origin  (Fawcett). 

The  Pituitary  Body  is  developed  between  the  trabeculae  cranii ;  the 
pre-sphenoid  is  formed  in  front  of  it  and  the  basi-sphenoid  behind  it.  A 
canal  may  remain  in  the  foetal  or  even  adult  bone  to  mark  the  point  of 
ingress  of  the  buccal  part  of  the  pituitary.^  The  wings  of  the  vomer  cover 
the  opening  of  the  pituitary  canal  on  the  pharyngeal  aspect  of  the  skull, 
if  it  be  present.  On  the  cerebral  aspect  it  opens  at  the  olivary  eminence 
which  also  marks  the  union  of  the  pre-  and  the  basi-sphenoids.  The 
writer  has  seen  a  child,  in  which  the  trabecular  cartilages  had  remained 

orbito-sph. 

presphenoidX         ^ .   J 
-^         optic  for. 

for.  rot 


ext  pteryg.  proc. 
for  ovale         I  \\    lingula 

int.  pter   ^^''-'P^' 

Fig.  144. — The  Sphenoid  in  a  Foetus  of  4  months.     The  Centres  of  Ossification  are 
deeply  shaded.     (After  Sappey.) 

apart,  leaving  a  wide  gap  through  which  the  pituitary  projected  within  the 
sej)tum  of  the  nose.  The  pre-sphenoid  and  afterwards  the  basi-sj^henoid 
are  much  altered  by  the  growth  of  the  sphenoidal  sinuses  which  commence 
to  expand  rapidly  about  the  7th  year.^  The  great  wings  support  the 
temporal  poles  of  the  brain,  their  size  depending  on  the  development  of  that 
part  of  the  brain.  They  are  much  larger  in  man  than  in  any  other  mammal, 
owing  to  the  great  size  of  the  human  temporal  lobes.  The  small  wings 
project  within  the  vallecula  Sylvii.  In  the  early  foetus  the  dorsum  sellae 
is  enormously  developed,  and  fills  the  deep  and  sharp  angle  between  the 
mid-brain  and  fore-brain  (Fig.  85). 

The  Ethmoid. — The  cartilaginous  basis  of  the  skull  is  completed  in 
front  by  the  ethmoid  ;  on  its  upper  surface  rest  the  olfactory  bulbs.  In 
the  primitive  skull  (Figs.  130,  133)  the  olfactory  capsule,  out  of  w^hich  the 
cartilaginous  ethmoid  has  been  evolved,  is  far  in  front  of  the  space  which 
contains  the  fore-brain.  It  has  been  brought  within  the  floor  of  the 
cranial  cavity  by  a  double  process — by  a  shortening  of  that  part  of  the 
trabecular  plate  which  unites  the  sphenoid  to  the  olfactory  or  ethmoidal 
capsule,  and  by  the  forward  extension  of  the  cerebral  vesicles  which  have 

1  H.  Wrai,  Anat.  Hefte,  1907,  vol.  33,  p.  411  (Cranio-pharyngeal  Canal). 

2  V.  Z.  Cope,  "  Ossific.  of  Sphenoid,"  Journ.  Anat,  1917,  vol.  51,  p.  127, 


150      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

pushed  their  way  into  the  forehead  until  they  project  beyond  the  olfactory 
region.  The  cribriform  plate  is  formed  in  the  4:th  month  ;  up  till  then  a 
gap  separates  the  lateral  mass  from  the  septal  or  trabecular  plate  (Fig.  143). 
Formation  of  Foramina  in  Bone. — The  foramina  of  the  skull  are 
formed  in  one  of  three  ways  (Bland-Sutton)  : 

(1)  By  the  union  of  two  bones  ;  examples  of  this  form  are  the  jugular 
foramen,  sphenoidal  fissure,  Glaserian  fissure,  etc. 

(2)  By  the  union  of  two  elements  of  one  bone  ;  the  anterior  condyloid 
foramina,  optic  foramina,  the  foramen  magnum,  aqueductus  Fallopii,  etc. 

(3)  By  the  enclosure  of  a  notch  on  the  edge  of  a  bone  of  which  the  fora- 
men ovale  is  the  best  example.  This  foramen  is  at  first  a  notch  in  the 
posterior  border  of  the  great  wing  of  the  sphenoid  (Fig.  144)  ;  it  remains 
in  this  condition  in  all  mammals  except  man.  In  him  the  margins  of  the 
bone  on  each  side  grow  out  and  fuse,  and  thus  convert  the  notch  into 
a  foramen.  Other  examples  are  the  foramen  spinosum,  the  foramen 
rotundum,  parietal  foramen,  mastoid,  etc. 

Wormian  Bones. — In  the  six  fontanelles  which  occur  at  the  parietal 
angles  ossific  centres  frequently  appear.  Fontanelle  ossifications  form 
Wormian  bones.  They  occur  most  frequently  at  the  posterior  angles  of  the 
parietal  (Lambda  and  Asterion) ;  they  are  also  common  at  the  Pterion 
(epipteric  Wormian)  but  rare  at  the  Bregma.  The  Wormian  at  the  last- 
mentioned  point  receives  the  name  of  os  anti-epilepticum.  Much  confusion 
has  been  caused  by  naming  a  large  Wormian,  which  may  occur  in 
the  lambdoidal  (posterior-superior)  fontanelle,  the  inter-parietal  bone. 
Wormian  or  sutural  bones  are  particularly  numerous  in  the  skulls  of 
infants  who  have  been  the  subjects  of  hydrocephaly.  It  is  possible  that, 
during  the  rapid  expansion  of  the  skull,  the  tips  of  ossifying  fibres  become 
detached,  thus  forming  separate  centres  of  ossification  in  the  sutures  and 
fontanelles. 

The  Inter-parietal  Bone. — It  has  already  been  shown  that  the  part  of 
the  supra-occipital  above  the  superior  curved  lines  is  developed  from 
membrane  by  four  centres  of  ossification,  and  is  at  first,  and  almost  until 
birth,  nearly  separated  from  the  lower  part  developed  from  cartilage 
(Figs.  137,  140).  The  membranous  part  of  the  supra-occipital  represents 
the  inter-parietal  bone.  In  marsupials,  ruminants  and  ungulates,  the 
inter-parietals  fuse  with  the  parietals,  and  not  with  the  occipital.  In 
rodents  they  fuse  with  both  occipitals  and  parietals.  In  primates  and 
carnivora,  as  in  man,  they  fuse  with  the  occipital.  It  is  extremely  rare 
to  find  the  whole  inter-parietal  as  a  separate  bone  in  man,  but  a  large 
Wormian,  partly  replacing  the  inter-parietal,  is  very  frequent.  Such  a 
Wormian  bone,  if  large,  is  named  variously,  os  epactal,  os  Incae,  os 
triquetrum,  or  pre-interparietal. 

The  Post-frontal  does  not  occur  in  mammals  as  a  separate  bone  ;  in 
them  it  has  fused  with  the  frontal,  and  forms  that  part  of  the  bone  which 
articulates  with  the  great  wing  of  the  sphenoid  and  malar.  A  Wormian 
bone — the  epipteric— which  is  occasionally  developed  in  the  fontanelle 
at  the  pterion,  may  be  mistaken  for  it.  Traces  of  a  true  post-frontal, 
partly  separated  from  the  frontal,  rarely  occur  in  man, 


THE  CRANIUM 


151 


The  Cephalic  Index. — Anthropologists  have  employed  the  shape  of  the 
head  as  a  character  in  classifying  the  races  of  mankind.  The  cephalic  index 
is  used  to  express  the  shaj)e  of  the  head.  It  states  the  proportion  that  the 
breadth  bears  to  the  length  of  the  skull  (Figs.  145,  A,  B).  The  length  or 
long  diameter  of  the  skull  is  usually  measured  from  the  glabella  to  the  most 
projecting  point  of  the  occiput^commonly  situated  over  the  occipital 
poles  of  the  brain  ;  the  breadth  or  widest  diameter  is  measured  between 
the  widest  points — usually  some  distance  below  the  parietal  eminences. 
If  the  length  of  a  skull  is  100  mm.  and  the  breadth  75,  the  cephalic  index 
of  that  skull  is  75,  i.e.  the  breadth  is  75  %  of  the  length.  Human  races, 
on  an  average,  are  either  Dolichocephalic  (long-headed),  the  breadth 
being  75  %  or  less  of  the  length  ;  Brachycephalic,  in  which  the  breadth  is 
80  %  or  more  of  length  ;  or  Mesaticephalic,  in  which  the  breadth  is  between 

100^ 

100\ 


60-75% 


..J  80-90% 


Fia.  145,  A. — Diagram  of  a  Long-head  (Dolichocephalic). 
B. — Diagram  of  a  Short-head  (Brachycephalic). 

75  %  and  80  %  of  the  length.  Various  methods  are  employed  in  estimating 
the  height  of  the  skull,  but  the  best  is  that  which  takes  the  upper  margin 
of  the  external  auditory  meatuses  as  representing  the  basal  plane.  The 
height  is  measured  from  this  plane  to  the  highest  point  in  the  sagittal 
suture,  when  the  skull  is  oriented  so  that  the  lower  border  of  the  orbit  and 
the  middle  of  the  meatus  are  in  one  plane  (see  Duckworth,  Morphology  and 
Anthropology). 

The  English  people  have  an  average  cephalic  index  of  78,  the  South 
Germans  83,  but  it  must  be  remembered  the  individuals  of  every  race  show 
a  wide  range  of  variation.  It  will  be  seen  that  the  topography  of  the 
brain,  worked  out  by  German  surgeons,  cannot  be  applied  to  the  longer 
English  heads  without  modification. 

Factors  which  determine  the  Shape  of  Head. — The  shape  of  the 
skull  depends  (1)  on  the  size  and  shape  of  the  brain  ;  (2)  on  the  size  and 
strength  of  the  muscles  which  arise  from  it — the  muscles  of  mastication, 
or  are  inserted  to  it — the  muscles  of  the  neck.  Brain  growth  is  by  far  the 
most  important  factor,  but  we  do  not  know  the  conditions  which  flatten 
the  brain  from  side  to  side  in  dolichocephalic  races,  or  shorten  it  frora 


152      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

frontal  pole  to  occipital  pole  in  brachyceplialic  races.  Muscular  action  can 
only  exercise  a  minor  effect.  Professor  Arthur  Thomson  ^  has  shown  that 
there  is  a  correlationship  between  dolichocephaly  and  the  size  of  the  tem- 
poral muscles — which  are  relatively  large  in  long-headed  races — and  the 
shape  and  mechanism  of  the  mandible.  It  is  to  be  remembered  that  (1) 
the  muscles  of  mastication  and  of  the  neck  undergo  their  greatest  develop- 
ment between  the  12th  and  28th  years  ;  (2)  before  that  time  the  brain  has 
almost  completely  attained  its  adult  size  and  shape  ;  (3)  all  the  evidence 
obtained  from  measurements  in  the  living  indicates  that  the  changes  in 

ACROCEPHAL.Y 

SCAPHOCEPHALY  w-f35^''?^^^^^r-pr^*^ 


Fig.  146. — Outlines  of  Abnormal  Skulls,  showing  a  contrast  in  shape. 

cranial  form  which  take  place  then  affect  its  external  contour,  leaving  the 
shape  of  the  cranial  cavity  unaffected. 

Abnormal  Crania.^ — It  is  possible  that  light  will  be  thrown  on  the 
factors  which  determine  head-form  by  the  study  of  certain  pathological 
conditions.  In  the  disease  known  as  Acromegaly,  where  there  is  always 
a  great  enlargement  of  the  pituitary  gland,  the  skull  undergoes  peculiar 
growth  changes.  The  supra-orbital  ridges  become  greatly  developed,  the 
face  elongates,  the  temporal  lines  from  which  the  temporal  muscles  arise, 
grow  upwards  on  the  side  of  the  skull,  thus  increasing  the  area  of  the 
temporal  muscles.  At  the  same  time  the  lines  which  mark  the  attach- 
ment of  the  muscles  of  the  neck — the  mastoid  processes,  superior  curved 

1  Arthur  Thomson,  Man's  Cranial  Form,  Oxford,  1903. 

2  For  skull  in  achondroplasia  see  Dr.  Murk  Jansen,  Achondroplasia,  Leyden,  1912  ; 
A.  Keith,  Journ.  Anat.  and  Physiol.  1913,  vol.  47,  p.  189.  For  AcromegaUc 
changes:  Keith,  Lancet,  1911,  vol.  1,  p.  993. 


THE  CRANIUM 


15J 


lines  and  external  occi2:)ital  protuberance — also  increase  greatly  in  size. 
In  achondroplasia  and  in  rickets  the  skull  assumes  characteristic  forms 
due  to  a  disturbance  in  the  growth  of  the  base  of  the  skull.  To  a  certain 
degree  the  growth  of  the  cranial  bones  is  regulated  by  internal  secretions. 
In  Fig.  146  two  common  types  of  abnormal  skull  forms  are  shown.  They 
are  contrasted  types  ;  in  one — Acrocephaly  or  steeple-skull — -the  base  is 
abnormally  short,  owing  to  an  arrest  of  growth  at  the  junction  of  the  pre- 
sphenoid  and  ethmoid.  Compensation  is  obtained  by  an  upward  growth 
of  the  brain,  thus  heightening  the  roof.  In  severe  cases  the  optic  nerves  may 
be  pressed  on,  and  blindness  thus  caused.  In  the  second  type — Scapho- 
cephaly, or  boat-shaped  skull — the  cranium  is  very  narrow  from  side  to 
side,  while  the  calvarial  arc — from  nasion  to  opisthion  (posterior  border  of 
foramen  magnum) — is  greatly  elongated.     In  scaphocephaly  there  is  an 


NA310N 
FACIAL  ANGLE 


PR05THI0N--' 


GLE.of  FLEXION 


Fig.  147,  A. — The  Facial  Angle  as  estimated  by  two  lines  drawTi  from  the  Nasion 
to  the  Basion  and  to  the  Prosthion  (incisor  alveolus). 
£.— Method  of  estimating  the  degree  of  flexion  and  extension  of  the  cranial 
axis,  a,  anterior  border  of  cribriform  plate  ;  b,  on  olivary  groove 
in  front  of  olivary  eminence,  a,  b,  trabecular  axis  ;  b — basion  =  the 
chordal  axis.  The  angle  of  flexion  is  contained  by  the  two  lines 
meeting  at  b. 

arrest  of  growth — often  a  synostosis — along  the  sagittal  suture.  In 
acrocephaly  the  coronal  suture  is  closed.  In  these  two,  and  in  allied 
conditions,  there  is  a  certain  amount  of  evidence  which  points  to  a  dis- 
turbance in  the  function  of  the  glands  of  internal  secretion. 

The  Facial  Angle  ^  is  the  angle  at  which  the  face  projects  from  the  axis 
of  the  skull  (Figs.  147,  148).  The  skull  consists  in  man,  as  in  all  mammals, 
of  two  parts — the  facial  part  (splanchnocranium),  which  carries  the  teeth  and 
is  developed  according  to  their  size,  and  the  brain  capsule  (neurocranium), 
which  depends  on  the  size  of  the  brain.  The  smaller  the  brain  and  the 
larger  the  face,  the  more  does  the  face  project  in  front  of  the  skull,  and, 
therefore,  the  greater  is  the  facial  angle,  and  vice  versa.  It  will  thus  be  seen 
that  the  facial  angle  is  to  a  certain  degree  an  index  of  brain  development. 
It  is  smallest  in  the  most  highly  developed  races  of  man  ;  it  is  larger  in  the 
lower  races,  and  larger  still  in  the  anthropoids  ;  it  increases  in  size  with  the 

^  For  a  description  of  the  various  methods  of  estimating  the  facial  angle  see  Duck- 
worth's Morphology  and  Anthropology,  2nd  Edition,  1915. 


154      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

advent  of  the  permanent  teeth  and  the  necessary  increase  in  the  size  of 
the  face.     It  is,  therefore,  greater  in  the  adult  than  in  the  newly  born. 

Flexion  of  the  Cranial  Axis. — In  Figs.  147,  A,  and  148  the  axis  of  the 
cranial  base  is  represented  by  a  line  drawn  from  basion  to  nasion,  but  it  is 
quite  apparent  that  this  line  does  not  represent  the  axis  accurately.  The 
truth  is  that  there  are  two  parts  in  the  cranial  axis  which  are  functionally 
as  well  as  morphologically  distinct,  the  chordal  and  prechordal  parts  (p. 136). 
In  the  higher  primates — especially  in  man — the  prechordal  part  is  bent 
downwards — or  flexed — on  the  chordal.  The  manner  in  which  the  degree 
or  angle  of  flexion  may  be  measured  is  shown  in  Fig'.  147,  B  ;  it  is  a  much 
openej"  angle  in  anthropoids  than  in  man.     The  degree  of  flexion  is  most 


TZ->»EXT:    INION 


^^teXi"*^- -  0  P 1  ST  H I  ON 
^f   TEMPORO   OCCIP:  CREST 

O'CCIP:  CONDYLE 


Fig.  148.— Profile  of  the  Cranium  of  an  Immature  Chimpanzee,  showing  the  ascent 
of  the  Temporal  Ridges,  the  formation  of  Occipital  Crests  and  the  lines  of  the 
Facial  Angle. 

variable  in  man  ;  in  cases  where  the  flexion  is  great  the  forehead  is  pro- 
jecting and  the  face  receding,  the  facial  angle  being  apparently  small.  If 
there  is  a  great  degree  of  extension  of  the  axis,  then  the  forehead  is  receding, 
and  the  lower  part  of  the  face  projecting  or  prognathous.  Thus  the  facial 
angle  is  not  a  safe  guide  to  the  degree  of  prognathism  or  face  projection, 
because  it  may  be  exaggerated  or  masked  by  the  extension  or  flexion  of 
the  cranial  base. 

The  Para-occipital  Process  is  sometimes  present  in  man,  and  projects 
downwards  from  the  jugular  process  of  the  occipital  bone.  The  rectus 
capitis  lateralis  is  inserted  to  it.  The  process  represents  the  para-occipital 
process,  which  is  so  highly  developed  in  four-footed  mammals.  The  para- 
mastoid  process  projects  from  the  temporal  bone  lateral  to  the  para-occipital 
(Parsons). 

Upgrowth  of  the  Temporal  and  Occipital  Ridges  or  Curved  Lines.— 
In  lower  animals,  such  as  the  ape  or  dog,  a  great  increase  in  the  develop- 


THE  CRANIUM 


155 


merit  of  the  temporal  and  nuchal  muscles  takes  place  with  the  eruption  of 
the  permanent  teeth,  the  area  of  their  origin  from  the  skull  being  neces- 
sarily enlarged.  The  ridges  of  bone  which  mark  the  limit  of  attachment 
of  these  muscles,  the  temporal  and  occipital  ridges,  ascend  on  the  skull  as 
waves  of  bone  before  the  growing  muscles.  The  ridges  may  meet,  as  in 
apes,  along  the  sagittal  and  lambdoidal  sutures  and  form  great  crest-like 
upgrowths.  In  Fig.  148  the  position  of  the  temporal  lines  in  a  juvenile 
chimpanzee  is  shown  ;  they  are  approaching  the  sagittal  suture.  They 
have  extended  backwards,  and  met  with  the  occipital  lines,  which  are 
ascending  above  the  attachment  of  the  growing  muscles  of  the  neck.  The 
temporal  and  occipital  lines  are  seen  to  be  fused  together  to  form  a  temporo- 
occipital  crest.  At  the  same  time  the  temporal  lines  spread  forwards  on 
the  frontal  region,  the  frontal  extension  being  accompanied  by  a  marked 
growth  of  the  supra-orbital  ridges  and  of  the  zygomatic  arches.     Thus  the 

hind  brain 
muscle  plate 
head  oauity ' 
notochord 
sclerotome 

-foregut 
visceral  arch, 
heart 
■pericardium 

Fig.  149. — Scheme  of  a  Segmental  Head  Cavity  and  of  the  various  parts  formed 

from  it. 

skull  is  modified  by  the  growth  of  the  muscles  of  mastication  and  of  the 
neck.  In  man  these  changes  also  take  place,  but  to  a  less  extent  than  in 
anthropoids.  At  birth  the  temporal  lines  are  just  above  the  lower  border 
of  the  parietal  bones.  During  the  second  year  the  mastoid  part  of  the  ridge 
for  the  attachment  of  the  neck  muscles  grows  downwards  into  a  pyramidal 
process — the  mastoid — which  is  peculiar  to  the  human  species.  In 
Neanderthal  man,  the  mastoid  process  is  shaped  as  in  anthropoids.^ 

Segmentation  Theory  of  the  Skull.^ — It  is  inferred  from  investiga- 
tions made  on  the  developing  heads  of  fishes  and  amphibians  that  each 

1  See  Keith,  Journ.  Anat.  and  Physiol.  1910,  vol.  44,  p.  251. 

^  Some  researches  on  the  morphology  and  segmentation  of  the  skull  are  :  W.  H. 
Gaskell,  Origin  of  Vertebrates,  London,  1910  ;  E.  S.  Goodrich,  Proc.  Zool.  Soc.  Lond. 
1911,  p.  101 ;  W.  E.  Agar,  Proc.  Roy.  Soc.  Edin.  1907,  Feb.  4th;  Schumacher,  .4 ?ia«. 
Anz.  1907,  vol.  31,  p.  145;  Gaupp,  Verhand.  Anat.  Oesellsch.  1907,  p.  129;  Greil, 
ibid.  p.  59  ;  F.  H.  Edgeworth,  Quart.  Journ.  Mic.  8c.  1911,  vol.  56,  p.  167,  Journ. 
Anat.  and  Physiol.  1903,  vol.  37,  p.  73  ;  J.  W.  van  Wijhe,  Petrxis  Camper.  1906,  vol. 
4,  p.  1  ;  A.  Meek,  Journ.  of  Anat.  and  Physiol.  1911,  vol.  45,  p.  357  (Dev.  Skull  of 
Crocodile) ;  W.  Wright,  Lancet,  1909,  vol.  1,  p:  669  (Morphology  and  Variations  of 
Skull).     See  also  references  on  p.  135. 


156 


HUMAN  EMBRYOLOGY  AND  MOEPHOLOGY 


primitive  cephalic  segment  contains  a  cavity  comparable  to  that  seen  in 
each  body  segment  (p.  67),  from  the  wall  of  which  are  developed  (see 
Fig.  149)  :  (1)  a  sclerotome,  (2)  muscle  plate,  (3)  skin  plate,  (4)  modified 
nephrotome,  (5)  a  ventral  part  of  the  walls  join  in  the  formation  of  the 
coelom.  A  part  of  each  segment,  on  the  lateral  aspect  of  the  fore-gut,  is 
modified  to  form  a  visceral  arch  (Fig.  149).  The  sclerotome  of  each  seg- 
ment forms  (1)  a  cartilaginous  sheath  for  the  notochord,  (2)  a  cartilaginous 
roof  for  the  neural  tube,  (3)  a  process  which  runs  into  the  branchial  part  of 
the  segment.  The  number  of  segments  in  the  mammalian  head  is  by  no 
means  settled  ;  on  the  evidence  of  the  cranial  nerves  the  number  appears 
to  be  seven  (p.  98),  but  certain  considerations,  specially  relating  to  the 
facial  and  branchial  structures,  which  we  proceed  to  examine  in  the  next 
chapter,  lead  us  to  suspect  that  the  number  is  nine — the  number  of  neuro- 
meres  which  are  marked  out  on  the  hind-brain. 


/.■'lat  vent/'. ) 


off.  b. 


.•■■^..^aerebellum 
i^i^^^-^arachorda/  cart 


Fig.  150. — A  schematic  diagram  of  the  segmental  elements  of  the  Skull.  The  num- 
bers refer  to  the  Cartilaginous  Bars  of  the  various  Visceral  Arches.  The  4th  and 
5th  are  combined  in  the  Hyoid  Bone,  the  6th  and  7th  in  the  Thyroid  Cartilage, 
the  8th  (and  9th  ?)  in  the  Arytenoid,  Cricoid,  and  Tracheal  Cartilages. 

In  Fig.  150  a  diagrammatic  representation  is  given  of  one  of  the  many 
segmental  theories  of  the  skull.  The  parachordal  plate  represents  the 
unseparated  centra  of  the  nine  segments.  The  primitive  neural  arches 
have  been  disturbed  by  (1)  the  enormous  enlargement  of  the  neural  tube, 
but  especially  by  the  expansion  of  that  tube  in  front  of  the  notochord  and 
parachordal  plate  to  form  the  cerebrum  and  basal  ganglia.  In  amphioxus 
the  neural  tube  does  not  extend  beyond  the  notochord.  All  that  remains 
of  the  neural  arches  of  the  nine  primitive  segments  are  the  lateral  occipital 
cartilaginous  processes  (Fig.  139).  Of  the  cartilaginous  processes  of  the 
nine  segments  the  1st  form  the  trabeculae  cranii  (Huxley,  Howes)  ;  with 
the  forward  protrusion  of  the  neural  tube  these  come  to  form  part  of  the 
base  of  the  skull ;  the  2nd  form  the  palato-quadrate  bars.  Both  of  these 
processes  are  preoral.  The  3rd  forms  the  mandibular  bar,  the  4th  the 
hyoid  bar,  the  5th,  6th,  7th,  8th  form  the  cartilaginous  bars  in  the  1st,  2nd, 
3rd  and  4th  branchial  arches.     The  reader  will  see  that  if  the  first  and  last 


THE  CRANIUM  157 

cartilages  are  rejected  as  having  no  segmental  significance,  the  theory  put 
forward  here  is  identical  with  that  formulated  in  connection  with  the 
cranial  nerves.  We  are  at  least  justified  in  assuming  that  the  parachordal 
part  of  the  skull  is  the  oldest,  and  is  therefore  known  as  the  palaeocranium  ; 
whereas  the  prechordal  part  is  more  recent  and  is  for  this  reason  known  as 
the  neocranium.  Further  details  relating  to  the  facial  and  pharyngeal 
parts  of  the  head  will  be  given  in  the  following  chapters. 

Gaskell's  Theory.^ — Gaskell  regarded  the  trabecular  or  prechordal  part 
of  the  vertebrate  head  as  a  derivative  of  the  j)rosoma,  and  the  parachordal 
part  from  the  mesosoma  of  an  invertebrate  form  such  as  is  now  exemplified 
by  the  Kingcrab  (Limulus).  The  prosoma  carries  7  pairs  of  appendages 
which  surround  the  mouth.  The  last  of  these  represents  the  mandible,  the 
first,  the  nasal  processes  ;  the  intermediate  appendages  are  combined  in 
the  maxillary  j)rocesses.  The  mesosoma  carries  processes  which  serve  for 
respiration  and  locomotion.  In  vertebrates  these  are  modified  to  form 
branchial  arches. 

1  See  Origin  of  Vertebrates,  London,   1910. 


CHAPTER  XII. 
DEVELOPMENT  OF  THE  PACE. 

Evolution  o!  the  Human  Face. — In  our  survey  of  the  neural  part 
of  the  human  cranium  we  have  seen  that  its  outstanding  features  are  the 
result  of  a  great  cerebral  development.  When,  however,  we  turn  to  the 
facial  and  pharyngeal  parts  of  the  skull  and  head,  we  find  that  the  factors 
which  have  determined  their  shape  are  related  to  the  functions  of  smell, 
\  respiration  and  of  mastication.  It  is  unnecessary  to  again  insist  on  the  fact 
that  the  human  embryo,  in  the  latter  part  of  the  first  month,  shows  a 
resemblance  to  a  generalized  type  of  fish  ;  it  possesses  the  basis  of  a 
branchial  arch  system.     As  in  the  fish,  the  olfactory  organ  is  represented  by 


MES:NA3:  FOLD 

OLFi  PIT 
LAT:  NA9:  PROe 
POST-NARES 

MAN  01 BLE.  -  -^ J 


Fig.  151. — The  Naso-Buccal  Grooves  of  a  Dog-Fish.     On  the  right  side  the  naso- 
buccal  channel  is  exposed. 

a  pair  of  pits  or  depressions,  which  at  first  have  no  communication  with 
the  mouth.  In  some  forms  of  fish — certain  rays  and  sharks  (Fig.  151) — 
a  channel  is  formed  between  each  olfactory  pit  and  the  mouth.  The 
functional  meaning  of  such  a  channel  is  evident ;  the  water  imbibed  is 
sampled  by  the  nose  before  entering  the  mouth.  When  pulmonary  breath- 
ing was  introduced,  as  in  Dipnoean  fishes,  the  open  naso-buccal  channel 
became  enclosed  by  the  union  of  its  bounding  folds.  In  amphibians, 
reptiles  and  birds  the  naso-buccal  channel  becomes  dilated  to  form  a  true 
respiratory  nasal  passage,  and  the  parts  bounding  the  passage  unite  on  the 
roof  of  the  mouth  to  form  the  primitive  palate.  In  Fig.  152  the  parts 
entering  into  the  formation  of  the  primitive  palate  are  shown.  •  They  are 
three  in  number  :  (1)  a  premaxillary  and  vomerine  part  developed  between 
the  nasal  passages  ;  (2)  a  right  and  left  maxillary  part,  laid  down  on  the 
lateral  or  outer  aspect  of  each  passage.  In  mammals  a  fourth  element  is 
added  to  the  primitive  or  reptilian  palate,  and  in  this  way  the  mammalian 
mouth  is  separated  from  the  nasal  respiratory  passage,  and  can  serve  the 
purposes  of  mastication  and  suction.     Thus  in  the  evolution  of  the  face 

158 


DEVELOPMENT  OF  THE  FACE 


159 


there  have  been  three  distinct  stages  :  (1)  a  piscine,  in  which  the  nose  and 
mouth  were  formed  independently  ;  (2)  an  amphibian  stage,  where  the 
nasal  respiratory  passage  opened  on  the  roof  of  the  mouth  ;  (3)  a  mam- 
malian stage,  in  which  it  opened  in  the  naso-pharynx.  In  the  development 
of  the  human  embryo  we  see  these  three  stages  reproduced.^ 

Processes  which  form  the  Face.^ — Towards  the  end  of  the  4th  week 
of  foetal  life,  five  processes  begin  to  spring  from  the  base  of  the  primitive 
cerebral  capsule,  which  by  the  end  of  the  second  month  have  completely 
united  together  to  form  the  facial  part  of  the  head.  In  Fig.  153,  a  dia- 
grammatic representation  is  given  of  the  condition  of  these  five  processes 
about  the  end  of  the  6th  week  of  development.     Of  the  five,  one,  the 


PREMAXILL^ 


MAV:  PAL 


ANTr  NARE9 
NA30-PAL:  PAP: 


JACOBSOn'3   CARTIl.1 


..    POSTrNARES 

-••MAX I  PAL; 
-•••  VOMER 

ALVeOLAR 


INT-  PT£RVG 


NASO-PHAR--  ' 


SOFT     PALATE. 


Fig.  152.— Roof  of  the  Mouth  of  a  Lion-Pup,  showing  the  condition  of  Cleft  Palate 
recalling  in  form  the  Palate  of  ReptUes.  On  the  right  side  the  bones  are  exposed 
by  removal  of  the  soft  parts. 

nasal  or  fronto-nasal,  composed  of  symmetrical  right  and  left  halves,  is 
median,  and  projects  beneath  the  fore-brain  ;  the  others  are  lateral,  two 
on  each  side,  the  mandibular  and  maxillary.  The  cavity  which  these  five 
processes  surround  is  the  stomodaeum,  a  space  ultimately  destined  to 
form  the  nasal  and  part  of  the  buccal  cavities.  The  representatives  of  these 
five  elements  are  recognizable  in  certain  fishes  (see  Fig.  151).  The  part  of 
the  adult  face  formed  by  each  process  is  shown  in  Fig.  154. 

Nasal  Region  of  Face. — In  reality  the  core  of  the  face  is  formed  by  the 
cartilaginous  capsule  which  encloses  the  organ  of  smell.  We  have  seen 
that  the  olfactory  capsule  occupies  the  terminal  part  of  the  prechordal 
plate,  and  in  primitive  vertebrates  forms  the  entire  snout  or  face  (Figs. 
130,  133).  Hence  the  first  step  in  the  development  of  the  human  face 
represents  the  upbuilding  of  the  nasal  cavities.     Three  stages  in  this  process 

1  See  Professor  J.  E.  Frazer,  Lancet,  1916,  vol.  2,  p.  45  ;  Berry  and  Legg,  Harelip 
and  Cleft  Palate,  1918 ;  Keith,  "  Malformations  of  the  Head  and  Neck,"  Brit.  Med. 
Journ.  1909,  vol.  2,  p.  310. 

-  K.  Peter,  Anat.  Anz.  1911,  vol.  39,  p.  41  (Development  of  Face). 


160 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


are  depicted  in  Fig.  155,  taken  from  a  recent  research  by  Professor  Frazer.^ 
At  the  end  of  the  5th  week  the  olfactory  organ  is  exposed  on  each  side  of 
the  fore-brain  as  a  plaque  surrounded  by  a  growing  raised  margin  or  fold. 
The  pituitary  recess,  opening  from  the  stomodaeum  (Fig.  155,  A)  lies  in 
contact  with  the  under  surface  of  the  fore-brain.  At  the  end  of  the  6th 
week  (B),  the  olfactory  plaque  has  become  a  pocket  by  the  upgrowth  of  the 
mesial  and  lateral  nasal  folds  or  processes  which,  being  united  above,  rise 
up  as  a  hood.  Below,  the  olfactory  pit  communicates  with  the  buccal 
cavity  by  an  open  naso-buccal  channel — just  as  in  the  dog-fish.  At  the 
same  time  the  maxillary  process  grows  forward,  and  applies  itself  to  and 
fuses  with  the  substance  of  the  lateral  nasal  fold.  In  the  7th  week  (C)  the 
maxillary  process  has  come  in  contact  and  fused  with  the  globular  end 


mid  brain 


cereb. 


cerebral  vesicle 


eye 
nasal  field 

fat.  nas.  proc. 
mes.  nas.  proc. 
maxillary  proc. 

mandib.  proc, 

hyoid  arch, 
cardiac  emin. 


maxillary  process 

nasal  field 
lat.  nas.  proc. 


mes.  nas.  proc.  (proc.  glob. 


mandib.  proc. 
hyoid  arch 


Fig.  153. — Showing  the  formation  of  the  Face  by  the  Nasal,  Maxillary  and  Mandibular 

Processes  in  an  Embryo  of  the  6th  week.     (After  His.) 

Fig.  154. — Showing  the  parts  of  the  Face  formed  from  the  Nasal,  Maxillary  and 

Mandibular  Processes. 

(globular  process)  of  the  mesial  nasal  fold,  and  thus  the  naso-buccal  channel 
is  covered  over  and  we  can  now  speak  of  anterior  nares  and  a  posterior 
opening  or  primitive  choana — at  first  closed  by  an  epithehal  membrane 
(Fig.  165,  C).  As  the  olfactory  pockets  enlarge  they  come  closer  together 
under  the  fore-brain  until  the  mesial  folds  and  the  tissues  between  them 
form  the  primitive  septum  of  the  nose — the  lateral  nasal  fold  and  inter- 
mediate tissue  of  each  side  being  sometimes  called  by  a  separate  name^ 
the  fronto-nasal  process.  Thus  the  nasal  cavities  which  form  the  founda- 
tion of  the  face  are  built  against  the  wall  of  the  fore-brain  and  the  nasal 
folds  represent  the  margins  of  the  outgrowing  edifice. 

Malformations  of  the  Face. — These  processes  may  fail  to  unite  in 
the  second  month,  and  in  this  manner  malformations  of  the  face  are  pro- 
duced.    The  most  common  anomaly  is  a  partial  failure  of  the  nasal  and 

^  See  references,  p.  165. 


DEVELOPMENT  OF  THE  FACE 


161 


maxillary  processes  to  fuse,  various  degrees  of  hare  lip  and  cleft  palate 
being  tlius  caused.  In  hare  lip/  the  cleft  appears  in  the  upper  lip  between 
the  middle  part  formed  by  the  mesial  nasal  processes  and  the  lateral 
parts  formed  by  the  maxillary  processes  (Fig.  154).  In  cleft  palate,  the 
failure  of  union  occurs  between  the  deep  parts  of  the  nasal  and  maxillary 
processes  (Fig.  171).  The  lateral  or  the  mesial  parts  of  the  nasal  process 
may  fail  to  fuse  with  the  maxillary  processes,  and  these  appear  on  the  face 
as  polypoid  or  irregular  projections  (Figs.  156,  157).  In  such  cases  the 
right  and  left  maxillary  processes  may  unite  and  form  the  whole  of  the 
upper  lip.  Macrostoma  is  due  to  a  partial  failure  of  the  mandibular  ta 
unite  with  the  maxillary  element.     Any  of  these  processes  may  be  under- 


r*lTUIT   PIT 

STOMODAEUM 

FOREBRAIN 

MES  NAS   FOLD 
OLFACr  PIT 
LAT  A/AS  FOLD 


MES. NAS  rOLO 
OLFACT  PIT 

LAT   NAS    FOLD 


MAX  PROC 


FRON  NAS.PROC 
MES  A/AS    FOLD 
Ah/T  NARES 

LAT  NAS  FOLD 


MANDIB    PPOC 

MAX    PROC 


POST.  NARES 
EUSTACH    OP. 

MAX   PROC 


(A)  7  m.m -(s^h  vvh)  {B)Jom.m.(e^f'w>f).    {Oiz  m.m.^r'^^W^J 

Fig.  155. — Three  stages  in  the  formation  of  the  Nasal  Cavities  and  Primitive  Palate. 
(Prof.  J.  E.  Frazer.) 

or  over-developed  ;  over-development  of  the  nasal  and  under-development 
of  the  mandibular  (micrognathia)  are  of  common  occurrence. 

The  cleft  in  the  lip  of  the  hare  is  exactly  in  the  middle  line,  and  is  due 
to  a  separation  of  the  right  and  left  parts  of  the  mesial  nasal  process. 
The  condition  of  median  hare  lip,  which  is  rare  in  man,  is  represented 
in  Fig.  158  ;  in  this  case  there  was  a  partial  cleft  of  the  palate,  and  the 
pituitary  body  formed  a  tumour-like  mass  within  the  septum  of  the  nose. 
A  median  cleft  in  the  lower  lip  is  also  rare,  and  is  due  to  a  failure  in  the 
union  of  the  right  and  left  mandibular  processes  of  the  lower  jaw  (Fig. 
159).  Another  remarkable  condition — cyclopia — is  shown  in  Fig.  202, 
p.  202,  where  the  nasal  processes  have  united  together  to  form  a  single 
proboscis-like  structure  projecting  above  the  eyes,  which  are  partly  fused. 

1  For  an  account  of  the  development  of  the  lips  see  :  L.  Bolk,  Anat.  Hefte,  1911, 
vol.  44,  p.  227  (describes  curious  pits  seen  in  abnormally  developed  lower  lips)  ;  M. 
Ramm,  Anat.  Hefte,  1905,  vol.  29,  p.  55  ;  W.  L.  H.  Duckworth,  Journ.  Anat.  and 
Physiol.  1910,  vol.  44,  p.  349  (Lips  of  Primates). 


\ 


162 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


In  this  condition  the  palate  and  upper  lip  are  formed  by  the  union  of  the 
maxillary  processes.  The  condition  is  not  uncommon,  and  shows  how 
adaj)table  the  various  embryological  parts  of  the  face  are.^ 

The  Method  of   Fusion. — The  manner  in  which  embryological  parts 
unite  is  similar  in  nature  to  the  healing  of  wounds.     Fig.  160  represents 


Fig.  ]  .56. — Face  of  a  Child  showing  the  left  Nasal  Process  and  Pocket  as  a  free  Poly- 
poid Body  and  the  left  Maxillary  Process  ununited  with  the  Mesial  Nasal  (left 
Hare  Lip).     (After  Kirchmayer.) 

Fig.  157. — Face  of  Child  in  which  the  Nasal  and  Maxillary  Processes  are  ununited. 
A,  Polypoid  Tubercle  in  line  of  Naso-Maxillary  Cleft ;  B,  Eight  Lateral  Nasal 
Process  ;  C,  Left  Lateral  Nasal  Process  ;  D,  Mesial  Nasal  Process  ;  E,  F,  Max- 
illary Process.    (London  Hospital  Medical  College.) 

a  coronal  section  of  the  head  of  a  human  embryo,  in  which  the  mesial 
nasal  process  containing  the  germinal  epithelium  of  the  upper  incisor 
teeth  is  about  to  unite  with  the  maxillary.  The  ectodermic  coverings 
of  the  processes  are  in  contact.     Across  the  epithelial  union  thus  formed 


7  V 


DEPRESSION 

MEDIAN    CLE.Pt 
MAX:PROC' 

PITUIT:  BODY 
CLEFT  ,M  PALATE 


Fig.  158. — Median  Hare  Lip  in  a  ChUd  with  Partial  Cleft  Palate  and  Ectopia  of  the 

Pituitary.     (Mr.  A.  R.  Tweedie's  case.) 
Fig.  159. — Median  Cleft  of  the  Lower  Lip  and  Jaw.     (Prof.  MacCormick's  case.) 

the  mesodermal  tissue  spreads,  the  two  processes  thus  becoming  intimately 
united.  We  know  that  the  process  of  healing  may  be  arrested  by  many 
pathological  conditions  ;   the  process  of  embryological  union  may  be  also 

1  F.  P.  Mall,  Contributions  to  Embryology,  1917,  vol.  6,  No.  15  ;  R.  J.  Gladstone, 
Journ.  Anat.  1920,  vol.  54,  p.  196 ;  Davidson  Black,  Journ.  G.  Neur.  1913,  vol.  23, 
p.  193. 


DEVELOPMENT  OF  THE  FACE 


163 


arrested,  but  the  exact  causes  of  the  arrest  we  do  not  yet  know.  If  union 
of  the  facial  processes  fails  to  take  place,  then  subsequent  growth  tends  to 
move  the  processes  apart,  and  union  becomes  impossible.  The  cleft  in  the 
lip  or  palate  increases  in  width  as  the  foetus  becomes  older.  The  tongue 
lies  between  the  maxillary  plates  (Fig.  161),  a  normal  position  during  the 
2nd  month.  It  is  extruded  as  the  palate  is  formed,  the  extrusion  being 
due  to  the  rapid  growth  of  the  mandibular  and  maxillary  processes  in  the 
earlier  part  of  the  3rd  month. 

Structures  formed  in  the  Mesial  Nasal  Processes. — We  have  already 
seen  that  the  mesial  nasal  processes,  which  represent  the  inner  walls  of  the 
nasal  pockets  or  cavities,  grow  out  from  the  base  of  the  fore-cranium, 
and  when  they  grow  together  to  form  the  primitive  septum  of  the  nose, 
the  cartilage  formed  in  their  united  substance  represents  a  direct  forward 


CERE.BR:  VES.: 


NASAu  SEPT: 


max:  PROC: 


JOCOBSONS  ORaAN 


MANDIBLE. 


MECKECS    CARTILAGE 


MANDIBLE 


FiQ.  160. — Coronal  Section  of  the  Head  of  a  Human  Embryo  in  the  6th  week  of  de- 
velopment and  14  mm.  long.  (After  J.  L.  Paulet,  Archiv.  fur  Mik.  Anat.  und 
Entwickl.  1911,  vol.  76,  p.  658.) 

Fig.  161. — Similar  Section  of  the  same  Embryo  further  back,  shomng  the  Tongue 
in  the  Palatal  Cleft.     (J.  L.  Paulet.) 

continuation  of  the  prechordal  plate.  From  the  united  substance  of  the 
mesial  nasal  processes  are  formed  the  septum  of  the  nose  (Fig.  162),  the 
premaxillary  part  of  the  upper  jaw,  and  the  middle  third  of  the  upper  lip 
"X^^gs.  152, 157) ;  in  their  anterior  inferior  angles^re  formed  the  premaxillae. 
The  remainder  forms  the  septum  of  the  nose.  Part  of  the  cartilage  of 
this  septum  remains  unchanged  as  the  septal  cartilage  (Fig.  162).  In  the 
septal  wall  are  also  developed  the  mesial  limbs  of  the  alar  cartilages,  which 
give  form  to  the  anterior  nares.  One  element  is  added  to  the  lower  anterior 
part  of  the  septum — just  above  the  opening  of  the  naso-palatine  canal — 
the  paraseptal  cartilages  (Fig.  142)  which  primarily  serve  for  the  protection 
of  an  isolated  area  of  the  olfactory  membrane — Jacobson's  organ — reduced 
to  a  vestige  in  man.  In  Fig.  155,  C  it  will  be  observed  that  the  lateral 
nasal  process  also  fuses  with  the  mesial ;  the  paraseptal  cartilages  are 
derived  from  the  lateral  nasal  processes  (Fawcett). 

The  vomer  is  developed  in  the  membrane  (perichondrium)  which  covers 
the  primitive  septum  (Fig.  169).     A  centre  of  ossification  appears  at  the 


164 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


end  of  the  2nd  montli  on  each  side  near  the  lower  border  of  the  septum  ; 
these  fuse  together  under  the  palatal  margin  of  the  cartilage.  Thus  the 
vomer  forms  at  first  a  shallow  trough  in  which  the  cartilage  of  the  septum 
appears  to  be  imjDlanted  (Fig.  163). 

The  Vertical  Plate  of  the  Ethmoid  is  formed  by  a  direct  ossification  of  the 
cartilage  of  the  primitive  septum.     Ossification  begins  in  the  4th  month. 


crista  galli 

uertical  plate  of  ethmoid 

position  ofcanalis  cranio- 
pharyngeus. 


sept.  cart. 


alar 
carta. 


Fm.  162.- 


vomer. 

^naso-pafatine  canal 
premaxilla 
mesial  port,  of  upper  lip. 

-Showing  the  structures  formed  in  the  Mesial  Nasal  Processes. 


The  crista  galli,  the  intra-cranial  part  of  the  septum,  is  formed  in  part  by 
the  ossification  proceeding  into  the  attachment  of  the  falx  cerebri. 

Fiemaxillary  Bones. — The  two  premaxillary  bones  form  the  sockets 
of  the  four  upper  incisor  teeth.  In  the  human  foetus  at  birth  the  suture 
between  the  premaxilla  and  maxilla  can  be  seen  on  the  hard  palate  ;  it 
runs  on  each  side  from  the  naso-palatine  foramen  to  the  alveolus  between 

trough  for  cartilaginous 
'  septum. 

bright  plate 


left  plate 


Fig.  163. — Showing  the  Trough-shaped  Vomer  of  the  newly-horn. 

the  lateral  incisor  and  canine  (Fig.  165).  As  is  illustrated  in  Fig.  165,  the 
relationship  of  this  suture  to  the  tooth  sockets  is  variable,  but  the  relation- 
ship just  mentioned  is  the  usual  one.  On  the  facial  aspect,  a  suture  be- 
tween the  premaxilla  and  maxilla  is  at  no  stage  distinct,  the  maxillae 
appearing  to  overlap  the  premaxillary  elements,  almost  completely  ex- 
cluding them  from  the  face.  The  nasal  spine  is  formed  by  the  premaxillae. 
The  palatal  plates  of  the  premaxillae  represent  the  prevomers  which  are 
seen  as  distinct  bones  in  the  primitive  palate  of  amphibia. 


DEVELOPMENT  OF  THE  FACE 


165 


In  mammals  generally  the  premaxillae  are  highly  developed,  separated 
throughout  their  whole  extent  by  a  suture  from  the  maxillae  and  form  the 
snout  part  of  the  face.  In  the  higher  Primates  the  face  becomes  less 
elongated,  less  prognathous,  or  projecting,  and  the  premaxillae  less  de- 
veloped. In  the  orang,  for  instance,  the  premaxillary  sutures  are  distinctly 
seen  on  the  face  at  birth  (Fig.  164),  but  as  the  permanent  canines  begin 
to  develop  they  fuse  with  the  maxillae.  The  premaxilla  is  more  reduced 
in  man  than  in  any  other  primate  ;  in  him  it  is  partly  fused  with,  and  over- 
lapped by,  the  maxilla  from  the  first  appearance  of  ossification ;  in  apes 
fusion  does  not  occur  until  the  eruption  of  the  permanent  teeth.  The 
vestigial  character  of  the  premaxilla  in  man  is  due  to  the  reduced  size  of  his 
masticatory  apparatus  and  the  consequent  retrogression  in  the  develop- 
ment of  the  facial  part  of  the  skull. 

Relationship  of  the  Premaxilla  to  Cleft  Palate.^ — It  is  usual  for  the 
sockets  of  all  four  incisor  teeth  to  be  formed  by  the  premaxilla.     In  many 


ant.  nares. 
left  premaxilla 


maxilla 


'    I"  canine. 

Fig.  164. — Showing  the  suture  on  the  face  between  the  premaxilla  and  maxilla  in 
the  Skull  of  a  Young  Orang. 

Fig.  165. — Palate  at  birth,  showing  varieties  of  the  suture  between  maxilla  and  pre- 
maxilla. On  the  right  side  (A)  the  suture  between  the  palatal  processes  of  pre- 
maxilla and  maxilla  ends  at  the  socket  of  the  canine  ;  on  the  left  (J5)  between  the 
mesial  (Z^)  and  lateral  (/*)  incisors  ;  D,  naso-palatine  foramen,  in  which  the 
anterior  end  of  the  vomer  appears  ;  E,  F,  palatal  processes  of  the  maxillary  and 
palate  bones. 

cases  of  cleft  palate  (see  Fig.  167)  only  the  two  central  incisors  are  situated 
on  the  premaxilla,  the  sockets  of  the  lateral  incisors  being  attached  to  the 
maxilla.  Even  in  the  normal  palate  (Fig.  165,  B)  this  may  be  the  case. 
Albrecht  supposed  that  each  premaxilla  was  made  up  of  two  bones— an 
outer  and  an  inner — and  that  in  cleft  palate  the  fissure  might  lie  between 
the  elements  of  the  premaxillary  or  to  their  outer  side.  We  now  know  (1) 
that  cleft  palate  is  not  due  to  a  failure  of  ossific  centres  to  join,  but  to  a 
non-union  of  two  embryological  masses — the  mesial  nasal  and  maxillary  ; 
(2)  that  the  partial  suture,  which  may  divide  the  palatal  part  of  the  pre- 
maxilla, is  due,  not  to  two  centres  of  ossification,  but  to  the  formation 

iM.  Inouye,  Anat.  Hefte,  1912,  vol.  45,  p.  471  (Premaxilla  in  Man);  1912,  vol.  46, 
p.  1  (Dev.  of  Palate,  Mammals)  ;  E.  Gaupp,  Anat.  Hefte,  1910,  vol.  42,  p.  311  (Evol. 
of  Palate) ;  G.  Schorr,  Anat.  Hefte,  1908,  vol.  36,  p.  69  (Dev.  of  Palate)  ;  E.  Fawcett, 
Journ.  Anat.  and  Physiol.  1906,  vol.  40,  p.  400  (Ossific.  of  Palate).  See  also  references, 
p.  159, 


166 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


of  the  palatal  part  by  two  processes — one  corresponding  to  the  middle 
incisor  socket,  the  other  to  the  lateral  incisor  ;  (3)  the  germ  or  bud  of  the 
lateral  incisor  is  formed  at  the  point  of  union  of  the  mesial  nasal  and 
maxillary  processes.  If  these  processes  fail  to  join,  the  bud  of  the  lateral 
incisor,  as  the  processes  move  apart  during  subsequent  growth,  may  be 
carried  away  by  the  maxillary  or  premaxillary  element,  or,  as  I  have  seen, 
be  left  stranded  in  the  cleft  between  the  processes.  If  the  lateral  incisor 
remains  attached  to  the  maxillary  process,  then  its  socket  is  formed  by  that 
element ;  if  by  the  premaxillary,  then  the  cleft  appears  in  the  more  usual 
situation,  and  the  socket  forms  part  of  the  premaxilla.  The  late  Mr. 
Clement  Lucas  observed  that  the  lateral  incisor  is  often  small  or  even  absent 
in  families  subject  to  cleft  palate. 

Naso-palatine    Foramen. — The    naso-palatine    foramina    are    formed 
where  the  mesial  nasal  and  two  maxillary  processes  unite  to  form  the 


riG.  166. — Facial  part  of  the  Skull  of  a  Cyclops  Eoetus,  in  which  the  nasal  processes 
formed  a  free  proboscis,  the  eyes  a  median  structure  and  the  maxillary  pro- 
cesses the  palate.  A,  orbital  plates  of  frontal;  ' B,  fused  optic  foramina;  C, 
orbital  plate  of  sphenoid ;  C,  basi-sphenoid  ;  E,  orbital  plate  of  maxUla ;  F, 
ear  ;  G,  superior  maxilla  ;    C,  canine  ;    ill',  first  milk  molar. 

Fig.  167. — Case  of  Cleft  Palate,  in  which  the  maxillary  and  premaxUlary  processes 
have  remained  ununited  on  the  left  side.  A,  septal  process  of  premaxilla  ;  B, 
nasal  septum  ;  C,  canine  ;  B,  palatal  process  of  premaxilla  ;  E,  palatal  process 
of  maxilla.     The  left  lateral  incisor  was  absent. 

palate  (Fig.  172).  In  animals  with  well-developed  premaxillae  the  two 
naso-palatine  (anterior-palatine)  foramina  are  large,  and  through  each 
passes  the  naso-palatine  duct,  which  allows  a  communication  between 
the  buccal  and  nasal  cavities.  The  odour  of  the  food  within  the  mouth 
thus  reaches  the  organ  of  Jacobson,  which  is  situated  on  the  septum, 
close  to  the  nasal  orifice  of  the  duct.  In  man  the  upper  ends  of  the  ducts 
remain  open  ;  they  terminate  blindly  below,  behind  the  mesial  incisor  teeth, 
in  the  naso-palatine  or  incisive  papilla. 

Nasal  Duct. — The  lachrymal  sac  and  nasal  duct,  through  which  tears 
pass  from  the  eye  to  the  inferior  meatus  of  the  nasal  cavity,  are  formed 
between  the  lateral  nasal  and  maxillary  processes  (Figs.  154,  155,  157). 
At  the  end  of  the  6th  week,  when  the  furrow  between  the  maxillary  and 
nasal  processes  is  obliterated,  the  nasal  or  naso-lachrymal  duct  is  represented 
by  a  solid  bud  or  core  of  ectoderm  embedded  at  the  inner  angle  of  the  eye 
and  in  the  site  of  the  upper  part  of  the  naso-maxillary  groove  or  fissure. 
This  bud  becomes  cord-like,  one  extremity  growing  towards  the  nasal 


DEVELOPMENT  OF  THE  FACE 


167 


cavity,  which  it  reaches  at  the  beginning  of  the  3rd  month,  while  the  orbital 
extremity  expands  to  form  the  lachrymal  sac.  The  canaliculization  of 
the  duct  begins  in  the  3rd  month,  but  is  not  complete  until  late  in  foetal 
life.  In  Fig.  157  the  lateral  nasal  and  maxillary  processes  have  not  fused  ; 
the  eye  is  separated  by  two  folds  from  the  nasal  cavity  ;  the  outer  repre- 
sents the  semilunar  fold,  the  inner  a  fold  in  which  the  lachrymal  canaliculi 
and  caruncula  lachrymalis  are  formed. 

Structures  formed  in  the  Lateral  Nasal  Process. — The  lateral  nasal 
process  is  developed  to  form  the  outer  wall  and  roof  of  the  chamber  con- 
taining the  olfactory  organ.  Within  it  develops  a  plate  of  cartilage  which 
represents  the  greater  part  of  the  cartilaginous  olfactory  capsule  of  lower 
vertebrates.     In  the  human  embryo  the  process  of  chondrification  begins 


nasal, 
carta,  which, 
disappears 

lateral  cart. 


alar  cart. 


from  mes.  nas.  proc. 


mid.  turb.  proc. 
sup.  turb.  proc. 


(int.  ptery.  pi. 
[(pterygo-pal.bar) 

I  open,  antrum 
[ofHighmore 

{ palate 

\( ptery go-palat.  bar) 


inf.  turb 
from  man.  proc. 

Fig.  168. — Showing  the  structures  formed  in  the  Lateral  Nasal  Processes. 

near  its  lower  border  and  spreads  up  towards  the  roof  (Frazer),  where  it 
joins  the  upper  edge  of  the  septal  cartilage,  developed  on  the  united  mesial 
nasal  processes  and  also  spreads  backwards  to  enfold  the  hinder  part  of  the 
olfactory  chamber  and  to  become  continuous  with  the  presphenoid  part  of 
the  prechordal  plate.  The  cribriform  area  is  the  last  to  be  formed  (see 
Figs.  143,  A,  B,  p.  148).  In  front,  the  lower  border  of  the  lateral  nasal 
process  joins  the  septal  process,  adding  to  it  the  paraseptal  cartilage 
(p.  163). 

What  becomes  of  the  Cartilage   of   the   Lateral   Nasal   Process^ 

(Fig.  168).— It  forms  on  each  side: 

(1)  The  cribriform  plate  around  the  olfactory  nerves  as  they  issue  from 
the  olfactory  bulb  ; 

(2)  The  lateral  mass  of  the  ethmoid,  at  first  merely  a  plate  of  cartilage  ; 
the  superior  and  middle  turbinate  processes  are  developed  from  the  plate 

^  See  Fawcett,  loc.  cit.  p.  135. 


168      HUMAN  EMBEYOLOGY  AND  MOEPHOLOGY 

(Fig.  169)  ;   ossific  centres  appear  in  the  cartilage  of  tlie  lateral  mass  and 
turbinate  processes  during  the  fourth,  month  of  foetal  life  ; 

(3)  The  inferior  turbinate  bone  (Fig.  169)  (maxillo-turbinal).  The  body 
of  the  superior  maxilla  is  developed  on  its  outer  side  in  the  maxillary 
process  (Fig.  169) ; 

(4)  The  lateral  and  part  of  the  alar  cartilages  of  the  nose  ; 

(5)  In  the  membrane  over  the  cartilage,  between  the  ethmoid  behind 
and  the  cartilages  of  the  nares  in  front,  are  developed  the  lachrymal  and 
nasal  bones,  and  the  ascending  process  of  the  superior  maxilla.  The 
cartilage  beneath  these  bones  disappears  after  birth  (Fig.  168).  Ossifica- 
tion of  the  nasal  bone  appears  at  the  beginning  of  the  3rd  month  ;    the 


sup.  turb. 
lot.  nas.  proc. 


-frontal, 
orbit  pi.  front. 


T^ ^septum 

mid.  fa^-\v    _^MM^^         #'"^«'  "as.  proc  J 
inf  turbr^^^^^mMgl^^^m-sup.  max.  (max.  proc.J 

-dental  sac 
uomer       palate  (max.  proo.) 

Fig.  169. — Coronal  Section  of  the  Skull  of  a  7th  month  Human  Foetus  to  show  the 
cartilages  of  the  Lateral  and  Mesial  Nasal  Processes  and  the  bones  formed  round 
them. 

centre  for  the  lachrymal  appears  late — at  the  beginning  of  the  4th  month 
(MaH). 

Arteries  and  Nerves  of  the  Nasal  Processes. — A  knowledge  of  the 
development  of  the  face  assists  one  to  unravel  the  complicated  distribution 
of  its  arteries  and  nerves.     Each  process  carries  its  own  vessels  and  nerves. 

1.  Mesial  Nasal  Process.  The  chief  artery  and  nerve  of  this  process 
are  the  naso-palatine,  but  branches  also  come  from  the  nasal  nerve  and 
its  accompanying  artery,  the  anterior  ethmoidal. 

2.  Lateral  Nasal  Process.  The  nerves  of  the  lateral  nasal  process  are 
derived  from  Meckel's  ganglion  and  from  the  descending  palatine  nerve. 
Vessels  accompany  these  nerves  from  the  descending  palatine  artery. 
The  nasal  nerve  and  anterior  ethmoidal  artery  supply  the  process  in  front. 

The  Parts  formed  from  each  Maxillary  Process. — The  maxillary 
process  springs  from  the  base  of  the  mandibular  process  at  the  end  of  the 
4th  week  of  development,  and  sweeping  forwards  below  the  eye,  separates 
that  structure  from  the  mouth  (see  Figs.  44^  45,  154).     In  front  it  comeg 


DEVELOPMENT  OF  THE  FACE 


169 


in  contact  and  fuses  with  the  lateral  nasal  process,  which  it  assists  to  form 
the  outer  wall  and  floor  of  the  nasal  cavity,  and,  in  the  7th  week,  with  the 
globular  process  of  the  mesial  nasal  which  forms  the  premaxillary  part  of 
the  palate  and  the  middle  part  of  the  upper  lip.  The  part  of  the  face 
formed  by  the  maxillary  process  is  shown  in  Fig.  154.  The  hard  palate 
(with  the  exception  of  the  j)reniaxillary  part)  is  formed  by  a  palatal  plate 
which  begins  to  grow  inwards  from  the  maxillary  process  in  the  6th  week 
(Fig.  170)  and  fuses  with  the  plate  of  the  opposite  side  about  the  10th 
week.  The  palatal  processes  separate  the  buccal  from  the  nasal  cavities, 
forming  the  roof  of  the  one  and  the  floor  of  the  other.  The  palatal 
plates  meet  first  with  the  premaxillary  part  (Fig.  171)  ;  behind  that  they 
come  in  contact  with  each  other  ;  the  process  of  fusion  spreads  backwards, 


tni'd  brain 


cerebral  uesi'cfe 

anterior  nares 
upper  Zip.  mes.  nas.  proc. 
eye 

■premax.  mes.  nas.  proc. 
upper  lip.  max.  proc. 
alueolus  max.  proc. 

pa/at.  proc. 

max.  proc.  (section) 

inner  recess  1st  cleft. 
(Eustacli.  tube) 
posterior  nares 
'roof  of  pharynx 
septal  part,  mes.  nas.  proc. 

Fia.  170. — Showing  the  ingrowth  of  the  Palatal  Plates  of  the  two  Maxillary  Processes 
at  the  end  of  the  6th  week.  The  openings  erroneously  indicated  as  "  posterior 
nares,"  are  the  primitive  choanae.     (After  Kollmann.) 

and  before  the  end  of  the  third  month  the  hard  and  soft  palates  form  a 
complete  naso-buccal  septum. 

Cleft  Palate. — To  understand  the  manner  in  which  the  various  forms  of 
clefts  arise  in  the  palate  it  is  necessary  to  note  the  manner  in  which  the 
septum  of  the  nose  grows  and  the  fate  of  the  primitive  choanae.  At  the 
end  of  the  6th  week  (Fig.  170)  the  nasal  septum  is  seen  to  be  relatively 
short  and  wide  and  to  form  the  mesial  borders  of  the  primitive  choanae 
or  posterior  nares  (see  Fig.  197),  By  the  9th  week  (Fig.  171)  the  septum 
has  grown  greatly  in  length,  pushing  the  primitive  palate  forwards  away 
from  the  base  of  the  skull,  and  thus  presenting  a  long  posterior  or  palatal 
border  which  still  forms  the  mesial  edges  of  the  primitive  choanae  ;  the 
choanae  stiU  extend  from  the  primitive  palate  to  the  sphenoidal  end  of  the 
septum.  The  dorsum  of  the  foetal  tongue  lies  against  the  palatal  margin 
of  the  septum  with  the  palatal  folds  tucked  under  its  lateral  margins 
(Fig.  161)  until  the  9th  week,  when  the  forward  growth  of  the  primitive 
palate  lifts  the  nasal  septum  off  the  dorsum  of  the  tongue  and  allows  the 


170 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


palatal  folds  to  come  in  contact  with  each  other  and  with  the  lower 
margin  of  the  septum.  The  manner  in  which  the  palatal  folds  are 
applied  to  the  septum  is  shown  in  Fig.  171,  5  ;  by  a  process  akin  to  the 
healing  of  wounds  the  palatal  folds  unite  with  each  other  and  with  the 
palatal  border  of  the  septum.  The  process  begins  behind  the  premaxilla 
and  passes  backwards,  but  the  posterior  part  oiE  the  septum  is  left  free  to 
form  the  partition  between  the  permanent  posterior  nares.  Thus  in  the 
formation  of  the  palate  a  Y-shaped  cleft  has  to  be  united  ;  the  short  limbs 
lie  on  each  side  of  the  premaxilla  in  the  primitive  palate,  the  long  limb 
in  the  middle  line  of  the  permanent  or  mammalian  palate.  All  three  parts 
may  remain  united  as  in  Fig.  157,  or  the  long  limb  with  one  short  as  in 
Fig.  167,  or  only  the  long  Hmb  as  in  Fig.  152.  Further,  it  sometimes  happens 
that  one  or  both  primitive  choanae  are  closed  permanently  by  the  plug 


PFtlMITI\/E     palate: 
PALATAL    FOLD 


ANT.  NARES 
LIP 

ALVEOLAR  PFtOC. 
PAL. FOLD 


PALATAL  FOLD  (cut) 


PREMAX  PROC. 
MAS.  PAL    CAN. 
PALAT.       POLO 


PA.L.PHAR .  FOLD 


Fig.  171. — Development  of  the  Maxillary  Palate.  A,  stage  reached  in  9th  week; 
B,  schematic  figure  to  illustrate  the  manner  in  which  the  maxillary  folds  are 
applied  to  the  nasal  septum.     (Prof.  Frazer.) 

of  epithelium  which  temporarily  occludes  them  becoming  organized  and 
forming  membrane  or  bone.  As  the  septum  and  choanae  expand  this 
occluding  membrane  or  partition  is  stretched  and  gives  rise  to  atresia  of 
the  posterior  nares.  The  wide  gap  and  bent  septum  seen  in  nearly  all 
cases  of  cleft  palate  are  due  to  changes  produced  by  growth  in  the  later 
months  of  foetal  life.  An  asymmetrical  growth  is  a  result  of  the  failure  in 
the  union  of  the  processes. 

The  Soft  Palate. — While  the  hard  palate  is  derived  from  the  palatal 
plates  of  the  maxillary  processes,  the  soft  palate  (Fig.  171,  A)  is  derived 
from  a  fold  which  arises  as  a  prolongation  backwards  of  each  horizontal 
plate  into  the  pharynx.^  Into  the  palatal  folds  spread  derivatives  of  the 
superior  constrictor  to  form  the  palato-pharyngeus,  palato-glossus  and 
azygos  uvulae,  and  possibly  also  the  levator  palati.  The  posterior 
pillars  of  the  fauces  are  continuations  of  the  palatal  folds  within  the  pharynx. 
A  divided  uvula  represents  a  failure  of  the  final  stage  in  the  formation  of  the 
palate. 

^  See  J.  Ernest  Frazer,  Journ.  Anat.  and  Physiol.  1911,  vol.  45,  p.  190. 


DEVELOPMENT  OF  THE  FACE 


171 


Bones  formed  in  each  Maxillary  Process.^ — The  zygomatic  process 
of  the  temporal,  the  malar,  and  the  greater  part  of  the  superior  maxillary 
are  formed  directly  in  the  connective  tissue  within  the  process.  They 
are  membrane-formed  or  dermal  bones.  The  centre  foT  the  maxilla  appears 
at  the  beginning  of  the  7th  week  in  that  part  of  the  process  which  lies  under 
the  eye.  Very  soon,  after  the  various  processes  of  the  face  are  fully  united, 
an  extension  passes  upwards  over  the  lateral  nasal  cartilage  towards  the 
frontal  bone  (frontal  process)  ;  the  orbital,  alveolar,  and  palatal  processes 
are  later  extensions  from  the  single  centre  of  ossification  (Mall,  Fawcett). 

Palato-Quadrate  Bar. — In  lower  vertebrates  the  maxillary  process  is 
supported  by  a  skeletal  bar  of  cartilage  known  as  the  palato-quadrate  bar, 
because  it  stretches  from  the  palate  to  the  quadrate  bone^  situated  at  the 
base  of  the  mandibular  arch  (Fig.  173).  Although  in  the  human  embryo 
this  cartilaginous  bar  is  at  no  time  clearly  differentiated  (Fawcett),  there 


premaxiiia 


naso-pal.  for. 

palatal  proc.  of  max. 
palatine  foramen, 
palatal  proc.  of  palatine 


Fig.  172. — Showing  the  Hard  Palate  at  birth.     The  premaxillary  part  is  formed 
from  the  Mesial  Nasal  Processes  ;    the  remainder  by  the  Palatal  Plates  of  the 

Maxillary  Processes. 

can  be  no  doubt  that  two  bones  have  arisen  in  connection  with  it — namely 
the  palate  and  internal  pterygoid  (Fig.  174).  The  internal  pterygoid  plate 
— the  first  part  of  the  sphenoid  to  ossify — is  formed  early  in  the  3rd  month 
in  membranous  tissue  which  overlies  the  position  of  the  middle  part  of  the 
bar,  while  the  vertical  plate  of  the  palate  is  developed  in  membrane  over  its 
more  anterior  part.  Ossification  extends  to  the  horizontal  plate,  within  the 
horizontal  plate  of  the  maxillary  process,  at  the  end  of  the  2nd  month. 

The  mandibular  process  has  also  a  cartilaginous  bar  developed  within 
it  known  as  Meckel's  cartilage  (Fig.  173).  Thus  each  of  the  processes 
which  grow  out  to  form  the  face  has  a  basis  of  cartilage,  but  while  the 
cartilages  within  the  nasal  processes  are  continuous  with  the  base  of  the 
skull,  the  cartilage  within  the  maxillary  process  comes  in  contact  by  its 
posterior  extremity  with  Meckel's  cartilage  (Fig.  173).  The  quadrate 
bone,  which  is  well  seen  as  a  separate  element  in  birds  and  reptiles,  forms  a 
movable  base  on  which  the  lower  jaw  articulates.  This  form  of  joint 
gives  birds  and  reptiles  an  easy  faculty  of  swallowing  unmasticated  food. 

^  E.  Fawcett,  Journ.  Anat.  and  Physiol.  1911,  vol.  45,  p.  378  (Ossification  of  Maxilla). 
See  also  MaU,  Amer.  Jour.  Anal.  1906,  vol.  5,  p.  449. 


172 


HUMAN  EMBRYOLOaY  AND  MORPHOLOGY 


With  tlie  development  of  grinding  and  chewing  teeth  in  the  very  early 
ancestry  of  mammals  a  more  stable  form  of  temporo-mandibular  articulation 
was  evolved,  the  mandible  during  the  change  coming  to  articulate  with  the 
temporal  bone,  thus  leaving  the  upper  end  of  Meckel's  cartilage  and  the 
quadrate  free  to  be  utilized  as  the  malleus  and  incus  by  the  organ  of  hearing. 


hyomandib.  cart. 


nasal  pit 
septal  carta 
palate 


quadrate 
Meckel's  cart. 


Fis.  173. — The  Cartilages  in  the  Nasal,  Maxillary  and  Mandibular  Processes  of  a  Shark. 

The  simplest  condition  of  the  cartilages  of  the  maxillary  and  mandibular 
processes  is  seen  in  certain  fishes.  In  the  common  base  of  these  two 
processes,  there  is  developed  a  cartilage  which  binds  the  basal  ends  of  the 
palato-quadrate  bar  and  Meckel's  cartilage  to  the  skull.  The  cartilage  of 
the  hyoid  arch  is  also  bound  to  it,  and  hence  it  is  known  as  the  hyo-mandi- 
bular  cartilage.     (Compare  Figs.  132,  133,  150  and  173.) 

Nerves  and  Arteries  o£  the  Maxillary  Process.— A  knowledge  of  the 
manner  in  which  the  maxillary  process  is  developed  explains  the  distribu- 


MALAft 
PALATB 


TYMPANIC 


meckeCs  cartil- 


Fig.  174. — Diagram  to  show  the  position  of  the  bones  in  the  Sliull  of  the  Human  Foetus 
which  are  formed  in  connection  witli  the  palato-quadrate  bar. 

tion  and  course  of  its  arteries  and  nerves.  The  second  division  of  the  5th, 
represented  by  the  infra-orbital,  descending  palatine,  pterygo-palatine, 
and  Vidian  nerves,  forms  its  nerve  supply.  Its  main  artery  is  the  internal 
maxillary. 


DEVELOPMENT  OF  THE  FACE  173 

Formation  of  Foramina  and  Canals  in  Bone. — The  development  of 
canals  and  foramina  in  the  bones  of  the  maxillary  process  illustrates  the 
manner  in  which  these  are  formed  in  the  skull  generally.  Many  foramina 
and  canals  occur  between  elements  which  unite  in  the  course  of  develop- 
ment (see  p.  150).  The  Vidian  nerve  lies  between  the  internal  pterygoid 
plate  (a  separate  bone)  and  the  external  pterygoid,  a  plate  which  is  formed 
in  the  maxillary  process  as  a  prolongation  of  the  great  wing  of  the  sphenoid. 
The  pterygo-palatine  canal  is  situated  between  the  pterygoid  and  palatal 
parts  of  the  palato-quadrate  bar.  The  descending  palatine  nerve  lies 
between  the  palate  bone  and  superior  maxilla.  These  are  canals  formed 
between  different  elements.  The  infra-orbital  nerve  at  first  passes  forwards 
in  a  groove  on  the  orbital  aspect  of  the  superior  maxilla,  but  in  the  later 
months  of  foetal  life,  upgrowths  from  the  centre  of  ossification  of  the 
maxilla  meet  over  the  nerve  and  convert  the  groove  into  a  canal. 

The  foramen  rotundum  and  foramen  ovale  are  at  first  notches  on  the  edge 
of  the  great  wing  of  the  sphenoid,  but  in  the  course  of  foetal  growth  the 
notches  are  converted  into  foramina.  Hence  wherever  a  nerve  foramen 
or  canal  is  found  one  may  conclude  that  it  marks  the  junction  of  two  ele- 
ments, originally  distinct,  or  that  it  is  originally  a  groove  or  notch  on  the 
edge  of  the  bone  (Bland-Sutton).  The  foramina  for  nerves  in  the  malar 
bone  appear  to  be  exceptions  to  this  rule.  Only  one  centre  appears  for  the 
ossification  of  this  bone  (7th  week),  and  the  nerves  evidently  become 
involved  during  the  ossification  of  the  membranous  basis.  The  malar 
bone  is  occasionally  ossified  from  two  centres  which  may  fail  to  unite  ; 
the  bone  is  then  divided  by  a  suture  passing  from  the  orbit  to  the  temporal 
fossa.  A  divided  malar  occurs  rather  more  frequently  in  Japanese  and 
Mongolian  skulls,  hence  the  name  of  Os  Japonicum. 

Palatal  Rugae. — ^In  all  classes  of  mammals  the  mucous  membrane  on 
the  hard  palate  is  ridged  transversely  ;  three  or  four  of  these  transverse 
ridges  are  seen  on  each  side  of  the  palate  of  the  newly  born  child  ;  they 
tend  to  disappear  in  the  adult.  Food  is  triturated  between  them  and  the 
rough  papillae  on  the  palatal  aspect  of  the  tongue.  Their  disappearance 
in  man  is  probably  due  to  the  soft  nature  of  his  food. 

Maxillary  Sinus. — It  will  be  seen  from  Fig.  175  that  the  maxillary 
process  is  at  first  a  thin  plate,  lying  between  the  orbit  and  mouth,  con- 
taining the  canine  and  molar  tooth  buds.  It  rests  on  the  outer  aspect 
of  the  lateral  nasal  process,  and  to  some  extent  assists  that  process  to  form 
the  outer  wall  of  the  nasal  cavity  (Fig.  169).  In  the  third  month  of  foetal 
life  the  mucous  membrane  in  the  middle  meatus  begins  to  bud  outwards, 
presses  before  it  and  bursts  through  the  lateral  nasal  plate  of  cartilage 
and  begins  to  distend  the  maxillary  process.  At  birth  the  sinus  is  only 
a  shallow  recess  on  the  outer  wall  of  the  middle  meatus,  above  the  germ 
of  the  first  milk  molar  (Fig.  175).  It  continues  to  grow  until  the  25th 
year,  and  is  the  only  one  of  the  air  sinuses  developed  from  the  nasal  cavity 
which  is  more  than  a  rudiment  at  the  time  of  birth.  In  the  years  of 
adolescence  the  sinus  expands  until  it  inflates  the  maxillary  part  of  the 
malar.  As  it  expands  backwards  the  posterior  border  of  the  maxilla, 
which  contains  the  buds  of  the  permanent  molar  teeth,  undergoes  a  rotation 


174 


HUMAN  EMBEYOLOGY  AND  MORPHOLOGY 


downwards,  so  that  what  was  situated  on  the  posterior  border  conies  to 
be  situated  on  the  alveolar  border  (Fig.  176).  If  the  processes  of  growth 
and  rotation  are  arrested,  the  last  molar  (wisdom)  tooth  is  left  on  the 
posterior  border  of  the  maxilla,  where  it  may  give  rise  to  pain  and  sup- 
jDuration.     The  maxillary  sinus  or  antrum  is  peculiarly  large  in  man  and 


hiatus  semiluri: 
antrum 


ethm.  cell 
turb. 
hiatus  semilun. 
antrum 

tooth  bud 


tooth  bud 


palate 


inf.  turb. 


Fig.  175.- 


-Coronal  Section  of  the  Nasal  Cavities  of  a  Newly-Born  Child,  showing  the 
development  of  the  hiatus  semilunaris  and  air  sinuses. 


in  the  anthropoid  apes.      It  is  small  in  monkeys,  a  greatly  expanded 
inferior  meatus  taking  its  place. ^ 

Mandibular  Process  and  Arch. — The  two  mandibular  processes  unite 
in  the  middle  line  and  form  the  mandibular  or  first  visceral  arch.  The 
arch  forms  the  lower  or  hinder  boundary  of  the  stomodaeum  (Fig.  177). 
The  right  and  left  processes  are  in  contact  in  the  4th  week  of  development, 


/KNTRUM    AT  BiHTH 
ANTRUM  OF  AOULT 


Fig.  176.- 


-Showing  the  manner  in  which  the  development  of  the  Maxillary  Antrum 
alfects  the  size  of  the  palate  and  position  of  the  molar  teeth. 


but  the  process  of  fusion,  which  may  be  arrested  (Fig.  159),  is  not  complete 
until  the  middle  of  the  second  month. 

Parts  formed  from  the  Mandibular  Arch.— Besides  the  lower  jaw, 
there  are  formed  from  this  arch  the  soft  parts  over  and  under  the  jaw,  the 
lower  lip,  the  muscles  of  mastication,  the  mylo-hyoid  and  anterior  belly  of 

^  See  Keith,  Proc.  Anat.  Soc.  Great  Brit,  and  Ir.  May,  1902,  Brit.  Journ.  Dent.  Sc. 
1902,  vol.  45,  p.  529  ;  J.  Parsons  Schaeffer,  Amer.  Journ.  Anat.  1912,  vol.  13,  p.  1  ; 
Ibid.  Amer.  Journ.  Anat.  1912,  vol.  13,  p.  1  (Formation  of  Nasal  Duct) ;  Ibid. 
Amer.  Journ.  Anat.  1910,  vol.  10,  p.  313  (Formation  of  Antrum).  See  also 
references,  p.  135. 


DEVELOPMENT  OF  THE  FACE 


175 


the  digastric,  the  tensor  palati,  and  the  tensor  tympani.  The  anterior 
two-thirds  of  the  tongue,  the  sublingual  and  submaxillary  glands  are 
formed  from  the  floor  of  the  primitive  pharynx  between  the  mandibular 
and  the  second  or  hyoid  arch.  These  parts  are  supplied  from  the  nerve 
of  the  mandibular  arch,  and  are  therefore  probably  derived,  in  part  at  least, 
from  the  substance  of  the  arch.  The  whole  arch  and  its  derivatives  are  set 
apart  primarily  for  the  purpose  of  mastication.  Only  in  mammals  are  the 
lips  separated  from  the  alveolar  processes.  In  the  human  embryo  the  lower 
lip  is  demarcated  from  the  alveolus  by  the  downgrowth  of  an  epithelial 
groove  (the  labio-alveolar  plate  or  groove)  about  the  middle  of  the  7th 
week. 

The  Mandibular  Arch  bounds  the  stomodaeum  behind,  and  is  the  fore- 
most of  the  visceral  arches  which  encircle  and  form  the  walls  of  the  primitive 
pharynx.    Meckel's  cartilage^  forms  its  skeletal  basis  (Figs.  173,  174). 


FORE   BRAIN 
STOMODAEUM 


MAK  :  PROC: 
MANOIB  :  PROC 


Fig.  177. — The  Mandibular  Arch  and  Stomodaeum  (primitive  mouth)  in  a  Human 
Embryo  of  the  5th  week.    (After  Rabl.)    A,  from  the  front ;  B,  from  the  side. 

The  3rd  division  of  the  5th  is  its  nerve,  but  its  artery,  the  first  aortic  arch, 
has  only  a  transient  existence,  although  the  inferior  dental  may  represent 
part  of  it. 

Development  and  Ossification  of  the  Lower  Jaw.^ — In  Fig.  178, 
which  represents  the  condition  of  the  human  mandible  at  the  beginning 
of  the  4th  month,  the  primitive  cartilaginous  skeleton  of  the  mandibular 
arch  can  still  be  followed  from  the  symphysis  to  the  tympanum.  Only 
one  part  of  the  cartilage  takes  a  direct  share  in  the  formation  of  the  man- 
dible— that  part  which  lies  near  the  symphysis  and  assists  to  form  the 
section  of  the  mandible  which  carries  the  first  premolar  and  canine  teeth. 
The  ventral  extremities  persist  through  foetal  life  as  cartilaginous  nodules  ; 
they  may  become  ossified.  The  proximal  end  of  Meckel's  cartilage  forms  the 
malleus  ;  all  the  rest  of  the  bar  disappears,  although  the  long  internal 
lateral  ligament  occupies  the  site  of  part  of  the  cartilage.     In  rare  instances 

1  E.  Gaupp,  Anat.  Anz.  1911,  vol.  39,  pp.  97,  433,  609  (Morphology  and  Mandible). 

'^  I  have  followed  the  account  given  by  Dr.  Alex.  Low,  Jour?},  of  Anat.  and  Physiol. 
1910,  vol.  44,  p.  83.  See  also  Professor  Fawcett's  account  in  Journ.  Amer.  Med.  Assoc. 
1905,  vol.  45,  p.  695.  For  abnormal  ossification  of  Meckel's  cartilage  see  Keith, 
Journ.  Anat.  and  Physiol.  1910,  vol.  44,  p.  161. 


FORE 

BRAIN 

V 

,   MAX: 

PROC: 

J 

^ 

MANDIB;  ARCH 
1?T 

p 

\ 

,HY0ID    ARCH 

2Np 

^ 

%r 

-  3"° 

? 

1 

i 

_  4TH 

176     HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

the  cartilage  may  undergo  complete  and  independent  ossification.  Thus 
the  lower  jaw,  which  shares  with  the  clavicle  the  distinction  of  being  the 
first  bone  in  the  body  to  ossify,  is  a  membrane  or  dermal  bone.  Late  in 
the  7th  week  a  centre  of  ossification  appears  in  each  half,  on  the  outer  side 
of  the  Meckel's  cartilage,  and  near  the  site  of  the  future  mental  foramen. 
Each  half  of  the  lower  jaw  is  ossified  by  the  extension  of  a  single  centre. 
Processes  grow  up  on  either  side  of  the  inferior  dental  nerve  which,  with 
the  tooth  buds,  comes  to  lie  in  a  primitive  alveolar  trough.  During  the 
third  month  the  ascending  ramus  begins  to  form.  In  the  condylar  and 
coronoid  processes  a  formation  of  secondary  cartilage  occurs  ;  thus  the 
condyle  and  coronoid  are  ultimately  laid  down  in  cartilage.  The  two  halves 
of  the  mandible  unite  at  the  symphysis  during  the  second  year  ;  in  some 
animals,  such  as  the  kangaroo,  the  symphysis  remains  open. 

Evolution  of  the  Mandible. — To  interpret  the  appearances  seen  during 
the  development  of  the  human  mandible  we  must  suppose  that  Meckel's 

CORONOID, 
MEMBRANE  BONE 
INNER    DENTAL  SHELF 
TIP  of  M:  CARTILAGE 


MECKELS    CARTILAGE 
SYMPHYSIS 

FiG.f  178. — Meckel's  Cartilage  and  Mandible  of  a  Foetus  in  the  4th  month  of  develop- 
ment, viewed  on  the  inner  or  lingual  aspect.  (From  a  drawing  and  reconstruction 
by  Dr.  Alex.  Low.)  A  and  B,  cartilaginous  ossicles  at  symphysis ;  C,  termina- 
tion of  Meckel's  cartilage. 

cartilage  is  the  primitive  skeleton  of  the  mandible — a  condition  we  know 
to  occur  in  various  forms  of  fishes  (see  Fig.  173).  The  malleus  formed  the 
upper  end  of  the  skeleton  of  the  jaw,  the  joint  between  the  malleus  and 
incus  representing  the  mandibular  joint.  The  second  stage  in  the  evolution 
of  the  jaw  is  the  formation  of  membrane  or  dermal  bone  to  strengthen  the 
cartilaginous  rod  and  form  supports  for  the  teeth.  This  stage  is  also  seen 
in  fishes.  The  third  and  final  stage  is  the  formation  of  an  ascending  ramus 
and  the  evolution  of  a  new  joint  between  the  condyle  of  the  ascending 
ramus  and  the  squamosal  part  of  the  temporal.  This  stage  evidently 
occurred  in  the  early  ancestry  of  the  mammals.  In  all  other  vertebrates 
— amphibians,  reptiles  and  birds — the  primitive  joint  persists. 

Growth  Changes  in  the  Jaw. — The  mandible  undergoes  great  changes 
in  the  course  of  growth.  As  the  permanent  teeth  erupt  behind  the  milk 
set,  increased  alveolar  space  is  required.  This  is  obtained  (see  Fig.  179) 
by  new  bone  being  deposited  along  the  posterior  border  of  the  ascending 
ramus,  while  absorption  takes  place  at  the  anterior  border.  Growth  in 
the  vertical  height  is  obtained  by  the  deposition  of  new  bone  along  the 
upper  border  of  the  ramus.     Growth  of  the  upper  jaw  and  of  the  antrum 


DEVELOPMENT  OF  THE  FACE  177 

of  Highmore,  by  pushing  downwards  the  body  of  the  lower  jaw,  leads 
to  an  elongation  of  the  ascending  ramus,  and  to  its  assuming  a  more 
vertical  position  to  the  body  of  the  jaw  (Fig.  179).  In  old  age,  when  the 
teeth  drop  out  and  the  alveolar  margins  are  absorbed,  the  ascending  ramus 
again  becomes  oblique,  to  allow  the  lower  jaw  to  come  in  contact  with  the 
upper  during  mastication.  The  mental  eminence  is  present  at  birth,  and 
is  a  human  characteristic.  In  apes  the  genioglossal  muscles  arise  from  a 
fossa,  in  place  of  a  tubercle  as  in  man,  on  the  lingual  aspect  of  the  sym- 
physis.    In  primitive  races  this  simian  fossa  occasionally  occurs.^ 

As  the  teeth  erupt,  growth  occurs  both  at  the  lower  and  alveolar  borders, 
and  also  over  the  mental  eminence  or  chin.  These  growth  changes  are  well 
exemplified  in  the  subjects  of  acromegaly  (Fig.  179).     In  this  disease  growth 


CORONOID    PROC>^   L'*' 


^AT    BIRTH 

NORMAL 

..ACROMEGALY 


Fig.  179. — Mandibles  of  a  Child  at  Birth,  of  a  Normal  Adult  and  of  a  Man,  the  subject 
of  the  disease  of  growth  known  as  Acromegaly,  superimposed  to  show  the  manner 
in  which  growth  takes  place. 

of  the  jaw  proceeds  after  adult  years  are  reached.  The  deposition  of  new 
bone  at  the  condylar  process  leads  to  the  chin  and  teeth  being  pushed  for- 
wards in  front  of  the  upper  jaw  and  teeth.  The  chin  and  lower  border  also 
increase  in  size. 

The  Temporomandibular  Articulation. — Two  types  of  this  joint 
are  found  in  mammals,  one  (see  Fig.  180,  A),  exemplified  in  the  carnivora, 
in  which  only  a  hinge  action  is  permitted,  and  hence  the  jaws  act  like 
scissor  blades  ;  the  second  (see  Fig.  180,  C),  in  which  a  gliding  movement 
is  allowed,  the  teeth  being  thus  able  to  act  as  grinders.  The  second  type 
occurs  in  all  vegetable  feeders.  The  human  articulation  combines  the 
characters  of  both  types  (Fig.  180,  B),  the  gliding  action  taking  place 
between  the  interarticular  cartilage  and  the  skull,  the  hinge  action  between 
the  cartilage  and  the  condyle.     In  rodents  the  glenoid  cavity  is  a  narrow 

^  For  the  morphology  of  chin  and  symphysis  see  Professor  Arthur  Thomson,  Journ. 
Anat.  1916,  vol.  50,  p.  43. 

M 


178 


HUMAN  EMBRYOLOaY  AND  MORPHOLOGY 


gutter  in  which  the  plate-like  condyloid  process  glides  backwards  and 
forwards.  The  interarticular  cartilage  is  developed  in  all  the  Mammalia 
except  the  monotremes,  and  one  or  two  marsupials  (Parsons).^  At  the 
end  of  the  third  month  the  cartilage  appears  as  a  condensation  of  fibrous 
tissue  between  the  coronoid  process  and  root  of  the  zygoma.  There  is  at 
this  time  no  articular  cavity  ;  the  disc  appears  to  arise  from  tissue  caught 
between  the  condylar  process  and  future  glenoid  cavity  ( Vino grad off). 

Development   of  the  Tympanic  Plate  and   Articular  Eminence. — 

If  the  chin  be  depressed  the  condyle  of  the  jaw  moves  on  to  the  articular 


inter-artic 
cart. 

condyle -^j, 


ext  aud. 
meat 


post-glemid  sp. 


post-glenoid  sp. 


-mastoid 
^    .-.^tympanic 

condyle 


ext  aud  meat, 
paramastoid 
"^post-glenoid  sp. 

cartilage 
condyle 

Fig.  180. — The  chief  types  of  the  Temporo-MaxUlary  Articulation. 
A.  Carnivorous  Type.    B.  Omnivorous  Type.     G.  Herbivorous  Type. 

eminence  (Fig.  180,  B)  ;  if  over-depressed  it  springs  over  the  eminence, 
and  a  dislocation  is  produced.  This  is  impossible  in  the  early  years  of  life, 
for  at  birth  there  is  no  eminence  and  no  glenoid  cavity  (see  Fig.  181,  A). 
At  birth  the  membrana  tympani  lies  exposed  on  the  surface  of  the 
skull  behind  the  condyle,  supported  in  a  fine  osseous  hoop,  the  tympanic 
ring.  The  ring  is  imperfect  above,  and  there  the  flaccid  part  of  the  mem- 
brane occurs.  By  the  second  year  the  ring  has  grown  into  a  plate  by 
sending  out  two  processes,  which,  as  they  grow  out,  unite  and  leave  a  gap 
between  (Fig.  181,  B).  This,  as  a  rule,  is  soon  filled  up.  By  the  20th 
year  the  tympanic  plate  is  three-quarters  of  an  inch  long,  forming  the 

1  "  Joints  of  Mammals,"  Journ.  of  Anat.  and  Physiol.  1900,  vol  34,  p.  41. 


DEVELOPMENT  OF  THE  FACE 


179 


bony  floor  of  the  external  meatus  and  the  posterior  wall  of  the  glenoid 
fossa,  which  in  man  is  remarkably  deep.  It  protects  the  meatus  from 
the  condyle,  and  must  be  regarded  as  an  accessory  part  of  the  mandibular 
joint.  Every  year  until  the  20th  the  bony  meatus  gets  longer,  while  the 
fibro-cartilaginous  part  becomes  relatively  shorter.  In  the  adult  the 
bony  part  forms  two-thirds  of  the  meatus.     As  the  tympanic  plate  grows 


post-glen.sp. 
Eustach. 


B 

post-glen.  sp. 


post-meat 
proc. 

petro-mast 


petro-squam. 
suture 


tymp.  plate^'^sr: 


r/f-^petro-mastoid 


post-meat, 
proc. 


post-glen.  sp. 


artic.  emin. 

tymp.  Plate"^  ^\^J^^^^_ 
styloid  petro-masto/d 

Fig.  181 . — Showing  the  chief  changes  after  birth  in  the  form  of  the  Temporo-Maxillary 

Articulation. 

A.  At  Birth.     B.  At  Two  Years.     C.  In  the  Adult. 

outwards,  the  membrana  becomes  less  easily  accessible  to  the  surgeon 
(Fig.  181,  C).  The  plate  also  grows  inwards  to  form  the  floor  of  the  bony 
part  of  the  Eustachian  tube  and  downwards  to  form  the  vaginal  process, 
to  which  the  upper  end  of  the  carotid  sheath  is  attached  (Fig.  181,  C). 

Fate  of  the  Stomodaeum. — Having  described  the  manner  in  which 
the  three  developmental  masses — nasal,  maxillary  and  mandibular — -are 
involved  in  the  upbuilding  of  each  side  of  the  face,  it  may  be  profitable 


180 


HUMAN  EMBKYOLOGY  AND  MOEPHOLOGY 


to  look  back  and  see  what  has  become  of  the  primitive  oral  cavity — ^the 
stomodaeum.  A  diagrammatic  section  of  this  cavity  is  given  in  Fig.  182  ; 
up  to  the  5th  week  it  is  separated  from  the  primitive  pharynx  by  the  oral 
membrane  ;    the  pituitary  evagination — Rathke's  pocket — is  seen  arising 


stomodaeum 
max.  proc. 

somatopleure 
mandibular  arch 


Fig.  182.- 


-Sagittal  Section  sliowing  the  Stomodaeum  and  position  of  the  Oral  Plate 
in  the  4th  week.     (Schematic.) 


from  the  stomodaeum  at  the  dorsal  margin  of  the  membrane.  When  the 
prechordal  plate  of  cartilage  is  formed  below  the  fore-brain,  the  -pituitary 
body  thus  becoming  an  intracranial  organ,  its  stalk  comes  to  be  situated 
at  the  hinder  or  sphenoid  end  of  the  nasal  septum  or  vomer.     This  vomerine 


Eustach. 
Ost  cleft). 


tonsil    u 
(2nd  cleft. 


from  max.  proc. 
^position  of  oral  plate . 

from  mandib.  proc. 


Fig.  183. — Showing  the  fate  of  the  Stomodaeum.    The  relative  position  of  the  Oral 
Plate  is  indicated. 

point  may  be  regarded  as  stationary  during  the  development  of  the  nasal 
and  buccal  cavities.  In  Fig.  183  the  position  is  shown  which  the  oral 
plate  would  assimie  were  it  to  persist  until  adult  life.  The  lips  and  teeth  are 
developed  in  front  of  it,  and  therefore  within  the  cavity  of  the  stomodaeum. 
The  hard  palate  is  developed  in  front  of  it  but  only  part  of  the  soft.     The 


DEVELOPMENT  OF  THE  FACE  181 

nasal  cavities  are  not  derived  from  the  stomodaeum.  It  is  true  tliat  the 
nasal  processes  grow  within  and  fill  up  the  primitive  space  as  it  expands, 
but  the  cavities  within  the  nasal  processes  represent  expansions  of  the 
primary  olfactory  pockets.  The  tongue  and  floor  of  the  mouth  arise  in 
the  pharynx,  behind  the  oral  plate. 

In  this  chapter  an  account  has  been  given  of  the  various  embryological 
elements  which  go  to  form  the  face.  In  the  chapters  dealing  with  the  eye, 
nose,  teeth  and  tongue  further  details  will  be  described.  The  chief  feature 
of  the  human  face  is  its  power  of  expression — due  to  the  high  differentiation 
of  its  subcutaneous  musculature,  and  to  the  elaborate  nervous  mechanism 
controlling  that  musculature.  The  muscles  of  expression,  we  shall  see,  arise 
in  connection  with  the  hyoid  arch  ;  their  wide  distribution  on  the  face 
occurred  with  the  evolution  of  the  pulmonary  respiratory  system. 


CHAPTEE  XTII. 

THE  TEETH  AND  APPARATUS  OF  MASTICATION. 

In  previous  chapters  dealing  with  the  Cranium  and  Face,  many  of  the 
changes  in  the  apparatus  of  mastication  have  already  been  mentioned. 
At  the  end  of  the  second  year  the  alveolar  parts  of  the  palate  and  mandible 
are  only  sufficiently  large  to  carry  the  milk  dentition — which  comprises 
20  teeth  altogether,  8  of  these  being  incisors,  8  milk  molars,  and  4  canines. 
During  the  eruption  of  the  permanent  teeth,  from  the  5th  to  the  22nd 
year,  space  has  to  be  found  for  the  12  permanent  molar  teeth,  the  place 
of  the  milk  teeth  being  occupied  by  the  permanent  incisors,  canine  and 
premolar  teeth.  Hence  the  rapid  growth  of  jaws,  the  enlargement  and 
strengthening  of  the  face,  the  development  of  supra-orbital  ridges  and  the 
upgrowth  of  the  temporal  line,  which  are  seen  to  take  place  as  the  per- 
manent teeth  come  into  position.  At  the  same  time  growth  changes  affect 
the  muscles  of  mastication. 

Evolution  of  Teeth.^ — The  teeth  are  products  of  the  skin.  Both  the 
cutis  or  dermis  and  the  epithelium  or  epidermis  enter  into  their  formation. 
A  tooth  is  a  papilla  of  the  dermis  which  has  undergone  a  peculiar  form  of 
ossification  (dentine) ;  it  is  coated  by  an  extremely  hard  substance — 
enamel — which  is  formed  by  the  epidermis.  Between  the  placoid  scales 
which  cover  the  skin  of  the  shark  and  the  complicated  molar  tooth  of 
an  elephant,  there  is  a  connecting  series  of  intermediate  forms.  The 
primitive  teeth  have  a  conical  or  peg-like  form,  but  with  the  evolution  of 
mastication  in  the  primitive  mammalian  stock  the  conical  teeth  became 
difierentiated  into  various  and  complicated  forms — the  molar  teeth 
departing  very  markedly  from  the  primitive  simple  type.  The  recognition 
of  the  true  nature  of  teeth  was  delayed  by  the  fact  that,  during  the  develop- 
ment, the  dental  papilla  and  its  epidermal  covering  are  submerged  beneath 
the  lining  membrane  of  the  mouth. 

The  Structure  of  a  Tooth. — A  tooth  may  be  considered  as  made  up  of 
five  parts  (see  Fig.  184)  : 

(1)  The  pulp,  situated  within  (2)  a  capsule  of  dentine  ;  the  exposed  part 
or  crown  of  the  dentine  is  coated  by — (3)  the  enamel ;  the  embedded  part 
or  root  by  a  layer  of  bone — (4)  the  crusta  petrosa.  The  root  is  secured 
within  its  socket  by  (5)  the  peridental  membrane,  which  acts  as  a  periosteum 
to  both  the  crusta  petrosa  and  bony  wall  of  the  tooth  socket.  An  account 
of  the  development  of  a  tooth  has  to  deal  with  the  origin  of  each  of  these 
five  parts. 

182 


THE  TEETH  AND  APPARATUS  OF  MASTICATION        183 


(1)  Origin  of  the  Enamel. — The  enamel  buds  are  formed  by  the  ecto- 
derm of  the  stomodaeum.  In  the  7th  week  the  ectoderm  within  the  labial 
margin  grows  within  the  iinderlying  mesodermal  tissues  so  that  a  narrow 
semicircular  invagination  of  epithelium  is  formed  within  the  mandibular 
arch  below,  and  within  the  premaxillary  and  maxillary  parts  of  the 
primitive  palate  above.  To  the  plate  of  ectoderm  thus  infolded  the  name 
of  dental  lamina  or  shelf  is  given.  As  may  be  seen  in  a  section  of  the 
foetal  lower  jaw  (Fig.  185)  the  dental  lamina  is  continuous  at  its  origin 
with  the  epithelial  downgrowth  which  separates  the  lip  from  the  alveolus. 
From  the  ingrowing  or  deep  margin  of  the  dental  lamina  ten  epithelial 
buds  arise  during  the  3rd  month,  both  in  the  upper  and  lower  jaw.  Each 
of  these  twenty  enamel  buds  or  organs  produces  the  enamel  to  cover  the 
crown  of  a  milk  tooth.     Each  bud  as  it  deepens  and  expands  comes  against 


enamel 


Fig.  184. 


-dentine 

odontO'blasts 

-pulp 

crusta  petrosa 
epidermis  of  gum 
peridental  membrane 

alveolus 


dental  canal 
and  artery 

-Showing  the  parts  of  an  Incisor  Tooth. 


a  condensed  formation  in  the  mesoderm  of  the  jaw — the  dental  papilla.^ 
On  the  papilla  the  enamel  bud  becomes  partly  invaginated,  the  meso- 
dermal or  odontoblastic  bud  coming  to  lie  within  the  invagination  (Fig.  185). 
During  the  4th  month  the  deeper  stratum  of  ectodermal  cells  which  cover 
the  papilla  change  into  columnar  enamel-producing  cells  or  ameloblasts. 
The  basal  part  of  the  ameloblasts  is  converted  gradually  into  enamel, 
or  to  put  it  somewhat  differently,  the  ameloblasts  form  and  deposit  enamel 
in  their  bases  and  thus  produce  a  coating  for  the  dental  papilla.  Each 
am^eloblast  is  gradually  converted  into  an  enamel  fibre,  their  more 
superficial  parts  are  never  so  converted,  but  persist  as  the  cuticular  mem- 
brane (Nasmyth's  membrane)  which  covers  the  enamel  at  birth  and  is  soon 
afterwards  worn  ofi.  The  enamel  of  the  milk  teeth  is  completely  formed 
before  birth  ;  and  that  of  the  first  permanent  molar  is  already  partly 
deposited.     From  the  5th  month  onwards  the  dental  lamina — between  the 

1  A.  Masur,  Anat.  Hefte,  1907,  vol.  35,  p.  263  (Dev.  of  Dental  Pulp) ;   J.  Howard 
Mummery,  The  Microscopic  Anatomy  of  the  Teeth,  1920. 


184      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

tooth  germs  and  the  surface  of  the  alveolar  margin — -undergoes  a  gradual 
disruption  and  absorption.  Isolated  masses  of  the  lamina  may  persist 
within  the  gums  and  in  certain  cases  give  rise  to  masses  of  dental  tissue — 
odontomes. 

(2)  Origin  of  the  Dentine. — The  dental  papilla  or  odontoblastic  germ, 
formed  from  the  mesoderm,  corresponds  to  a  depressed  skin  (dermal) 
papilla,  the  enamel  cells  representing  its  covering  of  epithelium.  The 
dental  papilla  determines  the  shape  of  the  tooth.  In  its  superficial  layers 
it  contains  numerous  cells,  odontoblasts,  with  branched  processes  radiating 
towards  the  enamel  epithelium.  By  the  agency  of  the  odontoblasts  a 
substance  is  deposited  which  becomes  calcified  into  dentine  or  ivory.  It  is 
deposited  in  the  matrix  round  the  processes  of  the  odontoblasts.  The 
cavities  in  which  the  processes  are  enclosed  form  the  tubules  of  the  dentine. 
In  rodents  especially,  but  also  in  all  mammals,  although  only  to  a  slight 

UABIO-DENTAL.   GROOVE 

LOWER  UP  I 

POSITION  or  ORAL    MCMB. 


MANDIB    ARCH 


DENTAL   PAPILLA 


Fig.  185. — Section  through  the  Lip  and  Mandible  of  a  Foetus  in  the  third  month  of 
development,  showing  the  down-growth  of  the  Dental  Shelf. 

extent  in  civilized  races  of  mankind,  the  odontoblasts  react  to  wear,  add 
new  layers  of  dentine  to  the  wall  of  the  pulp  cavity,  and  thus  prevent  the 
pulp  from  being  exposed.  The  dentine  is  deposited  first  in  the  crown  of 
the  tooth  beneath  the  enamel ;  the  neck  is  laid  down  next,  and  then  the 
root,  the  last  point  of  all  to  be  formed  being  the  narrow  canal  at  the  apex 
of  the  root  by  which  the  dental  vessels  and  nerves  reach  the  pulp  cavity. 
The  dental  crowns  reach  their  full  size  at  the  time  of  their  formation. 
Teeth  thus  differ  from  all  other  structures  of  the  body  in  undergoing  no 
growth  subsequent  to  the  period  of  their  development. 

(3)  The  Pulp. — The  pulp  represents  the  remnant  of  the  odontoblastic 
germ  enclosed  by  the  dentine.  It  is  made  up  of  a  matrix  of  branching 
cells  and  contains  the  ramifications  of  the  artery,  vein  and  nerve  of  the 
tooth.  Fine  processes  of  the  nerves  pass  into  the  dental  tubules,  while 
in  its  peripheral  zone  are  situated  cells  possessing  the  characteristics  of 
nerve  cells  (Mummery). 

(4)  The  Dental  Sac.^The  foetal  tooth,  as  may  be  seen  from  Fig.  186, 
lies  embedded  in  the  alveolus  surrounded  by  a  fibrous  capsule  known  as  the 
dental  sac.     The  sac  and  its  contents  form  a  dental  follicle.    When  the 


THE  TEETH  AND  APPARATUS  OF  MASTICATION        185 


enamel  bud  is  invaginated  by  the  dental  papilla,  the  invaginated  wall 
forms  the  enamel-producing  layer,  while  the  invaginating  or  parietal  wall 
becomes  surrounded  by  a  dense  layer  of  mesodermal  tissue.  The  parietal 
wall  is  converted  into  the  dental  sac.  At  first  (Fig.  185)  the  dental  sac  is 
continuous  with  the  odontoblastic  germ  ;  it  becomes  separated  from  the 
pulp  when  the  root  or  roots  of  the  teeth  are  completed.  Between  the 
enamel  (invaginated)  and  parietal  (invaginating)  layers,  filling  the  cavity 
of  the  sac,  lies  a  mass  of  jelly-like  epithelium  corresponding  to  the  corneous 
epithelium  of  the  skin.  As  the  crown  of  the  tooth  grows  it  rises  within 
the  sac  of  the  enamel  germ,  and  causes  the  absorption  of  the  gelatinous 
material  (Fig.  187). 

(5)  The  Peridontal  Membrane. — The  peridontal  membrane  (Fig.  184) 
is  formed  by  that  part  of  the  dental  sac  which  surrounds  the  fang  of  the 


-lower  lip 

,gum  ridgeiepidermis) 

dental  grooue 

.remnant  of 
[dental  shelf. 

-epithelial  remnants 

r—— tongue 

h  enamel  bud  perm,  incisor 


enamel  of 
milk  incisor 


dental  sac 
ameloblasts 

papilla  (pulp) 


section  of  lower  Jaw 


Fig.  186. — Showing  the  stage  of  development  in  an  Incisor  Tooth  of  a  Foetus  of 

six  months. 

tooth.     The  part  of  the  dental  sac  which  surrounds  the  crown  is  absorbed 
during  the  eruption  of  the  tooth. 

(6)  The  Crusta  Petrosa. — The  peridontal  membrane  is  of  the  nature 
of  periosteum,  and  contains  osteoblasts  which  deposit  the  crusta  petrosa 
(bone)  on  that  part  of  the  dentine  which  forms  the  fang  and  also  on  the 
inner  wall  of  the  alveolus.  The  centres  of  ossification  in  the  upper  and 
lower  jaw  spread  round  the  labial  and  lingual  aspects  of  the  dental  sacs, 
thus  enclosing  them  in  a  bony  gutter  or  trough.  Subsequently  septa  are 
developed  between  the  dental  sacs,  and  thus  the  developing  teeth  come  to 
be  situated  in  bony  crypts.  The  roof  of  a  crypt  is  never  completed  ; 
a  hole  or  window  persists  through  which  the  neck  of  the  dental  sac  emerges 
to  become  continuous  with  the  mucous  membrane  covering  the  alveolus. 
The  crowns  of  the  teeth  erupt  at  the  point  of  union  between  the  dental  sac 
and  alveolar  membrane. 

Epithelial  Remnants  of  Enamel  Organ. — Epithelial  remnants  of  the 
dental  lamina  are  to  be  found  in  the  substance  of  the  alveolus  up  to  the 
end  of  foetal  life  or  later,  and  may  give  rise  to  cysts  of  various  kinds. 


186 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


Besides  these  there  are  also  others  which  occur  within  the  sac  surrounding 
an  uncut  tooth,  representing  remains  of  the  enamel  organ.  In  Fig.  187 
is  depicted  a  section  of  an  unerupted  first  permanent  molar  tooth,  lying 
within  its  dental  sac,  remnants  of  the  enamel  organ  being  shown  distributed 
within  the  sac  from  the  crown  to  the  growing  ends  of  the  roots.  We  have 
seen  that  the  enamel  organ  represents  an  epithelial  sac,  only  the  inner  or 
invaginated  wall  being  concerned  in  the  formation  of  the  enamel,  the  outer 
or  enveloping  layer  becoming  broken  up  as  shown  in  Mr.  Mummery's 
figure,^  to  form  an  interrupted  epithelial  layer  sometimes  named  Hertwig's 
sheath. 

Origin  of  the  Permanent  Teeth. — From  the  dental  shelf,  besides  the 
buds  for  the  milk  teeth,  there  grow  inwards,  during  the  latter  part  of  the 


EPITHBL.  . 
DENTAL  SAC 

Fig.  187. — Epithelial  remnants  in  the  Dental  Sac  of  a  first  Permanent  Human  Molar. 
(Howard  Mummery.) 

3rd  month  of  development,  so  as  to  lie  on  the  lingual  aspect  of  the  milk 
buds,  processes  of  ectoderm  which  form  the  enamel  of  the  ten  teeth  which 
replace  the  milk  teeth  (Figs.  185,  156  and  158).  The  three  permanent 
molars  of  each  side  arise  from  a  process  which  prolongs  the  dental  lamina 
backwards  behind  the  part  from  which  the  enamel  buds  of  the  milk  teeth 
arise  (Fig.  188).  The  first  molar  is  the  earliest  of  all  the  permanent  teeth  to 
undergo  development.  The  permanent  teeth  are  formed  in  exactly  the 
same  manner  as  the  milk  set.  They  develop  on  the  lingual  aspect  of  the 
roots  of  the  milk  teeth  (Fig.  186),  and  if  the  milk  teeth  be  roughly  extracted 
the  permanent  bud  may  also  be  torn  out.  Being  developed  deeper  in  the 
alveolus  than  the  milk  teeth,  the  neck  of  the  dental  sac  is  more  elongated, 
and  has  been  named  the  gubernaculum  dentis  under  the  belief  that  it  serves 
to  guide  the  teeth  during  eruption.  The  opening  by  which  the  guberna- 
culum  emerges  from  the  crypts  of  the  permanent  incisors  and  canines  is 

1  J.  Howard  Mummery,  Phil.  Trans.  1919,  vol.  209  (B),  p.  305. 


THE  TEETH  AND  APPARATUS  OF  MASTICATION        187 


seen  on  the  lingual  side  of  the  alveolus  near  the  sockets  of  the  corresponding 
milk  teeth.  In  the  case  of  the  premolars,  the  openings  lie  within  the  crypts 
of  the  milk  molars  (Carter). 

Dentigerous  and  other  Cysts  of  the  Jaw.^ — Cysts  with  epithelial 
walls,  containing  fluid,  teeth  or  other  dermal  contents,  occasionally  develop 
in  the  jaw.     They  are  formed  from  epithelial  remnants  of  the  dental  lamina. 


CHE.EK    rOLD 

MOLAR     PROC 
FOR    Z"!?*  3r*  MOLARS 


MUC-.  MEMBRANE 
ALVEOLUS 


ENTAL    SHELF 


QERM    of 
Zn"^    PREMOLAR 


CROWN   0-f 
2"?^    MILK    MOLAR 


1^?  PERM:  MOLAR 


Fig.  188. — ^Mucous  Membrane  covering  the  posterior  part  of  the  Alveolus  of  a  newly 
born  Child  with  the  Dental  Shelf  still  attached  to  it.  Proceeding  backwards 
from  the  end  of  the  dental  shelf  is  seen  the  "  molar  process,"  which  gives  rise  to 
the  three  permanent  molar  teeth.  The  crown  of  the  second  milk  molar  and  the 
germ  of  the  second  premolar  are  also  shown.    (After  Rose.) 

which  normally  breaks  up  and  disappears  completely,  or  from  detached 
parts  of  the  enamel  buds. 

Number  of  Dentitions. — In  many  lower  vertebrates,  such  as  sharks,  the 
dental  lamina  gives  off  constantly  a  series  of  buds,  so  that  as  soon  as  one 
tooth  is  lost  another  springs  up  in  its  place  from  behind  (Fig.  189).  In 
mammals  generally,  as  in  man,  the  dental  lamina  gives  off  only  two  series  of 


OENTAL  SHELF 


prim,  cusp 
cusp  on  cing. 
cingulum 


i»T SERIES,  zr'f   3rd 


^.w.jr- 


FlG.  189.— Diagrammatic  Section  across  Dental  Shelf  of  a  Shark  sho-wing  a  Succession 

of  Dentitions.     (After  Vialleton.) 
Fig.  190.— Premolar  Tooth  of  a  Carnivorous  Mammal  to  show  the  Primitive  Cone, 
Cmgulum  and  Secondary  Cusps  springing  from  the  Cingulum.    (Marett  Tims.) 

buds — one  for  the  milk  set  and  another  for  the  permanent  set.  In  mar- 
supials it  gives  off  only  one  series,  so  that  the  first  set  of  teeth  is  never 
replaced  by  a  second.  Thus  in  the  most  primitive  vertebrates  there  is  a 
succession  of  teeth,  owing  to  the  fecundity  of  the  dental  shelf.  In  man 
there  are  only  the  primary  and  secondary  broods,  but  it  is  possible  that 

1  P.  Adloff,  Anat.  Anz.  1912,  vol.  40,  p.  177  (Abortive  Dental  Buds). 


188 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


occasionally  representatives  of  a  3rd  brood  may  be  produced,  for  there 
are  cases  on  record  where  a  permanent  tooth  has  been  replaced  by  another 
late  in  life. 

Morphology  of  Human  Teeth.^ — The  crowns  of  all  the  human  teeth 
seem  to  be  modifications  of  the  same  type,  all  being  evolved  from  the 
simple  conical  tooth  found  in  fishes  and  reptiles  (Figs.  190,  191).  The 
conical  peg-like  tooth  is  to  be  regarded  as  the  most  primitive  type,  and 
in  man  vestigial  teeth  of  this  type  occasionally  occur.  A  modified  example 
of  the  type  is  seen  in  the  premolars  of  carnivorous  mammals  (Fig.  190). 
Here  the  base  of  the  peg-shaped  crown  is  surrounded  by  a  ring  of  enamel — 
the  cingulum.  From  the  conical  tooth  was  evolved  the  tritubercular 
type,  one  in  which  the  crown  carries  three  tubercles  or  cusps,  two  on  the 
labial  side  of  the  crown  and  one  on  the  lingual  margin  (Fig.  191,  A). 
Secondary  cusps  arise  from  the  cingulum  (Marett  Tims),  and  by  the  fusion 
of  these  with  the  original  cone  the  two  outer  cusps  are  produced,  while 


a.e. 


1     2 


7+2 


A. 


B. 


D. 


E. 


F. 


Fig.  191. — A.  The  Tritubercular  Type  of  Tooth.  The  corresponding  cusps  are  shown 
in  the  crowns  of  an  Incisor  (B),  Canine  (C),  Bicuspid  (D),  Upper  Molar  (E),  and 
a  Lower  Molar  (F). 

the  inner  cusp  arises  within  the  cingulum.  The  canine  retains  the  conical 
form  of  crown  ;  the  prominence  or  heel  on  the  lingual  aspect  of  the  crown 
represents  the  inner  cusp  ;  occasionally  this  cusp  is  well  developed  on  the 
human  canine  (Farmer).  The  cutting  edge  of  the  incisors  represents  the 
two  outer  cusps  ;  when  newly  cut,  the  incisor  crowns  show  five  serrations 
,or  cuspules.  In  the  premolars  or  bicuspids  the  outer  cusp,  as  may  be 
seen  in  many  of  the  lower  primates,  is  really  double. 

In  the  upper  molar  teeth,  to  the  three  primary  cusps  which  form  a  cup, 
a  fourth  has  been  added  (see  Fig.  191,  E).  The  two  outer  or  buccal  cusps 
are  distinguished  as  the  A.E.  cusp  (antero-external),  the  P.E.  cusp  (postero- 
external) ;  the  two  inner  as  the  A.I.  (antero-internal)  and  P.I.  (postero- 
internal). In  the  upper  molars  the  cusps  are  situated  alternately  and  the 
P.E.  and  A.I.  cusps  are  united  by  an  oblique  enamel  ridge,  which  represents 
the  posterior  margin  of  the  crown  of  the  primitive  tritubercular  tooth 
(Fig.  191,  E).  In  the  upper  molar  teeth  of  civilized  races,  especially  in 
their  wisdom  teeth,  the  4th  or  posterior  internal  cusp  is  often  absent,  the 

1  A.  C.  F.  Etemod,  Verhand.  Anat.  Oesellsch.  1911,  p.  144  (Bicuspid  Theory  of  Teeth); 
Sir  C.  S.  Tomes,  Manual  of  Dental  Anatomy  ;  Prof.  L.  Bolk,  Versuch.  einer  Losung  der 
Oebissprobleme,  Jena,  1913 ;  Amer.  Journ.  Anat.  1916,  vol.  19,  p.  91  ;  Journ.  Anat. 
1921,  vol.  55,  p.  138 ;  T.  Wingate  Todd,  Introduction  to  Mammalian  Dentition, 
1918;  D.  M.  Shaw  on  use  of  dental  cusps,  Journ.  Anat.  1918,  vol.  52,  p.  97. 


THE  TEETH  AND  APPARATUS  OF  MASTICATION        189 

primitive  tritubercular  tooth  thus  reappearing.  In  the  lower  molars  two 
cusps  have  been  added  to  the  three  primary  ones,  making  five  in  all.  The 
fifth  cusp  is  situated  at  the  posterior  border  of  the  crown  ;  the  others  are 
arranged  in  opposite  pairs.  The  fifth  cusp  has  become  lost  in  the  2nd  and 
3rd  lower  molars  of  civilized  races.  Harrison  found  in  Sphenodon,  a 
primitive  type  of  lizard,  that  concrescence  or  fusion  of  the  simple  peg-like 
teeth  takes  place  in  the  posterior  part  of  the  jaw  ;  it  is  possible  that  the 
molar  teeth  of  mammals  may  have  originated  thus  (Marett  Tims).  Gem- 
mination  may  occur  in  human  incisors  ;  the  incisor  bud  divides  so  that 
two  crowns  are  produced  on  one  root.^ 

The  Roots. — The  upper  molar  teeth  have  three  roots,  two  outer  and 
one  inner,  but  in  the  wisdom  teeth,  especially  of  civilized  races,  the  roots 
are  usually  fused.  The  lower  molars  have  two  roots,  but  each  root  appears 
to  be  essentially  double  in  nature.  In  lower  primates  the  upper  bicusps 
have  three  roots,  but  in  man  these  are  usually  fused  so  as  to  form  one  or 
sometimes  two  roots.  The  lower  bicuspids  have  usually  one  root,  but  as  in 
lower  apes,  they  may  have  two.  The  roots  are  the  last  parts  to  be  formed. 
When  the  roots  of  the  molar  teeth  come  to  be  developed,  the  base  of  the 
dental  papilla  is  differentiated  into  three  parts — round  each  of  which  a 
root  is  formed  (Fig.  187).  In  that  peculiar  ancient  and  extinct  race  of  men 
— known  as  the  Neanderthal  race — the  dental  papilla  and  pulp  cavity  were 
very  large  and  the  roots  were  short  and  wide.  Thus  in  Neanderthal  teeth 
— the  condition  is  occasionally  seen  in  a  modern  tooth — the  pulp  cavity 
almost  descended  to  the  tips  of  the  roots.^ 

Eruption  of  the  Teeth.^ — The  eruption  of  the  milk  teeth  commonly 
covers  a  period  of  eighteen  months,  beginning  in  the  6th  with  the  lower 
incisors  and  ending  in  the  24th  or  30th  with  the  2nd  milk  molars.  The 
eruption  of  the  permanent  teeth  occupies  a  period  of  about  eighteen  years, 
beginning  with  the  1st  permanent  molar  in  the  6th  year  and  ending  about 
the  24th  with  the  3rd  molar.  The  milk  molars  are  replaced  by  the  per- 
manent premolars.  In  civilized  races  the  third  molars  or  wisdom  teeth 
frequently  remain  embedded  in  the  alveolus  or  may  be  quite  absent.  The 
upper  wisdom  tooth  is  developed  in  the  posterior  border  of  the  superior 
maxilla,  which  bounds  the  spheno-maxillary  fissure  in  front.  In  growth 
backwards  of  the  maxillary  antrum  the  posterior  border  of  the  superior 
maxilla  becomes  rotated  into  the  alveolar  border,  thus  bringing  the  wisdom 
teeth  into  position  (see  Fig.  176).  The  inferior  wisdom  teeth  are  developed 
in  the  alveolus  on  the  inner  aspect  of  the  ascending  ramus. 

A  fourth  molar  sometimes  appears  behind  the  third.  The  original 
primate  stock  is  supposed  to  have  had  three  incisors  and  four  premolars 
on  each  side,  yet  a  supernumerary  incisor  or  premolar  is  a  rare  abnor- 
mality. The  upper  lateral  incisor  may  be  very  small  or  even  absent, 
there  being  a  distinct  tendency  towards  the  disappearance  of  this  tooth 

1  J.  T.  Wilson  and  J.  P.  Hill,  Quart.  Journ.  Mic.  Sc.  1907,  vol.  51,  p.  137  (Tooth 
Formation  in  Monotremes) ;  W.  Ramsay  Smith,  Journ.  Anat.  and  Physiol.  1907, 
vol.  42,  pp.  126,  226  (Morphology  of  Teeth  of  Austrahan  Natives). 

2  Keith  and  Knowles,  Journ.  Anat.  and  Physiol.  1911,  vol.  46,  p.  12. 

3  G.  Fischer,  Anat.  Hefte,  1909,  vol.  38,  p.  617  (Eruption  of  Permanent  Teeth). 


190      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

in  civilized  races.  If  the  teeth  are  too  large  for  the  jaw,  a  not  uncom- 
mon condition  in  civilized  races  owing  to  a  diminished  growth  of  the  bony 
palate,  they  appear  in  irregular  positions. 

Mechanism  of  Eruption. — As  regards  the  mechanism  which  causes 
teeth  to  erupt  there  is  still  some  degree  of  uncertainty.  One  naturally 
infers  that  the  growth  of  the  root  will  tend  to  force  the  crown  upwards 
and  the  tissues  over  the  crown  to  atrophy.  The  process  of  eruption  is  a 
much  more  complex  one  than  the  mere  formation  of  a  root.  It  is  well 
known  that  a  rootless  tooth  may  cut  the  gum,  while  in  another  case  the 
root  may  form  and  yet  the  tooth  remain  embedded  in  the  jaw.  Eruption 
is  a  definite  growth  movement — allied  in  nature  to  the  mechanism  which 
leads  to  the  extrusion  of  a  foreign  body  by  the  tissues.  During  the  eruption 
of  a  tooth  there  is  not  only  an  absorption  of  the  overlying  tissues  of  the 
gum — probably  due  to  pressure — but  there  is  also  the  positive  growth  of 
the  peridontal  tissues  at  the  base  of  the  tooth-sac  which,  as  it  presses  the 
tooth  towards  the  surface,  moulds  the  surrounding  wall  of  the  dental 
crypt  into  a  suitable  alveolar  socket.  Thus  the  formation  of  the  socket 
or  alveolus  appears  to  be  part  of  the  mechanism  of  eruption.  Mr.  J.  T. 
Carter  regards  the  gubernaculum  dentis  as  playing  an  effective  part  in 
tooth  eruption. 1 

Effect  of  Civilization. — Mention  has  been  made  of  the  fact  that  the 
eruption  of  the  last  molars  in  highly  civilized  peoples  may  be  long  delayed 
or  arrested  ;  in  a  small  proportion  of  individuals  these  teeth  may  be  quite 
absent.  When  the  teeth  and  jaws  of  ancient  European  races  are  compared 
with  those  of  their  successors,  certain  changes  are  very  evident.  These 
are  (1)  the  crowns  of  the  teeth  in  the  ancient  races  are  much  worn  ;  (2) 
the  palate  is  well  formed,  and  large  enough  to  carry  the  teeth  without 
crowding  or  irregularity  ;  (3)  the  wisdom  teeth  are  in  position,  but  usually 
show  a  reduction  in  size  and  development ;  (4)  diseased  and  carious  teeth 
are  uncommon  ;  (5)  the  edges  of  the  incisor  teeth  come  into  apposition  in 
biting.  In  modern  Europeans  the  degree  of  wear  or  erosion  is  slight ; 
the  palate  is  often  vaulted,  contracted  and  the  teeth  crowded  and  mis- 
placed ;  the  wisdom  teeth  are  often  unerupted  or  absent ;  diseased  teeth 
are  extremely  common  ;  the  edges  of  the  lower  incisors  ascend  behind 
the  crowns  of  the  upper  (scissors  bite).  The  cause  or  causes  of  these  remark- 
able changes  are  ill-understood,  but  it  is  probable  that  some  or  all  will  be 
traced  to  the  nature  of  our  modern  dietary. 

Muscles  of  Mastication.2 — The  four  muscles  of  mastication — the 
temporal,  masseter,  external  and  internal  pterygoids  arise  in  the  man- 
dibular arch.  A  single  muscular  mass  is  apparent  at  the  end  of  the  first 
month  ;  during  the  second  month  it  is  differentiated  into  its  several  parts 
— the  internal  pterygoid  being  the  first  to  separate  from  the  common 
mass.  The  masseter  and  external  pterygoids  are  derived  from  the  primitive 
temporal  muscle.  The  external  pterygoid  is  a  late  addition  ;  even  in  man 
it  is  often  imperfectly  separated  from  the  temporal.     The  muscles  of 

1  See  Brit.  Dent.  Journ.  1904,  Feb. 

2  Professor  F.  H.  Edgeworth,  Quart.  Journ.  Mic.  Sc.  1914,  vol.  59,  p.  573. 


THE  TEETH  AND  APPARATUS  OF  MASTICATION         191 

mastication  differ  from  the  ordinary  striated  muscles  of  the  body  in  being 
derived  from  the  musculature  of  a  visceral  arch.  Their  motor  nerve — 
the  motor  root  of  the  Vth — represents  the  splanchnic  nerve  of  the  second 
segment  of  the  head  (see  p.  99).  The  somatic  motor  nerve  of  the  segment 
is  the  4th  or  trochlear  nerve  ;  the  somatic  musculature  of  this  segment  is 
represented  by  the  superior  oblique.  The  sensory  nerves  of  the  teeth — 
the  2nd  and  3rd  divisions  of  the  Vth  nerve — represent  the  skin  or  somatic 
sensory  fibres  of  the  second  or  mandibular  segment  of  the  head.  It  will 
be  thus  seen  that  the  apparatus  of  mastication  has  been  evolved  in  con- 
nection with  the  second  cephalic  segment — the  neuromere  of  this  segment 
being  the  second  of  the  mid-brain.  The  manner  in  which  the  muscles  of 
mastication  are  attached  to  the  skull,  and  the  extent  to  which  they  modify 
cranial  characters  have  been  already  mentioned  (p.  155).  The  evolution 
of  the  temporo-mandibular  joint  has  also  been  alluded  to  (p.  177). 


CHAPTER  XIV. 


THE  NASAL  CAVITIES  AND  OLFACTORY  STRUCTURES. 

Evolution  of  the  Nasal  Cavities. — Althougli  tte  sense  of  smell  is  a 
minor  one  in  the  economy  of  the  human  body,  it  is  very  evident  that  in 
the  root-stock  from  which  mammals  have  been  evolved  the  olfactory 
organ  must  have  held  a  foremost  place  amongst  the  sensory  structures. 
We  have  seen  that  the  great  superstructure  of  the  brain  rests  on  the  primary 
ganglia  connected  with  the  olfactory  nerves.  When  now  we  examine  the 
changes  connected  with  the  development  of  the  nose  and  nasal  cavities  in 


EYE 


FORE   BRAIN 


OLFACT:  PIT 
STOMODAEUM 


FORE  BRAIN 


NASAL  PROC: 
OLFACT:  PIT 
MAX:  PROC 
MANDIB: 


EYE 


Fig.  192,  A.— The  Olfactory  Pit  and  Face  of  an  Embryo  in  the  5th  week  of  develop- 
ment.    (After  Broman.) 

Fig.  192,  £.— The  Olfactory  Pit  and  Facial  Processes  in  an  Embryo  in  the  6th  week 
of  development.     (After  Hochstetter.) 

the  human  embryo,  we  shall  see,  behind  the  complicated  processes  at  work, 
a  recapitulation  of  conditions  which  are  to  be  seen  in  animals  occupying  a 
very  low  position  in  the  vertebrate  kingdom.  At  the  end  of  the  4th  week 
the  olfactory  membrane  appears  as  two  plaques  of  ectoderm  in  contact 
with  the  under  surface  of  the  fore-brain  (Figs.  155,  A,  192,  A)  ;  in  the 
5th  week  the  plaques  or  plates  become  two  pits — right  and  left,  the  usual 
condition  in  fishes  ;  in  the  6th  week  each  pit  becomes  connected  with  the 
primitive  mouth  or  stomodaeum  by  a  groove — a  condition  seen  in  the 
dog-fish  ;  in  the  6th  and  7th  weeks  the  pit  is  deepened  and  its  opening 
becomes  turned  towards  the  stomodaeum  owing  to  the  growth  of  its  lateral 
and  mesial  margins  which  form  the  lateral  and  mesial  nasal  processes 
(Fig.  155).  The  processes  unite  in  the  manner  already  described  and  a 
nasal  cavity  similar  to  that  of  the  air-breathing  or  dipnoan  fishes  is  estab- 
lished.    In  the  7th  and  8th  weeks  the  cavity  of  the  pit  is  rapidly  enlarged  ; 

192 


NASAL  CAVITIES  AND  OLFACTORY  STRUCTURES        193 

free  communication  with  the  mouth  is  established  ;  the  nasal  cavity  has 
then  become,  as  in  amphibians,  the  functional  vestibule  of  the  respiratory 
system.  In  the  3rd.  month  the  palate  is  complete,  and  the  stage  peculiar 
to  mammals  thus  established. 

In  tracing  the  development  of  structures  subservient  to  the  sense  of 
smell,  the  following  elements  have  to  be  dealt  with  : 

(1)  The  olfactory  sense  epithelium  and  olfactory  nerves  ; 

(2)  The  parts  of  the  brain  concerned  with  the  sense  of  smell ; 

(3)  The  capsule  which  contains  the  olfactory  epithelium  ; 

(4)  The  respiratory  tract  of  the  nasal  cavities. 

(1)  Origin  of  the  Olfactory  Sense  Epithelium.^ — At  the  end  of  the 

4th  week,  a  small  area  of  the  ectoderm  lying  under  the  fore-brain  becomes 
demarcated  on  each  side,  to  form  the  olfactory  plates.  Around  these  two 
plates  the  lateral  and  mesial  nasal  processes  grow  up  (Fig.  193),  the  plates 


lat  nas.  proc. 
mes.  nas.  proc, 

olfactory  pit  and  plate 

for  Jacobson's  organ 
stomodaeum 


,.^max.  proc. 
■''  mandib.  proc. 


Fig.  193. — The  Olfactory  Pit  and  Nasal  Processes  in  a  Human  Embryo  about 
5  weeks  old.     (After  Kollmann.) 

becoming  at  the  same  time  invaginated  to  form  the  olfactory  pits.  With 
the  growth  of  the  nasal  processes  the  cavities  of  the  expanding  olfactory 
pits  or  pockets  come  to  occupy  a  space  on  the  roof  of  the  stomodaeum, 
their  openings  being  turned  towards  that  cavity.  The  ectodermal  lining 
becomes  the  epithelial  membrane  of  the  nasal  cavities.  A  small  island 
is  detached  from  each  olfactory  plate  to  form  the  basis  of  Jacobson's  organ 
(Fig.  193).  The  sense  epithelia  in  the  olfactory  area  behave  as  nerve  cells 
and  send  out  nerve  processes  which  form  arborescences  round  the  neural 
cells  of  the  outgrowing  olfactory  bulb  (Fig.  194).  The  olfactory  nerves 
are  thus  formed.  At  first  the  olfactory  plates  are  directly  in  contact  with 
the  cerebral  vesicle,  but  later  on  they  are  separated  by  the  formation  of  the 
cerebral  membranes  and  cribriform  plates. 

In  the  foetus  the  olfactory  or  sense  epithelium  is  relatively  extensive, 
as  is  the  case  in  mammals  with  a  keen  sense  of  smell.     It  descends  almost 

^  For  development  of  Nasal  Cavities  see  J.  E.  Frazer,  Journ.  Anat.  and  Physiol. 
1912,  vol.  46,  p.  416  ;   K.  Peter,  Ergebnisse  der  Anat.  1911,  vol.  20,  p.  43. 


lU  HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

to  the  lower  border  of  the  middle  turbinate  on  the  outer  or  lateral  wall, 
and  to  the  junction  of  the  upper  two-thirds  with  the  lower  third  on  the 
mesial  or  septal  wall.  In  the  adult  the  distribution  is  much  restricted — ■ 
occupying  areas  only  about  one  finger  breadth  in  extent  below  the  cribri- 
form plate. 

(2)  The  Olfactory  Lobe. — As  the  olfactory  pits  are  being  thrust  into 
the  roof  of  the  stomodaeum  during  the  6th  week,  the  anterior  part  of  the 
floor  of  the  cerebral  vesicles  are  growing  out  as  hollow  protrusions  to  form 
the  olfactory  vesicles.  At  the  end  of  the  3rd  month  the  olfactory  vesicle 
has  assumed  the  form  shown  in  Fig.  194.  Its  cavity  is  at  first  continuous 
with  that  of  the  cerebral  vesicle,  but  this  connection  is  lost  in  the  3rd 
month  ;  it  becomes  solid,  and  forms  the  olfactory  bulb  and  tract  (Fig.  196). 


foramen  of  Monro 

(rudiment 
of  Corp.  callosum) 

caudate  nucleus 


^i^^  pineal  body 
mid  brain 


olfact.  lobe,  ant  part  (A) 

olfact.  lobe,  post,  patt  (B)  V         ^,$^^^'^^^^'''''' 


riG.  194. — The  Mesial  Aspect  of  the  Brain  of  a  Human  Foetus,  SJ  montiis  old,  show- 
ing the  Olfactory  Lobe.  A,  olfactory  bulb  ;  B  represents  the  paraterminal 
part  of  the  rhinencephalon. 

The  tip  of  the  anterior  horn  of  the  lateral  ventricle  marks  the  point  at  which 
the  cavity  of  the  olfactory  lobe  communicated  with  the  cerebral  vesicle. 

The  Rhinencephalon. — The  Rhinencephalon  is  made  up  of  the  parts 
of  the  cerebrum  which  are  primarily  connected  with  smell.  These  parts 
are  best  seen  in  a  typical  mammalian  brain  such  as  is  shown  in  Fig.  195. 
They  are,  following  the  classification  of  Elliot  Smith  :  (1)  the  olfactory 
bulb  and  peduncle  or  tract,  both  of  which  are  developed  from  the  olfactory 
lobe  ;  (2)  the  olfactory  tubercle,  represented  in  the  human  brain  by  a 
small  area  behind  the  trigone  ;  (3)  the  paraterminal  body  (Figs.  194,  195) 
which  is  represented  in  the  human  brain  by  the  gyrus  subcallosus  and 
septum  lucidum ;  (4)  the  hippocampal  formation  represented  in  the 
human  brain  by  the  supra-callosal  gyrus,  gyrus  dentatus,  hippocampus 
and  fornix  (Fig.  196)  ;  (5)  the  pyriform  lobe  (the  uncus  of  the  human 
brain)  ;  (6)  the  anterior  perforated  space.  In  man  these  parts  are  reduced 
in  size  owing  to  (1)  his  less  acute  sense  of  smell ;  (2)  the  great  development 
of  the  corpus  callosum  and  mantle  of  the  brain.  The  rhinencephalon 
represents  the  oldest  part  of  the  brain,  and  its  grey  matter  differs  from  the 
rest  of  the  cortex  in  structure. 


NASAL  CAVITIES  AND  OLFACTORY  STRUCTURES        195 


Morphology  of  the  Olfactory  Neural  Elements. — If  the  olfactory 
area  of  ectoderm  were  to  adhere  to,  and  form  part  of,  the  olfactory  bulb, 
then  the  olfactory  vesicle  would  be  comparable  to  the  optic  vesicle,  the 
rods  and  cones  representing  the  olfactory  epithelium,  the  ganglion  cells  of 
the  olfactory  bulb  those  of  the  retina,  while  the  lateral  and  mesial  olfactory 
tracts  would  correspond  to  the  optic  tracts.  This  homology  is  impaired 
by  the  fact  that  the  fibres  of  the  lateral  olfactory  tract  end,  not  in  a  gang- 
lionic mass,  but  in  true  cortex — that  of  the  pyriform  lobe  or  uncus  (Fig. 
195).  The  pyriform  cortex  is  linked  up  with  the  gyrus  dentatus  by  a 
second  relay  of  fibres,  while  the  dentate  gyrus  is  connected  in  turn  with  the 
hippocampal  cortex  by  a  third  relay.  The  fornix  and  the  hippocampal 
commissure  (see  p.  122)  represent  the  association  and  commissural  system 

neopallium 
Corp.  callos. 
hippoc.  form. 


dors.  com. 
chor.  fis. 


tubercle 

paraterm.  body 

lam.  term  and  ant  com. 


^pyriform  lobe  (uncus) 
ornix 
h)ppocampal  form, 
ant  perf.  sp. 


Fig.  195. — The  iresial  Aspect  of  a  typical  Mammalian  Cerebrum  shoeing  the 
parts  of  the  Rhinencephalon.     (Elliot  Smith.) 

of  the  hippocampal  formation.  The  anterior  commissure  was  originally 
made  up  of  fibres  passing  from  one  olfactory  bulb  to  the  other  (p.  121). 
Fibres  in  the  mesial  root  of  the  olfactory  tract  reach  the  dentate  gyrus  by 
means  of  the  fornix  and  supra-callosal  striae. 

The  Nasal  Cavities. — The  nasal  cavities  are  formed  by  the  expansion 
of  the  olfactory  pockets  within  the  substance  of  the  three  developmental 
masses  which  surround  each  of  them — the  mesial  nasal,  lateral  nasal  and 
maxillary  processes.  When  these  processes  unite  in  the  7th  week,  the 
primitive  nasal  cavity  rapidly  expands,  and  an  opening  temporarily  closed 
is  reformed  in  its  fundus  or  floor,  the  primitive  choanae,  situated  in  the 
roof  of  the  mouth  (Fig.  170).  The  choanae  are  separated  by  the  primitive 
nasal  septum,  and  are  at  first  in  front  of  the  pituitary  outgrowth — Rathke's 
pocket  (Fig.  197).  In  the  latter  part  of  the  2nd  month  and  the  earlier  half 
of  the  3rd  the  primitive  nasal  septum  and  the  choanae  on  each  side  of  it, 
extend  their  dimensions,  until  the  posterior  border  of  the  septum  reaches 
and  involves  the  mouth  of  Rathke's  pocket  (J.  E.  Frazer).     In  this  manner 


196 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


the  nasal  septum  is  secondarily  extended,  and  the  nasal  cavities  greatly 
deepened  (Fig.  197).  At  the  same  time  the  floor  of  the  nasal  cavities  is 
prolonged  backwards  by  the  formation  of  the  secondary  palate,  and  the 
secondary  choanae  are  established  within  the  region  of  the  naso-pharynx 
before  the  end  of  the  3rd  month.  The  process  of  chondrification  begins 
in  the  lower  part  of  the  lateral  nasal  process  during  the  period  at  which  the 
secondary  palate  is  being  formed.  The  chondrification  of  the  lateral  mass 
of  the  ethmoid  and  other  parts  of  the  olfactory  capsule  have  already  been 
described  (p.  149). 

gyrus 


^MPi-a-, 


sept,  lucid.. 


anterior  perforated  space       \   band  of  Giacomini 


collateral  fissure 


uncus 
temp,  incis. 

Fig.  196. — The  parts  of  the  Rhinencephalon  in  the  Human  Brain. 


Development  o£  Turbinates  and  Air  Sinuses.^ — Before  cartilage  has 
actually  been  formed  in  the  walls  of  the  primitive  nasal  cavities,  linear 
outgrowths  of  the  lining  epithelium  are  observed  to  occur  in  the  lateral 
wall  and  roof.  These  outgrowths  give  rise  to  the  meatuses  of  the  nose — 
the  inferior  under  the  maxillo-turbinal  appearing  first,  about  the  8th  week, 
the  superior  last,  about  the  12th  week.  In  the  lateral  wall  of  the  nasal 
cavity  of  a  foetus  20  mm.  long  and  in  the  8th  week  of  development  Dr. 
Milne  Dickie  ^  found  only  two  linear  depressions — ^the  lower  representing 
the  inferior  meatus,  the  upper  the  hiatus  semilunaris  (ethmoidal  infundi- 
bulum.  Fig.  198).  The  turbinate  processes  are  thus  carved  out  of  the  lateral 
wall  and  roof  of  the  nasal  cavity.  The  usual  number  is  five  in  mammals, 
but  in  man  the  4th  and  5th  are  only  temporary.  The  inferior  or  maxillo- 
turhinate  is  developed  on  the  lateral  wall,  but  the  middle  and  upper  appear 
on  the  roof  and  septal  wall,  their  lateral  position  being  attained  in  the  course 
of  development.     The  sphenoidal  turbinate  also  belongs  to  the  ethmoidal 

^  See  references,  p.  174. 

^  Journ.  Anat.  1914,  vol.  48,  p.  445. 


NASAL  CAVITIES  AND  OLFACTOKY  STRUCTURES        197 

series,  but  becomes  applied  to  the  body  of  the  presphenoid.  The  turbinates 
and  meatuses  are  developed  in  connection  with  respiration.  They  in- 
crease, it  is  true,  the  olfactory  area,  but  their  chief  use  is  apparently  to 
filter  and  warm  the  inspired  air. 

The  manner  in  which  the  nasal  mucous  membrane  pushes  its  way  from 
the  middle  meatus  into  the  maxillary  process  to  form  the  antrum  of  High- 
more  has  been  already  described  (p.  173).  The  other  air  sinuses — ^the 
frontal,  lachrymo-ethmoidal,  anterior,  middle  and  posterior  ethmoidal, 
and  sphenoidal  sinuses — six  in  all,  arise  in  the  same  way  as  the  antrum, 
but  begin,  with  the  exception  of  the  last  named,  to  enlarge  at  a  much  later 
date.  Although  they  begin  to  bud  out  about  the  time  of  birth,  they 
assume  their  active  growth  in  the  earlier  years  of  puberty,  and  reach  their 
full  size  before  the  30th  year. 


PRIMITIVE    5EPTL(M 

SECONDARY    SEPTUM 


NOSE 

ANT:  NARES 
PRIMITIVE    PALATE. 


Rathke's  Pocket 


Eustachian  tube 


Secondary  palate 


Fig.  197. — The  Primitive  Nasal  Cavities  and  Ciioanae  at  the  end  of  the  6th  week. 
The  formation  of  the  secondary  septum  and  palate  are  indicated.  (After  J.  E. 
Frazer.) 

At  birth,  the  lateral  mass  of  the  ethmoid  is  a  thin  plate,  carrying  the 
superior  and  middle  turbinate  processes,  which  almost  fill  the  nasal  cavity 
(Figs.  169,  198).  The  entire  ethmoid  is  narrow,  and  hence  the  proximity 
of  the  eyes  in  children.  Beneath  the  middle  turbinate  is  a  thumbnail-like 
impression — the  hiatus  semilunaris,  or  ethmoidal  infundibulum,  one  of  the 
earliest  formations  (8th  week).  The  maxillary  sinus  buds  out  near  its 
posterior  end,  the  point  at  which  the  bud  arises  becoming  the  site  at 
which  the  sinus  opens  in  the  middle  meatus  (Fig.  199).  The  uncinate 
process  of  the  lateral  mass  of  the  ethmoid  forms  the  prominent  lower 
margin  of  the  hiatus  (Fig.  198).  A  second  opening  may  be  present 
below  the  level  of  the  uncinate  process,  or  this  may  be  the  only  one 
developed. 

In  Fig.  198  part  of  the  middle  turbinate  has  been  removed  to  expose  the 
frontal  recess  of  the  middle  meatus — an  expansion  of  the  meatus  formed 
in  the  4th  month  of  foetal  life.  At  birth  ^  four  furrows  are  present — - 
representing  the  buds  of  air  sinuses.     One,  or  occasionally  two,  of  these 

^  I  have  followed  the  account  given  by  Professor  J.  Parsons  Schaefier,  Amer,  Journ, 
Anat.  1916,  vol.  20,  p.  125. 


198      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

enlarge  to  form  the  frontal  sinuses,  the  others  becoming  cells  of  the  ethmoid. 
The  duct  or  mouth  of  the  frontal  sinus  may  become  secondarily  continuous 
with  the  hiatus  semilunaris  or  the  bud  of  the  frontal  sinus  may  arise  from 
the  upper  end  of  the  hiatus.  The  bud  of  the  frontal  sinus,  as  it  expands, 
pushes  its  way  into  the  frontal  bone,  separating  the  outer  from  the  inner 
lamella.  The  bud  is  formed  in  the  first  year,  but  is  nascent  until  the 
fifth.  A  second  frontal  bud  may  arise  and  partially  or  completely  sup- 
plant the  primary  frontal  outgrowth.  As  a  rule,  by  the  25th  year  the 
sinus  reaches  outwards  over  the  inner  two-thirds  of  the  orbital  roof,  and  is 
an  inch  or  more  both  in  height  and  depth  at  its  mesial  part.  It  is  smaller 
in  women  than  in  men,  but  it  may  be,  and  often  is,  arrested  at  an  early 
stage  of  development,  or  it  may  be  absent  altogether.  The  size  of  the 
glabellar  prominence  is  no  index  to  its  development. 

The  stalk  of  the  frontal  bud  forms  the  infundibulum  or  naso-frontal 
duct,  which  is  narrow,  half  an  inch  long,  and  difficult  of  catheterization 

MIO.TURB.  (CiJTj 

rRONTAL.  rURROWS(7-^) 
SUP.  TURB. 

NASO-TURB. 


INrUMDIB    ETTHM.       /        BULLA.   ETH. 
ChiaT   SEMILUN.)  I  INF  TURB. PROC. 

UNCIN  PROC 

Fig.  198.— The  Lateral  Wall  of  the  Nasal  Cavity  of  a  child  at  birth.     (J.  Parsons 

Schaeffer.) 

from  the  nose.  Into  it  open  (or  sometimes  into  the  hiatus)  the  lachrymo- 
ethmoidal  and  anterior  ethmoidal  cells  which  surround  the  infundibulum. 
They  are  developed  as  outgrowths  from  the  infundibulum  (Fig.  199). 
Occasionally  the  maxillary  sinus,  as  is  frequently  the  case  in  the  gorilla, 
sends  a  process  to  form  part  of  the  frontal  sinus,  and  hence  there  may  be 
a  communication  between  the  sinus  and  the  antrum. 

The  development  of  the  frontal  sinuses  and  supra-orbital  ridges  leads 
to  a  marked  change  in  the  face  at  puberty.  By  the  formation  of  the 
frontal  sinuses  the  basal  area  of  the  skull,  to  which  the  face  is  attached,  is 
largely  increased  in  extent.  Such  an  increase  is  necessary  to  support  the 
palate,  which  grows  rapidly  in  size  at  puberty.  Up  to  the  fifth  year  the 
upper  jaw  has  to  carry  only  ten  milk  teeth  ;  in  the  adult  it  has  to  carry 
sixteen  permanent  teeth.  To  support  these  the  face  and  palate  have  to  be 
enlarged.  The  formation  of  the  frontal  sinus  gives  the  necessary  increase 
in  the  area  of  the  base  of  the  skull  for  their  support.  It  should  be  re- 
membered that  the  growth  of  the  brain  and  of  the  cranial  cavity  is  com- 
paratively slight  after  the  fifth  year.     Only  the  gorilla  and  chimpanzee 


NASAL  CAVITIES  AND  OLFACTORY  STRUCTURES        199 

show  an  arrangement  of  frontal  and  ethmoidal  sinuses  comparable  to  that 
of  man. 

Above  the  hiatus  lies  the  bulla  ethmoidalis,  which  is  inflated  by,  and 
commonly  carries  the  opening  of,  the  middle  ethmoidal  cell  (Fig.  199). 
The  posterior  ethmoidal  sinus  opens  beneath  the  superior  turbinate  process, 
and  IS  developed  from  the  superior  meatus.  The  ethmoidal  sinuses  are 
produced  in  the  cartilage  of  the  ethmoidal  or  lateral  nasal  plate  (Fig.  175). 
They  inflate  the  ossifying  cartilaginous  plate  until  it  becomes  a  cellular 
mass,  thus  increasing  the  breadth  of  the  intra-orbital  septum.  The 
sphenoidal  sinus  (Fig.  199)  is  formed  during  the  3rd  month  by  the  mucous 
membrane  growing  into  and  expanding  the  sphenoidal  turbinate  bone, 
which  is  a  small,  slightly  ossified  cartilage  lying  beneath  the  presphenoid 
at  birth,  and  forming  the  uppermost  (sixth)  of  the  nasal  turbinate  processes. 


infundibulum 
ant.  ethm.  sinus 
mid.  ethm.  sinus 


lachrymo-ethmoid. 
naso-turbinai 
hiatus  semilunari, 

lateral  recess 
of  pharynx 


vestigial  turbinates 

post.  ethm.  sinus 

sphen.  sinus 

splien.  turb. 

Eustachian  tube 


Fig.  199. — A  Diagram  of  the  Lateral  Wall  of  the  Nasal  Cavity,  showing  the  position  of 
the  Air  Sinuses.  The  parts  beneath  the  turbinate  processes  are  indicated  by 
stippled  lines. 

Latterly  the  sinus  grows  into  and  expands  the  presphenoid  and  part  of  the 
basi-sphenoid,  the  sphenoidal  turbinate  remaining  as  its  anterior  wall. 
The  sphenoidal  turbinate  is  a  detached  part  of  the  ethmoidal  cartilage. 

It  will  thus  be  seen  that  all  the  nasal  air  sinuses  are  produced  primarily 
by  a  budding  outwards  of  the  nasal  mucous  membrane  into  the  carti- 
laginous basis  of  the  lateral  nasal  processes.  Disease  may  readily  spread 
to  these  sinuses  from  the  nasal  cavities.  By  means  of  the  sinuses  the  area 
of  the  face  is  increased  to  support  the  adult  palate  bearing  the  permanent 
teeth.  Most  of  them  open  on  the  respiratory  tract  of  the  nasal  cavity. 
They  are  ventilated  with  every  breath.  They  act  also  as  resonance 
chambers. 

Vestigial  Turbinates. — There  is  frequently  to  be  seen  in  the  adult  one, 
or  even  two,  secondary  meatuses  above  the  superior  ;  these  are  constantly 
present  in  the  chimpanzee  and  in  mammals  with  a  keen  sense  of  smell. 
In  the  human  foetus  of  four  months  six  turbinates  are  usually  present, 
besides  secondary  processes  in  the  meatuses  beneath  them.     The  upper- 


200      HUMAN  EMBKYOLOGY  AND  MORPHOLOGY 

most  of  these,  the  sixth,  becomes  the  sphenoidal  turbinate  ;  the  fifth 
disappears  ;  the  third  and  fourth  may  remain  separate  or  become  united  ; 
the  first  and  second  form  the  inferior  or  maxillo-turbinal  and  middle 
turbinate  processes.  The  agger  nasi  (naso-turbinal,  Fig.  198),  in  front  of 
the  attachment  of  the  middle  turbinate  process,  is  a  vestige  of  the  naso- 
turbinal,  a  process  well  developed  in  most  carnivora  and  animals  with  a 
strong  scent.  The  uncinate  process,  which  forms  the  lower  border  of  the 
hiatus  semilunaris,  is  continuous  at  its  base  with  the  naso-turbinal. 
Through  the  hiatus  semilunaris  acting  as  a  gutter,  the  antrum  may  become 
a  cesspool  for  a  suppurating  frontal  sinus. 

Organ  of  Jacobson.^ — Mention  has  already  been  made  of  the  organ 
of  Jacobson — situated  on  the  nasal  septum  above  the  naso-palatine  canals. 
During  development  (Fig.  193)  a  part  of  the  olfactory  plate  becomes 
detached,  and  is  afterwards  invaginated  in  a  pocket  in  the  septum  and 


Fig.  200. — Nasal  Septum  of  a  Child  at  Birth,  showing  a  rod  inserted  in  the  pocket 
of  Jacobson's  organ  {A).  B,  closed  naso-palatine  canal;  C,  presphenoid ; 
D,  vomer. 

guarded  by  a  scroll  of  cartilage.  It  reaches  its  maximum  development 
in  the  human  foetus  at  the  5th  month,  and  afterwards  becomes  a  mere 
vestige- — often  unrecognizable.  It  sometimes  persists  and  forms  a  very 
evident  structure  on  the  septum.  A  pocket  can  usually  be  seen  on  the 
septum  at  birth  (Fig.  200).  This  special  development  of  the  olfactory 
organ  is  highly  developed  in  all  herbivorous  vertebrates  in  whom  the  naso- 
palatine canals  are  widely  open,  and  thus  the  juices  and  odours  of  the  mouth 
have  free  access  to  the  organ.  Professor  Broman  ^  has  suggested  that  it 
is  for  sampling  substances  dissolved  in  fluid,  as  is  the  case  with  the  olfactory 
organ  of  fishes. 

Nervus  Terminalis.^-^Amongst  the  fibres  of  the  olfactory  nerve,  par- 
ticularly in  the  branch  to  Jacobson's  organ,  there  occur  nerve  cells, 
apparently  of  the  same  nature  as  those  belonging  to  the  sympathetic  system. 
From  these  cells  issue  fibres  which  connect  the  olfactory  areas  of  sense 

1  E.  Zuckerkandl,  Ergebnisse  der  Anat.  1908,  vol.  18,  p.  801. 

2  Ivar  Broman,  Jubilee  Festschrift  of  the  University  of  Lund,  1918. 

^  For  references  to  Literature  see  Olof  Carsall,  Journ.  Comp.  Neur.  1918,  vol.  30, 
p.  1  ;  R.  McCotter,  ibid,  1913,  vol,  23,  p.  145  ;  H.  Ayrers,  ibid^  1919,  vol.  30,  p.  323, 


NASAL  CAVITIES  AND  OLFACTORY  STRUCTUEES        201 

epithelium  with  grey  matter  near  the  lamina  terminalis  of  the  fore-brain. 
The  fibres  constitute  the  nervus  terminalis  which  is  well  developed  in  low 
vertebrates  and  of  which  there  remains  a  vestige  in  man. 

Nasal  Duct.^ — Although  in  no  way  connected  with  the  sense  of  smell, 
the  nasal  duct  is  closely  related  to  the  nasal  cavities.  It  is  formed  between 
the  lateral  nasal  and  maxillary  processes  (Figs.  155,  193).  It  is  laid  down  as 
a  solid  epithelial  cord  along  the  naso-maxillary  groove  at  the  end  of  the 
second  month.  It  becomes  canaliculized  during  the  3rd  month. ^  Three 
bones  bound  it  :  the  superior  maxilla  on  the  outer  side,  formed  in  the 
maxillary  process  ;  the  inferior  turbinate,  formed  in  the  cartilage  of  the 
lateral  nasal  process,  and  the  lachrymal,  formed  over  the  lateral  nasal 
cartilage,  bound  it  on  the  inner  side.  The  formation  of  the  palate  cuts  the 
duct  off  from  the  mouth.  The  hamulus  of  the  lachrymal  varies  much  in 
size,  and  is  the  vestige  of  a  large  process,  which  in  lower  primates  enters 


frontal  sinus 
lachrymal  crest 

pars  facialis 


uncinate  process 


Fig.  201. — Showing  on  the  Inner  Wall  of  the  Orbit  (1)  the  Position  of  the 
Infundibuliim,  (2)  the  Pars  Facialis  Lachrymalis. 

into  the  formation  of  the  inferior  margin  of  the  orbit.  This  pars  facialis 
sometimes  occurs  in  man  (Fig.  201).  Occasionally  the  frontal  and  superior 
maxillary  bones  may  push  towards  each  other  between  the  lachrymal  in 
front  and  lateral  mass  of  the  ethmoid  behind,  and  thus  form  a  fronto- 
maxillary  articulation  on  the  inner  wall  of  the  orbit. 

Malformations  of  the  Nose. — In  Figs.  156  and  166  two  malformations 
of  the  nose  are  represented.  In  Fig.  156  the  rare  condition  is  shown  in 
which  one  olfactory  pit  and  its  corresponding  processes  form  a  polypoid 
body  ;  in  Fig.  202  the  condition  of  Cyclops,  where  Iboth  nasal  cavities  are 
enclosed  in  a  proboscis  is  represented.  The  eyes  are  also  fused.  The 
condition  of  the  facial  skeleton  in  such  a  case  is  represented  in  Fig.  166. 
In  such  cases  there  has  been  an  arrest  of  growth  of  the  cephalic  end  of  the 
embryonic  plate,  with  a  fusion  of  the  olfactory  bulbs  and  also  of  the  optic 
vesicles.  The  two  olfactory  plates  and  pits  are  united  in  a  single  median 
structure.     In  this  condition  we  seem  to  have  represented  a  pure  develop- 

1  See  references,  p.  174. 

2  See  Schaeffer,  Amer,  Jovrn.  Anat.  1912,  vol.  13,  p.  1. 


202 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


mental  abnormality — not  a  reversion  to  some  past  stage  in  evolution 
(see  p.  161). 

Two  otlier  malformations  require  mention.  During  the  3rd,  4tli  and 
5th  months  of  foetal  life  an  epithelial  plug  is  formed  within  the  anterior 
nares — where   the   cutaneous   and   nasal   epithelial   coverings   meet.     In 


-G 


Pig.  202. — Median  Sagittal  Section  of  the  Head  and  Face  in  a  Case  of  Cyclops.  A, 
frontal  bone  ;  B,  single  median  nasal  cavity  contained  in  a  proboscis  formed 
by  the  nasal  processes ;  C,  median  or  fused  eye  ;  D,  palate  formed  by  the 
maxillary  processes  only ;  E,  median  cerebral  vesicle ;  F,  single  optic  nerve ; 
G,  Eustachian  tube  ;  E,  palate  bone. 

rare  cases  the  plug  becomes  organized,  and  forms  a  dense  septum  within 
the  nares.  A  similar  obstruction,  often  containing  bone,  may  be  formed 
near  the  posterior  nares.  The  posterior  narial  occlusion  represents  an 
organization  and  persistence  of  the  epithelial  membrane  which  at  first 
closes  the  primitive  choanae  (see  p.  170). 


CHAPTER  XV. 

DEVELOPMENT  OF  THE  STRUCTURES  CONCERNED 

IN  THE  SENSE  OF  SIGHT. 

The  Nature  of  the  Eye. — It  is  in  vain  that  we  appeal  to  comparative 
anatomy  for  light  on  the  various  stages  in  the  evolution  of  the  eye  ;  the 
eye  of  vertebrates  is  already  fully  formed  in  the  earliest  form  known.  Our 
knowledge  of  its  origin  and  nature  rests  on  an  embryological  foundation  ; 
during  the  4th  and  5th  weeks  of  human  development  we  see  the  eye  com- 
pounded from  three  sources  :  (1)  the  retina  and  optic  nerve  arise  as  an 
outgrowth  of  the  neural  tube  ;  (2)  the  lens  arises  from  the  ectoderm  or  body 
covering  ;  (3)  the  tunics  and  mechanism  of  accommodation  from  the 
mesoderm.  The  union  of  these  three  tissues  to  form  the  most  marvellous 
contrivance  of  the  human  body  is  a  product  of  countless  ages  of  evolution. 
A  comparison  with  the  olfactory  organ,  already  mentioned  in  the  last 
chapter,  assists  us  in  understanding  the  peculiar  nature  of  the  eye.  The 
olfactory  plates  are  neural  in  nature  ;  their  sensory  cells  give  rise  to  the 
fibres  of  the  olfactory  nerves.  The  plaques  of  olfactory  epithelium  are 
situated  near  the  open  anterior  end  (neuropore)  of  the  neural  tube  ;  one 
can  easily  understand  how  they  might  shift  towards  the  neural  tube, 
merge  with  it,  and  become  enfolded  with  the  part  which  forms  the  olfactory 
bulb.  Were  we  to  implant  the  olfactory  epithelium  in  the  olfactory 
bulb  we  should  produce  a  structure  comparable  to  the  retina.  During  an 
early  part  of  the  4th  week  the  two  retinal  plates  are  represented  by  de- 
pressions on  the  sides  of  that  part  of  the  medullary  folds  which  are  enclosed 
to  form  the  fore-brain  (Fig.  203).  The  epithelium  which  lines  the  optic 
evagi nations,  clearly  parts  of  the  original  surface  covering  of  the  embryo, 
does  not  become  ependymal  cells  but,  like  the  olfactory  plates,  gives  rise  to 
those  highly  modified  sensory  cells — rods  and  cones.  Besides  the  rods  and 
cones  the  optic  evagination  gives  rise  to  nerve  and  other  cells,  in  this 
respect  resembling  a  typical  part  of  the  neural  tube.  It  is  thus  clear  that 
the  olfactory  and  optic  nerves  are  of  a  totally  different  nature  to  the  other 
cranial  nerves.  We  must  seek  the  origin  of  the  retina  as  a  superficial 
sense  organ,  which  has  become  so  modified  in  the  course  of  evolution  that 
its  primitive  simple  nature  is  hard  to  detect. 

The  structures  concerned  in  the  sense  of  sight  are  : 

(1)  The  Eyeball  and  the  Optic  Nerve  ; 

(2)  The  Eyelids  and  Lachrymal  Apparatus  ; 

(3)  The  Orbit,  and  the  Muscles,  Nerves  and  Vessels  contained  in  it ; 

(4)  The  Nerve  Centres  and  Tracts. 

203 


204 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


The  Eyeball. — The  condition  of  the  eye  in  the  4th  week  of  foetal  life  is 
shown  diagrammatically  in  Figs.  203,  204.  The  three  elements  which 
unite  to  form  the  eyeball  are  as  yet  separate.     They  are  : 

(1)  Ectoderm,  which  forms  (a)  the  epithelium  of  the  cornea,  (&)  the  lens, 
and  probably  (c)  the  capsule  of  the  lens. 

(2)  Neuroderm,  which  forms  {a)  the  optic  nerves,  (b)  sensitive  retina, 
(c)  pars  ciliaris  retinae,  (d)  uvea,  (e)  pigmentary  layer  of  retina,  (/)  the 
hyaloid  membrane. 

(3)  Mesoderm,  which  forms  {a)  outer  tunic  (sclerotic  and  fibrous  cornea)  ; 
(6)  middle  tunic  (choroid,  ciliary-choroid  and  iris)  ;  (c)  the  vitreous  humour 

forebmin 
optic  uesicle 
^epiblast 
-neuroblast 
mesoblast 


ECTODE.RM 
OPTIC   EVAQINAiT 
FUTURE    RETI 


MESOBLAST 


mid-brain 
hind-brain 


Fig.  203. — Diagrammatic  Section  across  Fore-brain  of  a  Human  Embryo  in  early- 
part  of  4th  week  to  show  the  Optic  Evaginations.     (After  Professor  Bryce.) 

Fig.  204. — Diagram  of  the  Elements  which  form  the  Eyeball. 

L  Structures    derived    from    the    Ectoderm.^     (a)  The  lens. — The 

lens  is  developed  by  a  saccular  invagination  of  the  ectoderm  situated  over 
the  optic  vesicle  at  the  beginning  of  the  5th  week  (Fig.  205).  About  a 
week  later  it  becomes  a  closed  sac  by  the  severance  of  its  connection  with 
the  ectoderm,  its  wall  being  formed  by  a  single  layer  of  epithelial  cells. 
The  cavity  of  the  lenticular  vesicle  is  gradually  obliterated  by  the  cells  of 
the  posterior  wall  becoming  elongated  (Fig.  206)  until  they  reach  the 
anterior  wall  (7th  and  8th  weeks).  Each  elongated  cell  is  transformed 
into  a  lens  fibre. 

The  cells  of  the  anterior  wall  retain  their  primitive  form  (Fig.  206). 
New  lens  fibres  are  added  by  the  cells  at  the  margin  (equator)  becoming 
multiplied  and  elongated.  The  central  fibres,  which  are  formed  first, 
are  the  shortest,  the  fibres  of  every  additional  layer  produced  become 
longer  than  those  of  the  previous  layer,  hence  the  concentric  arrangement 
of  fibres.  Further,  the  fibres  of  each  layer  are  so  graduated  in  length  that, 
when  produced,  they  meet  along  certain  lines  which  radiate  from  the 
anterior  and  posterior  poles  of  the  lens.  The  lens  is  relatively  large  at 
birth,  being  two-thirds  of  its  final  size  ;    growth  continues  until  puberty, 

1  E.  Kallius,  Ergebnisse  der  Anat.  1904,  vol.  14,  p.  234 ;  1906,  vol.  16,  p.  746  ;  1907 
vol.  17,  p.  463  (Development  of  Eye)  ;  F,  Keibel,  Keibel  and  Mali's  Manual  of  Human 
Embryology,  1912,  vol,  2. 


SENSE  OF  SIGHT 


205 


.ECTODERM 

MESODERM 


LENTIC   VESICLE 

OPTIC    VESICLE 

WALL  or  FOREBRAIN 


CORNEA 
LENS 

INVAGIN.  tVALL(RET/NA) 

'      COVERING  WALL 
(PIGMENT  LA^EK) 

OPTIC   STAUK 

WALL  OP 
FORE&RAIN 


MESODERM 


(a)  6-3  m.m. 


OP-nC  STALIN 

3rd  VENT. 
HYALOID  AHT, 

K 

FLOOPI  OF  FOREBRAIN 


OPTIC  CUP 

HYALOID   ART 

FLOOR  OF  FOREBRAIN 


(b)  7 m.m. 


Fig.  205,  A. — Depression  of  the  Ectoderm  to  form  the  Lenticular  Vesicle,  early  in  the 
6th  week.     (Hochstetter.) 
■JS. — Separation  of  the  Vesicle  later  in  the  6th  week.     Both  figures  represent 
Coronal  Sections  of  the  Fore-Brain  and  Optic  Vesicle  in  Human 
Embryo.    (After  Hochstetter.) 

and  even  then  has  not  ceased,  for  Priestley  Smith  found  that  there  is  an 
appreciable  addition  to  its  weight  with  each  decade  of  life.     It  wiU  thus 

-ECTODERM 

MESODERM 


LENS   CAVITY 

LENS  FIBRES 

Pupillary 

EJ<1BRANE 


ANT   CHAMBER 


OPTIC   CUP 


BA3l3i7f  IRIS 


76  mm. 

Fig.  206. — The  Formation  of  the  Lens  Fibres  from  the  Epithelium  on  the  Posterior 
Wall  of  the  Lenticular  Vesicle  and  the  ingrowth  of  mesoderm  to  form  the  sub- 
stance of  the  Cornea  and  Vascular  Capsule  of  Lens,  7th  week.     (After  Lindahl.) 

Fig.  207. — Diagrammatic  Section  of  the  Anterior  Part  of  the  Eyeball  to  show  the 
state  of  the  Anterior  Chamber  and  Iris  in  the  5th  month.     (After  Broman.) 

be  s(}en  that  the  lens  is  an  area  of  modified  epidermis,  and  in  manner  of 
development  closely  resembles  the  sense  organs  in  the  skin  of  fishes  and 
amphibians.     Like  the  epidermis,  it  shows  a  tendency  in  the  aged  to  be 


206 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


transformed  into  keratin.  The  oldest  cells  .(the  central  or  nuclear  fibres) 
alter  first ;  hence  the  central  position  of  the  cataract  which  occurs  so 
frequently  in  old  people. 

(b)  The  cornea. — The  epithelial  covering  of  the  cornea  is  continuous  with 
the  surrounding  ectoderm.  It  becomes  transparent.  The  mesoderm 
which  grows  in  between  the  lens  vesicle  and  ectoderm  forms  the  connective- 
tissue  basis  of  the  cornea  and,  by  a  later  invasion,  the  vascular  capsule  of 
the  lens  (Fig.  206). 

(c)  The  capsule  of  the  lens  is  a  cuticular  membrane  formed  by  the 
lenticular  cells.  Outside  the  proper  capsule  a  vascular  tunic  is  formed 
from  the  mesoderm  (Fig.  207). 

2.  Structures  formed  from  the  Optic  Vesicles  (neurodermal  element). 

■ — Each  vesicle  is  well  developed  soon  after  the  commencement  of  the  4:th 


cerebr.  vesicle 


chor.  fis.       f^^^  ^;^^^^ 


3rd  uentn'c/e 


Olfact  hbe 
lamina  term, 
choroid,  dep. 

fens. 


optic  recess 


pituitary 


]  rig  lit  op.  ues. 
turned  down 


Fig.  208. — Diagram  showing  the  connection  with  the  Fore-Brain  and  condition  of  the 
Optic  Stalls  and  Vesicle  at  the  end  of  the  6th  week  of  development.     (After  His.) 

week  (see  Figs.  203,  204)  ;  even  before  the  medullary  plates  have  quite 
met  to  enclose  the  cavity  of  the  fore-brain  the  optic  vesicles  have 
commenced  as  evaginations  of  those  plates.  They  form  a  great  lateral 
diverticulum  on  each  side  of  the  fore-brain — a  cavity  which  becomes  the 
third  ventricle  in  the  adult.  The  condition  of  the  right  optic  vesicle  at 
the  end  of  the  6th  week  is  shown  diagrammatically  in  Fig.  208.  The 
stalk  or  neck  remains  constricted  to  become  the  optic  nerve  while  the 
vesicle  enlarges  and  becomes  invaginated  to  form  the  optic  cup. 

Invagination  of  the  optic  vesicle. — Almost  as  soon  as  it  begins  to  grow 
out  the  optic  vesicle  becomes  invaginated,  one  half  being  pushed  within  the 
other  (Figs.  205,  A,  B).  The  lenticular  bud  lies  within  the  indentation. 
The  remarkable  fact  was  discovered  by  Dr.  Warren  Lewis  that  the  optic 
vesicle,  if  transplanted,  can  cause  overlying  ectoderm  to  produce  a  new 
lenticular  bud.     The  invaginated  vesicle  is  known  as  the  optic  cup.     Fine 


SENSE  OF  SIGHT  207 

fibres  unite  the  neuroblastic  cells  which  line  the  optic  cup  with  the  deep 
aspect  of  the  lenticular  vesicle  (Cirincione).  The  invagination  of  the 
vesicle,  which  takes  place  in  an  oblique  manner — as  if  pressure  had  been 
applied  from  below  and  behind — leads  to  the  closure  not  only  of  the  cavity 
of  the  vesicle,  but  also  to  that  of  the  distal  half  of  the  stalk  (optic  nerve). 
The  point  at  which  the  central  artery  enters  the  optic  nerve  marks  the 
upper  limit  of  the  invagination  of  the  optic  stalk  (Fig.  209).  By  the  5th 
week  the  optic  vesicle  no  longer  communicates  with  the  cavity  of  the 
fore-brain,  but  the  recessus  opticus  in  the  floor  of  the  third  ventricle, 
above  the  chiasma,  remains  to  mark  the  point  at  which  the  original  evagina- 
tion  took  place  (Fig.  208).  The  parts  formed  from  the  optic  vesicles  are  : 
(a)  The  optic  nerve  is  formed  from  the  stalk  of  the  optic  vesicle.  The 
wall  of  the  stalk  is  at  first  composed  of  a  single  layer  of  columnar  epithelium ; 
in  the  second  month  these  cells  produce  a  sponge-work  of  fibres  on  the 
surface  of  the  stalk.^     During  the  8th  week,  the  optic  fibres,  developed  as 

RETINA 
LENS  FIBRES       j  CHOROID 

ANT.  STRATUM 
CORNEA 


OPTIC  NERVE 


,HVALOIO   ART. 
(cent.  ART.  RET.) 


LENS 

HYALOID  ART 


Fig.  209. — Certain  parts  of  the  Eye  during  the  7th  week  of  development.     (After  His.) 

processes  of  the  neuroblasts  of  the  invaginated  layer,  begin  to  grow  into  the 
brain  from  the  retina  along  the  sponge-work  of  the  optic  stalk.^  Thus  are 
formed  the  greater  number  of  the  fibres  in  the  optic  nerve.  The  optic 
fibres  also  form  the  chiasma  in  the  floor  of  the  third  ventricle  and  the 
optic  tracts  on  the  wall  of  the  fore-brain  (Fig.  224).  It  will  thus  be  seen 
that  the  optic  nerves  and  vesicles  are  of  the  same  origin  as  the  cerebral 
vesicle — both  representing  modified  parts  of  the  wall  of  the  fore-brain. 

(b)  The  pigmentary  layer  of  the  retina  is  formed  from  the  ensheathing 
or  outer  layer  of  the  optic  cup  (Fig.  210).  At  first  the  wall  of  the  optic 
vesicle  is  composed  of  a  single  layer  of  epithelium  ;  the  outer  or  pigmentary 
layer  of  the  retina  retains  this  embryonic  form.  Pigment  appears  as  early 
as  the  6th  week,  commencing  at  the  marginal  border. 

(c)  The  uvea  is  the  layer  of  pigmented  epithelium  which  covers  the 
posterior  surface  of  the  iris.  It  is  formed  out  of  both  outer  and  inner  layers 
of  the  optic  cup,  and  represents  the  rim  of  the  cup  (Fig.  211). 

{(l)  The  pars  ciharis  retinae  is  formed  out  of  that  part  of  the  inner  or 
invaginated  layer  of  the  optic  cup  which  lies  in  the  shadow  of  the  iris,  and 

^  Prof.  Robinson,  Journ.  Anat.  and  Physiol.  1896,  vol.  30,  p.  319. 
^  Prof.  Cameron,  Journ.  Anat.  and  Physiol.  1905,  vol.  39,  p.  135. 


208 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


is  therefore  inaccessible  to  light  rays.  It  also  retains  the  primitive  columnar 
or  partly  transitional  form  of  the  epithelium  (Fig.  211).  The  ora  serrata 
marks  the  junction  of  the  pars  ciliaris  retinae  and  sensitive  retina. 

Ciliary  Processes.^ — At  the  commencement  of  the  4th  month,  the  pars 
ciliaris  retina's  becomes  plicated  or  puckered  into  60  or  70  small  folds 


uuea 


lens,      optic,  cup 


optic,  stalbi 
nerue  j 


pars  ciliar.  ret 


cauity  ofuesia 


outer  or  pigment  layer.        inner  or  inuag.  layer 


Fig.  210. — Diagrammatic  Section  of  the  Optic  Cup  and  Lens.  The  cavity  Is  repre- 
sented as  gaping,  whereas  from  the  5th  week  onwards  the  outer  and  inner  walls 
are  in  contact. 

(Fig.  211)  ;  mesoderm  of  the  middle  tunic  (choroid)  grows  into  the  puckers 
and  forms  the  ciliary  processes.  It  should  be  observed  that  the  lens  lies 
within  the  optic  cup  and  the  ciliary  processes  are  formed  round  the  equator 
or  circumference  of  the  lens.  The  retinal  epithelium  which  covers  the 
ciliary  processes  is  secretory  in  nature.     It  forms  the  aqueous  humour,  thus 


CILIAPY  BODIES 


OUTEK  WALL 


INNER    WALL- RETINA 


Fig.  211. — Section  of  the  Iris,  showing  the  folding  of  the  marginal  part  of  the  Optic 
Cup  to  form  the  ciliary  processes  and  the  origin  of  the  Sphincter  Muscle  of  Iris 
from  the  anterior  or  outer  layer  of  cup,  in  Human  Foetus  in  6th  month.     (Szily). 

recalling  the  ependyma,  which  covers  the  choroid  plexuses  of  the  ventricles 
of  the  brain.  It  is  strange  that  from  the  same  layer  as  gives  origin  to  nerve 
cells  there  should  also  arise  supporting  (neuroglial)  and  secretory  cells, 
and  as  we  shall  see  anon,  the  unstriped  muscle  of  the  iris  (Fig.  211). 

(e)  The  sensitive  retina  is  formed  out  of  the  inner  or  invaginated  layer 
of  the  optic  cup  (Fig.  212).  At  first  the  inner  wall  is  composed  of  a  single 
layer  of  epithelium.     The  ciliary  part  of  the  retina  retains  this  form.     What 

^  M.  von  Lenhossek,  Verhand.  Anat.  Gesellsch.  1911,  p.  81  (Dev.  of  Ciliary  Body). 


SENSE  OF  SIGHT 


209 


is  called  the  outer  aspect  of  tlie  primitive  retina  is  directed  towards  the 
pigmented  layer,  but  is  separated  from  that  layer  by  what  remains  of  the 
cavity  of  the  optic  vesicle  (Fig.  210).  That  cavity,  it  will  be  remembered, 
is  a  prolongation  of  the  neural  canal  or  ventricular  cavity  of  the  brain. 
The  inner  or  vitreous  aspect  of  the  retina,  corresponding  to  the  outer 

^  cauity  of 
uesicle. 

retina 


Fig.  212. — Diagrammatic  Section  across  Optic  Cup  to  siiow  tlie  manner  in  which 
the  Cells  of  the  Inner  Layer  of  the  Optic  Cup  are  differentiated  to  form  the 
Retina.     (After  Fiirst.) 

A,  B,  C,  D,  E,  show  stages  in  the  development  of  the  Retina  from  the  simple  layer 
of  Cells. 

1.  The  outer  stratum  of  Sense  Cells  (rods  and  cones). 

2.  The  middle  stratum  connecting  (bipolar)  Nerve  Cells. 

3.  The  inner  stratum  of  Ganglionic  Cells  and  Fibres. 

The  cavity  of  the  Optic  Vesicle,  which  is  closed  by  the  invagination  of  the  retinal 
layer  within  the  cup  and  obliterated  by  the  outgrowth  of  the  rods  and  cones, 
is  represented  by  a  wide  black  zone  in  the  diagram. 

aspect  of  the  neural  tube,  is  directed  towards  the  lens.  The  manner  in 
which  the  complicated  strata  of  the  retina  arise  from  the  single  layer  has 
been  investigated  by  Professor  Fiirst,  and  is  represented  diagrammatically 
in  Fig.  212.  Differentiation  starts  at  the  centre  of  the  optic  cup  and  spreads 
towards  the  periphery.     The  original  layer,  while  dividing  and  producing 


hyaloid  art. 


optic 


optic  cup 


lens. 


margin  of  pupil 


choroidal  fissure 


Fig.  213. — The  Optic  Stalk  and  Cup,  viewed  on  tlie  lower  and  lateral  aspect, 
showing  the  Closure  o£  the  Choroidal  Fissure. 

broods  of  cells,  still  retains  its  position,  the  daughter  cells  being  pushed 
towards  the  vitreous  aspect  of  the  retina,  and  by  the  middle  of  the  7th 
month  of  foetal  life  all  the  retinal  elements  are  present,  the  fovea  centralis 
being  the  last  feature  to  appear.  As  far  as  mammals  are  concerned  the 
fovea  centralis  is  a  characteristic  of  the  higher  primates. 

o 


210 


HUMAN  EMBEYOLOGY  AND  MOEPHOLOGY 


On  each  surface  of  the  retina  is  developed  a  cuticular  or  limiting  mem- 
brane. Some  of  the  original  epithelial  cells  are  elongated  between  the 
limiting  membranes  and  form  the  fibres  of  Miiller.  On  passing  from  the 
margin  of  the  cup  to  its  centre  all  stages  will  be  seen  between  the  single 
layer  and  the  multi-stratified  condition.  Ultimately  three  strata  can  be 
recognized  in  the  retina.  Beneath  the  outer  limiting  membrane  the 
original  cells  remain  as  the  retinal  sense  epithelium  ;  processes  from  these 
cells  break  through  the  outer  limiting  membrane  to  form  the  rods  and 
cones  ;  the  middle  stratum  forms  bipolar  cells  ;  beneath  the  inner  limiting 
membrane  ganglionic  cells  are  formed.  The  middle  stratum  by  its  pro- 
cesses links  together  the  sense  epithelium  and  the  ganglionic  cells,  and  thus 
stands  in  the  same  relationship  to  the  sense  epithelium  and  ganglionic 
cells  as  a  posterior  root  ganglion  does  to  the  touch  corpuscles  of  the  skin 
and  the  cuneate  and  gracile  nuclei  of  the  medulla.  In  many  ways  the 
development  of  the  retina  recalls  the  development  of  the  spinal  cord. 
Both  form  part  of  the  neural  tube. 

The  Choroidal  Fissure. — Occasionally  congenital  fissures  are  seen  in 
the  lower  segment  of  the  iris  (coloboma  iridis)  or  choroid  (coloboma  chor- 
oidea)  (Fig.  214).  A  white  line,  due  to  absence  of  pigment,  may  be  seen 
in  the  corresponding  segment  of  the  retina  when  the  interior  of  the  eye  is 
examined.  These  are  due  to  imperfect  closure  of  the  choroidal  fissure. 
The  choroidal  fissure  is  the  result  of  the  peculiar  mode  in  which  the  optic 
vesicle  is  cupped  or  invaginated.  The  lens  grows  into  it  from  the  malar 
or  lower  lateral  aspect  and  becomes  lodged  in  the  anterior  part  of  the 
depression  ;    the  posterior  part  becomes  the  choroidal  fissure  (Fig.  209). 


IRIS 


PUPIU 


COLOBOMA 


PUPIU 


Fig.  214.— Coloboma  or  Cleft  of  Iris.    (After  Seggel.) 

Fig.  215. — Remains  of  Pupillary  Membrane.    (After  Prof.  Hippel.) 


The  margins  of  the  fissure  unite,  fusion  commencing  near  its  middle  and 
spreading  distally  to  the  margin  of  the  cup  and  proximally  until  it  reaches 
the  point  of  entrance  of  the  hyaloid  artery  (Fig.  209).  By  the  8th  week 
all  traces  of  the  fissure  should  have  disappeared.  Its  union  recalls  the 
closure  of  the  fissures  in  the  upper  lip.  Coloboma  and  harelip  are  lesions  of 
a  similar  nature.  With  the  closure  of  the  choroidal  fissure  the  optic  cup 
is  completed.     Its  brim  or  margin  becomes  the  site  of  the  pupil. 

Binocular  Vision.— At  first  the  optic  vesicles  are  directed  laterally  in  the 
human  embryo,  and  in  mammals  generally  the  eyes  are  so  directed,  each 
eye  having  its  own  field  of  vision.  In  the  Primates  the  eyes  swing  forwards 
during  the  second  month  ;   binocular  vision  is  thus  made  possible.     With 


SENSE  OF  SIGHT  211 

binocular  vision  and  the  combination  of  images  appear  in  the  highest 
primates  : 

(1)  A  fovea  centralis  and  macula  lutea  (L.  Johnston)  ; 

(2)  A  partial  crossing  of  the  optic  fibres  at  the  chiasma  ; 

(3)  Certain  alterations  in  the  attachments  of  the  oblique  muscles  of  the 
eyeball. 

The  primitive  cavity  of  the  Optic  Vesicle  (Fig.  210)  is  of  some  clinical 
importance.  It  is  obliterated  by  the  invagination  of  the  vesicle  ;  the 
rods  and  cones  formed  in  the  inner  or  invaginated  layer  grow  out  across 
the  cavity  into  the  outer  or  ensheathing  pigmented  layer  of  the  retina 
(Fig.  212).  From  accident  or  disease  the  retina  may  be  detached,  thus 
causing  blindness  ;  the  separation  takes  place  between  the  pigmented 
epithelium,  which  remains  in  situ,  and  the  rods  and  cones,  which  fall 
inwards  with  the  nerve  layer.  Fluid  then  collects  in  the  site  of  the  primi- 
tive cavity  of  the  optic  vesicle.  The  optic  part  of  the  medullary  plate 
in  amphibian  embryos  has  been  transj)lanted  and  produced  a  retina  in 
its  new  site.  Some  experimenters  found  that  the  ectoderm  over  the  optic 
graft  gave  rise  to  a  lens.^ 

3.  Parts  of  the  Eyeball  formed  from  the  Mesoderm. — After  the  optic 
vesicle  has  been  invaginated  against  the  lens,  a  continuation  of  the  same 
layer  of  mesoderm,  which  surrounds  and  forms  the  coverings  of  the 
brain,  envelops  the  optic  cup  and  spreads  inwards  between  the  ectoderm 
and  the  lens.  As  may  be  seen  from  Figs.  205,  A,  B,  the  lens  at  first  lies 
in  contact  with  the  inner  or  retinal  wall  of  the  optic  cup,  no  mesoderm 
intervening.  When  they  move  apart  in  the  3rd  month  a  connecting 
network  of  fibres  appears  between  them. 

The  structures  formed  from  the  mesoderm  are  : 

(1)  The  vascular  tunic  of  the  lens. — While  the  choroid  fissure  is  still  open, 
mesodermal  tissue  passes  into  the  cup  and  in  it  is  formed  the  hyaloid 
artery,  which  is  enclosed,  when  the  lips  of  the  fissure  fuse.  Mesodermal 
cells  also  enter  by  the  pupillary  margin  (Fig.  206),  and  in  this  way  the 
actively  growing  lens  becomes  surrounded  by  a  vascular  tunic,  in  which 
the  hyaloid  artery  terminates.  Beneath  this  tunic  lies  the  proper  capsule 
of  the  lens,  which  is  formed  from  the  epithelium  of  that  body. 

(2)  The  vitreous  humour  is  formed  out  of  the  mesoderm  which  passes 
into  the  optic  cup  behind  the  lens.  KoUiker  was  of  opinion  that  the 
mesodermal  cells  were  absorbed  and  that  the  vitreous  was  wholly  pro- 
duced from  the  lenticulo-retinal  fibrillar  network  mentioned  m  a  previous 
paragraph.  The  closure  of  the  choroidaL  fissure  cuts  the  vitreous  humour 
off  from  the  mesoderm  which  covers  the  outer  layer  of  the  optic  cup  and 
becomes  transformed  into  the  tunics  of  the  eyeball.  The  vitreous  humour 
— like  Wharton's  jelly  of  the  umbilical  cord — represents  an  early  form  of 
embryonic  tissue.     It  consists  of  cells  embedded  in  a  jelly-like  matrix. 

(3)  The  hyaloid  artery  is  the  vessel  which  supplies  the  mesodermal  tissues 
within  the  optic  cup  ;    it  terminates  in  the  vascular  capsule  of  the  lens 

^  Most  of  these  instructive  experiments  have  been  carried  out  by  American  investi 
gators.  For  a  recent  list  of  researches  see  Spemann,  Zool.  Jahrbuch,  1912,  vol.  32, 
Heft  1.     W.  H.  Lewis,  Amer.  Journ.  Anat.  1907,  vol.  7,  p.  259. 


212 


HUMAN  EMBEYOLOGY  AND  MORPHOLOGY 


(Figs.  209,  216).  In  the  7th  month  foetus  a  trace  of  the  artery  can  still  be 
seen  passing  through  the  vitreous  humour  from  the  optic  disc  to  the  lens. 
With  the  gradual  obliteration  of  the  artery,  the  mesodermal  capsule  of  the 
lens  becomes  thin  and  clears  up.  A  foetus  born  in  the  seventh  month  is 
blind,  because  the  vascular  capsule  of  the  lens  has  not  quite  disappeared. 
The  anterior  part  of  the  capsule — -filling  the  pupil — is  the  membrana 
pupillaris.  A  trace  of  the  membrane  may  occasionally  be  seen  crossing 
the  pupil  (Fig.  215).  The  part  of  the  hyaloid  artery  within  the  optic 
nerve  persists  as  the  central  artery  of  the  retina.     The  canal  of  the  artery 


conjunctiua 
epiblast 


Outer  or  \ 
pigment  layer] 

inner  layer, 
of  retina  >'■ 


Vitreous 


hyaloid  art.~li 


optic  nerue 


mesoblast 

epithel.  of  cornea 

'^S^basis  of  cornea  and 
capsule  of  lens. 

lens. 

lentic.  cavity 

uvea 

pigment  layer 
of  retina 


optic  fibres     I  '""^'^  ^^^^^  ^f  ^"^^'"^ 
i  basis  of  middle  and  outer 
I  tunics  of  eyeball 

Fig.  216. — Diagrammatic  Section  of  the  Eye  showing  the  parts  formed  from  the 
Mesoblast  or  Mesoderm.  (After  His'  Model  of  the  Eye  of  a  3rd  month  human 
foetus.) 

within  the  vitreous  humour,  from  the  optic  disc  to  the  lens,  remains  as  the 
hyaloid  canal — a  lymph  path.  The  hyaloid  artery  may  persist  and  cause 
partial  or  complete  blindness.  It  disappears  some  days  after  birth  in  cats 
and  rabbits. 

(4)  The  aqueous  chamber  is  formed  between  the  cornea  and  lens,  its 
walls  being  entirely  of  mesodermal  origin.  In  Fig.  216  the  mesoderm  which 
invades  the  space  between  the  ectoderm  and  lenticular  vesicle  is  represented 
as  forming  not  only  the  basis  of  the  cornea  but  also  the  anterior  wall  of  the 
vascular  tunic  of  the  lens,  these  two  parts  being  supposed  to  become 
separated  by  the  formation  of  the  aqueous  chamber.  Dr.  Lindahl  ^  finds, 
however,  that  these  two  parts  are  formed  separately,  the  mesodermal  basis 

^Anat.  Hefte,  1915,  vol.  52,  p.  195. 


SENSE  OF  SIGHT 


213 


of  the  cornea  in  the  6th  week  and  the  lenticular  capsule  later — at  the  9th 
week,  the  aqueous  chamber  being  the  potential  chamber  between  these  two 
formations  (Fig.  206).  Fluid  begins  to  collect  in  the  pupillary  area  of  this 
space  in  the  6th  month  and  spreads,  so  that  in  the  7th  month  the  chamber 
has  extended  to  the  corneo-scleral  junction.  Almost  to  the  time  of  birth,  the 
anterior  chamber  of  the  aqueous  is  very  shallow  (Fig.  217),  the  lens  lying 
near  the  cornea.  Even  so  late  as  the  6th  month  (see  Fig.  207)  the  posterior 
part  of  the  aqueous  chamber — the  part  which  lies  between  the  iris  in  front 
and  the  lens  behind — is  not  opened  up.  We  must  regard  the  aqueous 
system  as  strictly  comparable  to  the  cerebro-spinal  and  not  as  part  of  the 
lymph  system. 

(5)  The  choroid,  ciliary  processes  and  iris  form  the  middle  or  vascular 
tunic  of  the  eye,  and  are  developed  out  of  the  mesoderm  which  covers 


presphen 
orbit  connect  tis. 

basis  of  Tenon 


cornea 
ant  chamb. 

iris 
lens. 


dental  sacs.    sup.  max. 

Fig.  217. — Section  of  the  Eye  and  Orbit  at  birth. 

the  optic  cup.  They  form  a  vascular  and  pigmented  covering  through 
which  the  optic  cup  is  nourished,  and  correspond  to  the  combined  pia  mater 
and  arachnoid  membranes  of  the  brain.  The  ciliary  muscle  is  formed  in 
this  tunic  in  the  4th  month.  The  iris  is  late  in  its  development.  The 
uvea  on  its  deep  surface  is  formed  from  the  brim  of  the  posterior  surface 
(Fig.  211).  In  the  6th  month  the  sphincter,  and  then  the  dilator  muscles, 
are  produced — their  origin  being  peculiar.  The  muscle  fibres  arise  from 
the  epithelial  cells  of  the  uveal  part  of  the  optic  cup  (Fig.  211).  The  iris 
is  fully  formed  in  the  7th  month  and  can  then  react  to  light. 

(6)  The  sclerotic  is  derived  from  the  outer  mesodermal  envelope  of  the 
optic  cup  and  is  strictly  comparable  to  the  primitive  cranial  capsule.  It 
is  continuous  in  front  with  the  cornea  ;  behind,  with  the  sheath  of  the 
optic  nerve  and  dura  mater.  In  some  vertebrates,  but  not  in  mammals, 
plates  of  bone  are  developed  in  the  anterior  half  of  the  sclerotic,  recalling 
the  deposition  of  dermal  bones  in  the  primitive  capsule  of  the  brain. 

The  tapetum  lucidum  is  absent  in  the  human  and  primate  eye.  It  gives 
the  metallic  lustre  seen  on  the  retinal  surface  of  the  eye  of  the  ox,  and  is 


2U 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


formed  by  a  layer  of  fine  fibres  whicli  are  developed  on  the  retinal  surface 
of  the  cboroid. 

(7)  The  capsule  of  Tenon,  the  bursa  or  connective-tissue  socket  of  the 
eyeball,  is  developed  in  the  mesoderm  surrounding  the  eyeball.  A  lymph 
space  separates  it  from  the  sclerotic,  which  is  but  slightly  marked  until 
after  birth.  The  choanoid  muscle  (retractor  bulbi  or  orbital  muscle) 
which  surrounds  the  sclerotic  part  of  the  eyeball  as  a  muscular  hood  in 
mammals  and  vertebrates  generally  and  arises  in  common  with  the  external 
rectus,  has  become  greatly  reduced  in  man  and  the  higher  primates.  Re- 
mains of  the  retractor  bulbi — a  striated  muscle — have  been  described  by 
Prof.  Whitnall  in  the  human  orbit. ^  The  unstriped  muscle  of  the  orbit  occurs 
in  two  jDlaces  ;  the  orbital  part  (Miiller's  muscle)  bridges  the  spheno- 
maxillary fissure  ;  the  palpebral  part  forms  the  non-striated  musculature 
found  in  the  insertions  of  the  levator  palpebrae  (Groyer).  The  non-striated 
muscle  is  supplied  by  sympathetic  nerves.  Its  function  is  obscure,  but 
is  probably  designed  to  regulate  the  pressure  and  circulation  of  the  venous 
blood  of  the  orbit. 

Growth  of  the  Eyeball. — The  eyeball  is  relatively  large  at  birth,  its 
diameter  (17-18  mm.)  being  three-fourths  of  the  adult  diameter  (24  mm.). 
In  rate  and  precocity  of  growth  it  is  comparable  to  the  brain.  The  macula- 
lutea  and  fovea  centralis  are  said  to  have  reached  their  full  size  at  birth. 
A  child  born  at  the  end  of  the  7th  month  is  sensitive  to  light  and  darkness  ; 
appreciation  of  form  comes  towards  the  end  of  the  1st  year,  while  colours 
are  not  recognized  until  the  2nd  or  3rd  years — or  in  some  cases  the  colour 
sense  is  not  developed.  The  colours  at  the  opposite  ends  of  the  spectrum 
(red- violet)  are  the  first  to  be  recognized  (Edridge  Green). 

frontal  {brain  capsule)         / 


ethmoid 
\\  {lat.  nas.  proG.) 


nasal 
(lat.  nas.  proc.) 


asc.  proc.  max. 
(lat.  nas.  proc.) 


malar  (max.  process) 

sup.  max.  (max.  process) 
position  of  nasal  duct 

Fig.  218. — The  Origin  of  tiie  Bones  entering  into  Formation  of  the  Orbit. 

Formation  of  the  Orbit  (Fig.  218). — The  orbit  is  formed  (1)  above  by 

the  capsule  of  the  fore-brain  in  which  the  frontal  bone  is  developed  ;    (2) 

externally  and  below  by  the  maxillary  process.     In  the  maxillary  process  the 

malar  bone  and  superior  maxilla   (except  the  ascending  nasal  process) 

1  Journ.  Anat.  and  Physiol.  1912,  vol.  46,  p.  36. 


SENSE  OF  SIGHT 


215 


are  developed.  (3)  The  inner  wall  is  formed  by  the  lateral  nasal  process, 
in  which  the  nasals,  lachrymals  and  lateral  mass  of  the  ethmoid,  are 
formed.  The  optic  nerve  enters  the  orbit  between  the  orbito-  and  pre- 
sphenoids,  both  of  which  help  to  form  the  orbit.  The  orbital  surface  of 
the  great  wing  is  formed  at  a  later  period  in  a  membranous  basis  (see 
Fawcett,  p.  135).  The  orbital  plate  of  the  malar  cuts  the  orbit  off  from  the 
temporal  fossa  ;  it  is  develojDed  in  higher  primates  only.  The  nasal  duct 
is  formed  between  the  maxillary  and  nasal  processes  (Figs.  154  and  219). 
In  lower  primates  and  mammals  generally  the  hamular  process  of  the 
lachrymal  appears  on  the  margin  of  the  orbit ;  the  pars  facialis  lachrymalis 
is  sometimes  seen  in  the  human  skull  (Fig.  154,  p.  160).  Mention  has  been 
made  of  the  division  of  the  orbital  region  of  the  primitive  skull  (Fig.  133) 


Fig.  219. — ^Malformed  Face  of  a  newly  born  Child  in  which  the  Double  Formation  of 
'  the  Eyelid  is  seen.  The  Lateral  Nasal  and  Maxillary  processes  have  not  fused. 
Two  folds  separate  the  Eye  from  the  Nasal  Cavity.  The  inner  fold  represents 
the  Caruncula  Laclirymalis  and  the  outer  the  Plica  Semilunaris. 

into  orbital  and  temporal  parts  during  the  evolution  of  the  temporo- 
mandibular joint  (see  page  139).  The  division  is  effected  by  the  upbuildiAg 
of  a  lateral  wall  to  the  mammalian  orbit  ;  the  lateral  orbital  wall  must 
be  regarded  as  part  of  the  bony  scaffolding  for  giving  attachment  to  the 
muscles  of  mastication. 

The  eyelids  are  formed  in  the  earlier  weeks  of  the  3rd  month  by  folds 
of  ectoderm  which  commence  above  and  below  the  superficial  part  of 
the  eyeball.  Mesoderm  grows  into  the  folds  and  forms  the  tarsal  plates. 
The  upper  eyelid  is  formed  from  the  capsule  of  the  fore-brain,  the  lower 
from  the  maxillary  process.  About  the  middle  of  the  3rd  month  the  edges 
of  the  lids  meet,  adhere,  and  remain  adherent  until  the  end  of  the  sixth 
month.  In  rabbits,  mice,  kittens  and  puppies  the  lids  are  still  closed  at 
birth.  The  upper  eyelid  is  developed  in  two  parts — outer  and  inner  ; 
occasionally  a  notch  remains  on  the  margin,  and  marks  the  point  at  which 
the  two  parts  unite  (Fig.  219).     The  upper  end  of  the  plica  semilunaris 


216 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


is  attached  in  the  embryo  at  the  position  of  the  notch.  The  ectoderm  on 
the  deep  surface  of  the  lids  retains  a  columnar  shape,  and  forms  the  palpe- 
bral conjunctiva.  It  is  continuous  with  the  ectodermal  stratum  of  the 
cornea.  From  the  ectoderm  between  the  adherent  edges  of  the  lids,  buds 
grow  during  the  4th  and  5th  months,  and  form  the  eyelashes,  Meibomian 
and  other  glands,  in  the  same  manner  as  hairs  and  sweat  glands  are  de- 
veloped. The  Meibomian  glands  represent  modified  sebaceous  glands, 
but  the  hair  or  cilia  from  which  they  primarily  arose  have  vanished.  The 
curious  epicanthic  fold  is  shown  in  Fig.  220.  It  is  represented  in  all  races 
during  foetal  life. 

The  plica  semilunaris  (Fig.  221),  a  fold  of  conjunctiva  in  the  inner 
canthus  of  the  eye,  is  a  vestige  of  the  third  eyelid  (membrana  nictitans) 
which  is  fully  developed  in  birds  and  reptiles.  In  the  snake's  eye  Mayou 
found  that  this  membrane  formed  what  is  commonly  called  the  anterior 

plica  semilunaris 

canaliculus 


Fig.  220.— Epicanthic  or  Mongolian  fold.     (After  Meckel.) 

Fig.  221. — Diagram  of  the  Plica  Semilunaris  and  Lachrymal  Oanaliculi. 

lamina  of  the  cornea  ;  it  is  the  epithelium  of  this  membrane  which  desqua- 
mates and  renders  the  animal  temporarily  blind.  The  plica  semilunaris 
is  relatively  large  in  the  human  foetus,  reaching  its  maximum  development 
in  the  5th  month.  It  is  well  seen  in  the  cat,  partially  crossing  the  cornea 
as  the  lids  are  shut.  The  lachrymal  papillae  in  man  rub  in  the  grooves 
at  the  outer  and  inner  margins  of  the  fold. 

The  Lachrymal  Gland  ^  arises  at  the  beginning  of  the  3rd  month  as  a 
number  of  ectodermal  buds  which  spring  from  the  fornix  of  the  conjunctiva 
beneath  the  upper  lid,  and  gTOW  into  the  tissue  of  the  outer  and  upper 
segment  of  the  orbit  (Fig.  222).  The  outer  buds  form  the  orbital  part  of  the 
gland  ;  the  more  internal  buds  form  the  palpebral  part.  Smaller  lachrymal 
glands  may  occasionally  be  found  at  the  outer  angle  of  the  eye,  which  is 
the  position  occupied  by  the  lachrymal  glands  of  birds  and  reptiles 
(Wiedersheim).  The  lachrymal  oanaliculi  and  sac  and  nasal  duct  are 
formed  out  of  solid  epithelial  cords  enclosed  between  the  maxillary  and 
lateral  nasal  processes  (see  p.  201).  The  canaliculi  are  formed  during  the  3rd 
month  as  sprouts  from  the  upper  end  of  the  solid  rod  of  epithelium  repre- 

1  Development  of  lachrymal  gland,  F,  Ask,  Anat.  Hefte,  1910,  vol.  40,  p.  489,  1908, 
vol.  36,  p.  189. 


SENSE  OF  SIGHT 


217 


senting  the  nasal  duct.  While  the  bud  of  the  upper  canaliculus  opens  at 
the  inner  end  of  the  upper  lid  (Fig.  222,  A),  the  inferior  canaliculus  extends 
some  way  along  the  lower  lid  before  it  conies  to  the  surface  (Ask).  It  may 
form  a  secondary  communication  nearer  the  inner  angle  of  the  eye,  thus 
giving  rise  to  a  congenital  lachrymal  fistula.     With  the  formation  of  the 


Fig.  222,  A. — Sho-ning  the  Termination  of  the  Lower  Lachrymal  Canaliculus  some 
distance  from  the  Mesial  End  of  the  Lower  Eyelid,  in  a  foetus  2 
months  old.  The  tubular  outgrowths  of  the  lachrymal  gland 
are  also  shown. 
B. — The  Mesial  Extremity  of  the  Lower  Eyelid  cut  off  to  form  the  Carun- 
cula.  The  lachrymal  outgrowths  are  more  complex  in  structure. 
From  a  foetus  in  4th  month  of  development.     (After  Ask.) 

lachrymal  canaliculus,  part  of  the  lower  eyelid  is  cut  off  and  forms  the 
caruncula  (Fig.  222,  A  and  B). 

The  Orbital  Muscles.^ — We  have  already  seen  that  the  head  is  com- 
posed of  nine  segments,  at  least  four  of  these  being  occipital ;  also,  that 
each  segment  gives  rise  to  a  muscle  plate  (Fig.  149).     The  muscle  plate 


to  sup.  rectus,  etc. 


to  sup. 

to  ext.  rectus 


-for.  Monro 

3rd  vent. 

Corp.  quad, 
motor  nucleus 
1st  segment 

of  2nd  segment 

4th  uentrtcle 
of  3rd  segment 


Fig.  223. — Diagram  of  the  Motor  Nerves  of  the  Muscles  of  the  Eye  derived  from 
the  1st,  2nd,  and  3rd  Cephalic  Segments. 

of  the  maxillary  or  premandibular — usually  called  the  first — segment 
forms  the  muscles  supplied  by  the  third  cranial  nerve — which  is  the  motor 
nerve  of  that  segment.  The  mesencephalon  (crura  cerebri)  contains  the 
corresponding  segment  of  the  neural  tube.     The  ciliary  muscle  and  sphincter 

^  For  an  account  of  the  development  of  orbital  vessels  see  F.  Dedekind,  Anat. 
Hefte,  1909,  vol.  38,  p.  1.     See  also  Dr.  Eliz.  A.  Fraser,  Proc.  Zool.  Soc.  1915,  p.  299. 


218      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

of  the  iris  also  belong  to  this  segment,  and  are  supplied  by  the  Ilird  nerve 
(Fig.  223).  The  muscle  plate  of  the  mandibular,  usually  named  the  second 
head  segment,  produces  the  superior  oblique.  The  dorsal  decussation 
of  the  IVth  nerves  is  evidently  the  result  of  a  mutual  migration  of  their 
nuclei — following  Kapper's  neuro-biotactic  law.  The  muscle  plate  of  the 
hyoid  or  third  cephalic  segment  gives  rise  to  the  external  rectus  ;  the  Vlth 
nerve  is  the  nerve  for  the  somatic  musculature  of  the  segment,  the  Vlith 
supplying  the  splanchnic  muscles. 

The  sensory  nerves  of  these  three  segments  are  fused  together  in  the 
three  divisions  of  the  Vth  nerve.  The  ciliary  ganglion  is  the  splanchnic 
(sympathetic)  ganglion  of  the  premandibular  segment.  The  nerves  for 
the  retractor  muscle,  the  non-striated  muscle  of  the  upper  eyelid,  and  the 
dilator  fibres  of  the  iris,  issue  from  the  upper  three  dorsal  segments  of  the 
spinal  cord,  and  reach  the  eye  by  the  cervical  sympathetic  chain  and 
cavernous  plexus.  The  nerve  fibres  for  the  orbicularis  palpebrarum 
pass  out  with  the  facial,  but  they  are  said  to  arise  from,  or  have  connection 
with,  cells  in  the  first  segment  of  the  neural  canal  (oculomotor  nucleus). 
The  ophthalmic  division  of  the  fifth  represents  the  sensory  somatic  nerve 
of  the  same  segment  to  which  the  third  nerve  belongs  ;  hence  the  reflection 
of  pain  along  this  nerve  (frontal  headache)  in  disorders  of  accommodation, 
the  muscle  of  accommodation  being  the  ciliary,  and  its  nerve,  the  oculo- 
motor, both  also  derivatives  of  the  first  segment.  Mention  has  been  made 
of  the  origin  of  the  retractor  muscle  with  the  external  rectus  from  the 
3rd  segment.  The  levator  palpebrae  superioris  is  a  late  delamination 
from  the  superior  rectus. 

Development  of  the  Nerve  Centres  concerned  with  Sight. — Five  parts 
of  the  brain  are  concerned  with  vision.     They  are  : 

(1)  The  optic  tracts. 

(2)  The  basal  centres  surrounding  the  termination  of  the  aqueduct  of 
Sylvius  in  the  3rd  ventricle. 

(3)  The  optic  radiations. 

(4)  The  occipital  lobes — in  part  at  least. 

(5)  The  angular  gyri. 

(1)  The  optic  tracts  are  made  up  of  fibres  developed  from  the  ganglionic 
cells  of  the  retina  and  also  in  part  of  efferent  fibres  developed  from  cells 
of  the  basal  ganglia  in  which  the  optic  tracts  are  seen  to  terminate.  The 
fibres  grow  in  by  the  optic  stalk,  those  from  the  nasal  fields  of  the  retina 
decussating  in  the  floor  of  the  third  ventricle  between  the  origins  of  the 
optic  vesicles,  and  thus  form  the  chiasma.  The  optic  fibres  grow  backwards 
on  the  surface  of  thalamencephalon  (see  Fig.  224)  and  on  the  optic  thalamus 
to  reach  the  nerve  centres  which  afterwards  form  the  pulvinar,  lateral  geni- 
culate bodies  and  the  superior  corpora  quadrigemina.  In  these  centres 
the  optic  fibres  end.  It  is  said  that  80  per  cent,  of  the  fibres  from  the 
central  area  of  the  retina  terminate  in  the  lateral  geniculate  bodies. 

(2)  The  basal  ganglia. — The  corpora  quadrigemina. — Almost  in  every 
structure  the  human  embryonic  condition  resembles  the  adult  condition 
of  lower  vertebrates.     A  good  example  is  seen  in  the  corpora  quadrigemina 


SENSE  OF  SIGHT 


219 


The  human  foetus  at  the  end  of  the  2nd  month  (Fig.  224)  shows  the  corpora 
quadrigemina  represented  by  a  prominent  thickening  in  the  roof  of  the 
cavity  of  the  mid-brain,  which  forms  subsequently  the  aqueduct  of  Sylvius. 
The  thickening  is  divided  into  lateral  halves  by  a  median  sulcus,  each 
half  being  nearly  as  large  as  the  cerebral  vesicle  of  that  period.  In  Fig. 
225  is  shown  the  condition  in  an  adult  lizard  ;  there  is  one  body  on  each 
side — the  optic  lobes  or  corpora  bigemina.  As  the  human  foetus  grows 
older,  each  lateral  lobe  becomes  divided  into  an  upper  and  lower  part  by 
the  formation  of  a  transverse  groove,  the  upper  and  lower  pairs  of  the 
corpora  quadrigema  being  thus  formed.  The  upper  pair  are  connected 
with  sight.     In  the  mole  they  are  vestigial,  but  in  compensation  the  inferior 

puluinar 


thalamencephalon 


sup.  Corp.  quad. 


int.  genie, 
ext  genie. 


atli  of  optic  fibres 


off.  lobe 

optic  stalk 

optic  cup 

Fig.  224. — Diagram  of  the  Foetal  Brain  at  the  end  of  the  2nd  month,  showing  the 
position  in  which  the  Optic  Tracts  are  developed. 

corpora  are  well  developed  as  they  are  connected  with  the  sense  of  hearing, 
which  is  very  acute  in  that  animal. 

The  pulvinar  and  lateral  geniculate  body,  in  which  the  upper  division 
of  the  optic  tract  ends,  are  developed  in  the  wall  of  the  3rd  ventricle  (thala- 
mencephalon). The  mid-brain  is  the  part  primarily  connected  with  sight ; 
in  the  floor  of  its  cavity — the  aqueduct  of  Sylvius — are  situated  the  motor 
nuclei  for  the  muscles  of  the  eye  ;  on  its  roof — the  terminal  centres  for  the 
optic  tract  (see  p.  95).  As  the  vertebrate  scale  of  animals  is  ascended, 
the  termination  of  the  optic  tracts  is  found  to  be  transferred  more  and 
more  to  the  centres  on  the  thalamencephalon.  The  projection  of  retinal 
stimuli  to  the  occipital  cortex  from  the  nucleus  of  the  pulvinar  is  shown  in 
Fig.  113. 

(3)  The  optic  radiations  ^  connect  the  basal  optic  centres  just  named 
with  the  mesial  surface  of  the  occipital  lobes,  and  vice  versa.     The  fibres 

^  For  fuller  details  of  optic  tracts  see  Prof.  Elliot  Smith,  Cunningham'' s  Text-Booh 
of  Anatomy. 


220 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


join  the  posterior  part  of  the  internal  capsule,  and  pass  under  and  round  the 
posterior  horn  of  the  lateral  ventricle  to  end  in  the  cortex  of  the  calcarine 
fissure  and  neighbourhood.  The  cortex  in  which  the  optic  radiations 
terminate  is  divided  by  a  narrow  white  stratum — the  line  of  Gennari — 
into  a  superficial  and  deep  layer. 

(4)  Tiie  occipital  lobe  and  calcarine  fissure. — A  mesial  view  of  the  5th 
month  foetal  brain  is  shown  in  Fig.  226.     The  occipital  lobe  is  already  well 


cerebrum 


Corp.  bigem  {optic  lobe) 


cerebellum 


lamina  termin 


optic  nerue     oituit 


post  horn 


Fig.  225. — Mesial  Section  of  the  Brain  of  a  Lizard,  showing  the  resemblance  to  the 
Human  Foetal  Brain  (Fig.  224),  especially  in  the  development  of  the  Corpora 
Bigemina. 

formed  ;  its  inner  aspect  shows  the  calcarine  and  jDarieto-occipital  fissures. 
A  section  across  the  occipital  lobe  is  shown  in  Fig.  226  ;  the  posterior  horn 
is  large  ;    the  calcarine  fissure  indents  its  inner  wall,  giving  rise  to  the 

Jine  of  section 
par-OGcip.  fis. 
ca/car.  fis. 
calcarauis 


calcarine  fis, 

'fascia  dentata 
uncus 

Fig.  226,  A. — View  of  the  Mesial  Surface  of  the  Brain  in  the  5th  month. 
B. — Section  of  the  Occipital  Lobe  at  the  position  marked  in  A. 

calcar  avis  or  hippocampus  minor,  a  feature  which  is  seen  in  the  brains  of 
nearly  all  mammals  (Elliot  Smith). 

The  calcarine  is  one  of  the  first  fissures  to  be  formed  on  the  brain  ;  it 
appears  early  in  the  fifth  month.  This  and  the  hippocampal  depression, 
which  is  connected  with  the  sense  of  smell,  are  the  two  fissures  most 
commonly  present  in  the  mammalian  brain.  The  posterior  part  of  the 
calcarine  fissure  is  a  later  formation,  a,nd  is  distinguished  as  the  retro- 


SENSE  OF  SIGHT  221 

calcarine  (see  Fig.  127,  p.  131).  The  optic  radiations  end  in  the  cortex  of 
the  retro-calcarine  fissure.^  In  Fig.  227  the  condition  of  the  occipital 
lobe  in  the  5th  week  is  shown.  The  lateral  ventricle  is  as  yet  undifferenti- 
ated into  horns,  and  only  the  rudiment  of  the  occipital  lobe  is  present. 
The  occipital  lobe  is  produced  by  a  backward  growth  of  the  cerebral  vesicle, 
the  posterior  horn  being  produced  as  a  diverticulum  of  the  cavity  of  the 
vesicle.  By  the  5th  month  the  occipital  lobe  has  reached  far  enough 
back  to  overlap  the  cerebellum.  The  striate  or  visuo-sensory  area  of  the 
human  brain  is  not  larger  than  that  of  the  anthropoid  ape,  but  the  associa- 
tion or  visuo-psychic  area  is  infinitely  more  extensive.  "  Thus,  we  can 
take  it  that  the  superiority  of  the  human  over  the  ape's  brain  as  a  psychical 
organ  must  be  the  result  mainly  of  the  higher  development  of  the  association 
or  peri-striate  areas  "  (Elliot  Smith). 

(5)  The  angular  gyrus  is  connected  with  the  calcarine  region  by  associa- 
tion fibres.     In  it  are  seated  the  word-seeing  and  word-understanding 

rudiment  of  occi p.  lobe 


olf.  lobe 


optic  recess 
foramen  of  H/lonro 


Fig.  227. — Mesial  Section  of  the  Brain  at  the  4th  week,  showing  the  rudiment  of 
the  Occipital  Lobe.     (After  His.) 

centres.  It  is  developed  round  the  posterior  end  of  the  1st  temporal  or 
parallel  fissure  (Fig.  123).  It  is  part  of  the  wall  of  the  cerebral  vesicle.  The 
first  temporal  or  parallel  fissure  appears  during  the  sixth  month  and  is  one 
of  the  primary  fissures.  It  is  found  in  the  brains  of  all  primates  except 
the  lowest. 

Summary. — It  will  thus  be  seen  that  three  parts  of  the  neural  tube  are 
specialized  in  connection  with  sight. 

(1)  The  optic  vesicle,  an  outgrowth  from  the  fore-brain  (thalamence- 
phalon). 

(2)  The  occipital  region  of  the  cerebral  vesicle,  which  receives  fibres 
projected  from  the  basal  nuclei  connected  with  the  eyes. 

(3)  The  walls  of  the  3rd  ventricle  (thalamencephalon)  and  mid-brain 
(mesencephalon),  in  which  the  terminal  nuclei  of  the  optic  fibres  are 
developed. 

The  tunics  of  the  eye  are  extensions  of  the  embryological  coverings  of 
the  brain.     The  choroid  coat  and  the  vitreous  humour  spring  from  the 

^  For  a  description  of  the  cortex  of  the  visual  areas  see  Elliot  Smith,  Journ.  Anat. 
and  Physiol.  1907,  vol.  41,  p.  237.     See  also  references  on  p.  116. 


222      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

same  layer  as  forms  the  pia  mater  and  arachnoid.  The  sclerotic  is  a 
prolongation  of  the  primitive  cerebral  capsule,  in  which  the  skull  bones 
are  formed.  The  optic  vesicle  carries  with  it  a  prolongation  of  the  arteries 
and  veins  of  the  fore-brain.  Part  of  the  oj)tic  vesicle  is  transformed  into 
a  secretory  epithelium  over  the  ciliary  processes  in  the  same  way  as  the 
wall  of  the  neural  tube  becomes  a  covering  for  the  choroidal  villi  of  the 
brain. 


CHAPTER  XVI. 
THE   ORGAN   OF  HEARING. 

The  Nature  of  the  Labyrinth. — It  often  happens,  when  we  seek  to 
interpret  the  developmental  changes  which  give  rise  to  an  organ  or  system 
of  the  human  body,  that  a  reference  to  the  condition  seen  in  certain  groups 
of  fishes — especially  those  belonging  to  the  shark  kind,  selachians — gives 
us  great  assistance.  This  is  true  as  regards  the  organ  of  hearing.  In 
a  shark  or  ray  every  part  of  the  internal  ear — the  labyrinth  with  its  semi- 
circular canals — is  already  evolved  with  the  exception  of  one  part— the 
canal  of  the  cochlea  ;  it  is  represented  by  a  mere  rudiment.  The  labyrinth 
of  the  shark  is  not  an  organ  of  hearing,  for  it  is  generally  admitted  that 
fishes  are  insensitive  to  sound-waves,  but  for  the  balancing  or  orientation 
of  the  body.  Most  men  who  have  investigated  the  nature  of  the  labyrinth 
of  fishes  agree  that  it  represents  a  specialization  of  one  of  a  series  of  super- 
ficial sense  organs  set  on  the  sides  of  fishes — the  organs  of  the  lateral  line — 
these  also  being  connected  with  the  functions  of  balancing  and  movement. 
Hence  we  find  that  the  labyrinth  begins  as  a  pocket-like  invagination  of 
the  ectodermal  covering  in  the  head  region.  The  essential  element  of  the 
labyrinth  is  its  ciliated  epithelium  ;  movements  of  the  cilia,  produced  in 
various  ways,  give  rise  to  stimuli  which  pass  by  the  Vlllth  nerve  to  the 
hind-brain.  The  auditory  or  cochlear  part  of  the  labyrinth  appeared  when 
the  land-forms  of  vertebrates  were  evolved.  In  vertebrates  above  fishes 
the  rudiment  of  the  cochlea  begins  to  be  differentiated  and  an  apparatus 
for  converting  sound  waves  into  mechanical  waves  in  the  labyrinth  is 
evolved.^  A  vibrating  drum  was  established  in  the  site  of  the  first  of  the 
pharyngeal  or  visceral  clefts.  We  must  also  suppose  that  in  the  piscine 
type,  which  gave  origin  to  the  ancestry  of  the  mammals,  the  mammalian 
form  of  mandible  was  already  evolved,  for  it  is  from  remains  of  the  primitive 
cartilaginous  skeleton  of  the  lower  jaw  that  the  malleus  and  incus  are 
differentiated  in  the  human  and  mammalian  embryo. 

The  Structures  which  form  the  Organ  of  Hearing. — In  Fig.  228 
is  shown  diagrammatically  the  derivation  of  the  five  elements  which  unite 
together  to  make  up  the  organ  of  hearing.     The  five  elements  are  : 

(1)  The  otocyst — an  area  or  plaque  of  ectoderm  covering  the  head  of  the 
embryo  above  the  first  visceral  cleft  which  becomes  invaginated  in  a  saccular 

1  G.  L.  Streeter,  Journ.  Experiment.  Zoology,  1906,  vol.  3,  p.  543  ;  1907,  vol.  4,  p.  431  ; 
1914,  vol.  16,  p.  149  (Results  of  Experiment  on  Developing  Internal  Ear) ;  A.  Keith 
Proc.  Roy.  Soc.  Med.  1919,  vol.  xiii.  p.  1  (Otological  Section). 

223 


224 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


form,  to  become  the  epithelial  lining  of  the  membranous  labyrinth.     Some 
of  its  lining  cells  become  differentiated  into  ciliated  sensory  epithelium. 

(2)  A  ganglion  of  somewhat  uncertain  origin,  one  view  being  that  it  arises 
from  the  neural  crest  as  is  represented  in  Fig.  228,  but  there  is  a  growing 
conviction  that  some  at  least  of  the  ganglionic  cells  arise  from  the  ecto- 
derm of  the  otocyst.  The  nerve  cells  form  the  cochlear  and  vestibular 
ganglia.  Each  cell  sends  out  two  processes,  one  to  become  connected 
with  the  epithelium  of  the  otocyst,  the  other  to  end  in  groups  of  nerve 
cells  in  the  floor  of  the  hind-brain,  their  collective  fibres  forming  the  Vlllth 
nerve.  The  development  of  the  auditory  nerve  thus  resembles  that  of 
the  posterior  or  sensory  root  of  a  spinal  nerve. 

(point  at  which  acoustic 
ganglia  arise) 


periotic  capsule 
ganglia 

tgmp. 
Eustack 


acoustic  gang. 
4 1st  cleft  recess 
Meckel's  cart. 

~Ext.  audit  meatus 
(1st  cleft  depress, 
tragus 

andib  arch 


I'IG.  228. — Diagrammatic  Section  through  the  Cephalic  Region  of  an  Embryo, 
showing  the  origin  of  the  Auditory  System. 


(3)  The  otocyst  (membranous  labyrinth)  becomes  surrounded  by  a 
capsule  of  cartilage — the  periotic  capsule.  This  ossifies  from  several 
centres,  and  forms  the  bony  labyrinth  and  petro-mastoid. 

(4)  The  Eustachian  tube,  the  tympanum  and  antrum  of  the  mastoid 
arise  in  connection  with  the  pharyngeal  pocket  between  the  mandibular 
and  hyoid  arches  ;  the  corresponding  external  cleft  depression  forms  the 
point  of  origin  for  the  external  auditory  meatus  ;  while  out  of  the  tissue 
between  the  internal  pocket  and  external  cleft,  representing  in  position  a 
"  cleft-membrane,"  is  formed  the  membrana  tympani. 

(5)  The  hyomandibular  cartilage  (Fig.  173),  which  served  primarily  to 
bind  the  cartilages  of  the  maxillary  process,  mandibular  and  hyoid  arches 
to  the  base  of  the  skull,  becomes  the  stapes.  The  incus  and  malleus  arise 
from  the  upper  end  of  the  mandibular  bar  of  cartilage  (Fig.  132). 


THE  ORGAN  OF  HEARING 


225 


In  fishes  the  auditory  apparatus  is  composed  of  the  three  elements 
named  first.  In  amphibians,  reptiles  and  birds  a  membrana  tympani 
is  developed,  Avhich  is  connected  with  the  inner  ear  by  an  unjointed  deriva- 
tive of  the  hyomandibular  cartilage,  the  columella.  In  mammals  a  tym- 
panic cavity,  external  auditory  meatus,  and  auditory  ossicles  appear. 

External  Auditory  Meatus. — A  section  along  the  external  meatus  of 
a  newly  born  child  shows  that  it  is  divided  by  a  constriction  into  outer  and 


I  roof  of  meatus 

fegmen  tympani 
.attic  of  tympanum 


fYoofof  meatus 
petro-squamosal  suture 
311  tympani 


int.  aud  meat 


membrana  tympani 


meatus 


tympanic  ring 
fibrous  plate 


tympanic  plate 

Fia.  229,  A. — A  Section  of  the  External  Auditory  Meatus  of  the  Adult. 

B. — A  Section  of  the  External  Auditory  Meatus  at  Birth.     (After  Symington.) 

inner  parts  (Fig.  229,  B).  The  outer  part  is  derived  directly  from  the 
first  cleft  depression  ;  the  inner  part  arises  during  the  2nd  and  3rd  months 
by  a  solid  ingrowth  of  epithelium  which,  commencing  from  the  cleft  depres- 
sion or  pit,  grows  inwards  until  it  comes  in  contact  with  the  handle  of  the 
malleus,  when  it  expands  to  form  the  fundus  of  the  meatus  (Figs.  230,  236). 

TYMP.  RECESS 


(a)    16  mm.  STAGE. 


PLATE 

MEATUS 

fB)    30  mm.  STAGE. 


Fia.  230. — Showing  the  growth  of  the  external  meatal  plug  and  its  relationship  to 
the  tympanic  recess  of  the  pharynx.     (Prof.  Frazer.) 

During  the  7th  month  the  deeper  part  of  the  meatus  and  outer  aspect  of 
the  drum  are  formed  by  a  breaking  down  of  the  central,  and  therefore 
older,  cells  of  this  ingrowth.  Cartilage  surrounds  the  part  of  the  meatus 
derived  from  the  cleft ;  the  floor  of  the  deeper  part  is  formed  at  birth  by  a 
fibrous  plate  continuous  with  the  tympanic  ring.  In  the  adult  the  tym- 
panic ring  has  grown  outwards  in  the  fibrous  tissue,  as  we  have  already 
seen  (p.  178),  to  form  the  tympanic  plate  and  the  inner  two-thirds  of  the 


226 


HUMAN  EMBRYOLOaY  AND  MORPHOLOGY 


meatal  floor.  The  squamous  part  of  the  temporal,  which  is  developed  in 
its  roof,  also  grows  outwards,  and  forms  a  thick,  horizontal  plate  in  the 
inner  two-thirds  of  the  meatal  roof  (Fig.  229,  A  and  B).  Over  the  roof 
lies  the  third  temporal  convolution. 

The  meatus  is  supplied  in  front  by  the  nerve  of  the  mandibular  arch 
(auriculo-temporal  branch)  and  also  by  a  branch  from  the  nerve  of  the 
hyoid  arch — the  facial.  Why  the  vagus  should  supply  it  with  a  branch 
(Arnold's  nerve)  is  obscure.  In  fishes  a  branch  of  the  vagus  passes  back- 
wards beneath  the  skin  on  each  side  and  supplies  the  sense  organs  of  the 
lateral  line.  Many  regard  the  auricular  branch  of  the  vagus  as  a  vestige 
of  such  a  branch. 

In  the  newly  born  child  the  membrana  tympani  is  so  obliquely  set  that 
its  outer  surface  is  almost  in  contact  with  the  meatal  floor  (Figs.  229,  B, 
236).     With  the  development  in  length  of  the  meatus,  it  becomes  more 


otocyst 

margin 
antihelix 

antitragus 
lobule 

1st  visceral  cleft 
mandible 
''maxilla 


Fig. 


231. — Showing  the  Tubercles  which  arise  round  the  First  Visceral  Cleft  to 
form  the  External  Ear. 


vertical  in  position.  The  deeper  part  of  the  meatus  may  fail  to  form,  or 
the  whole  cleft  may  become  closed.  In  such  a  case  there  is  commonly  a 
corresponding  absence  of  development  of  the  middle  and  internal  ear. 

The  External  Ear. — Six  tubercles  appear  on  the  mandibular  and  hyoid 
arches  round  the  1st  cleft  depression  during  the  6th  week  and  form  the  basis 
of  the  external  ear  (Figs.  231  and  232).  Three  of  these  tubercles  grow 
from  the  mandibular  arch  and  form  the  tragus,  crus  of  the  helix,  and  helix  ; 
three  from  the  hyoid  to  form  the  lobule,  antitragus  and  antihelix.  The 
posterior  margin  of  the  ear,  or  descending  helix,  with  the  lobule,  arise  as  a 
mere  thickening  or  elevation  of  the  skin  behind  the  tubercles  on  the  hyoid 
arch.  During  the  latter  part  of  the  2nd  month  and  first  part  of  the 
3rd,  the  pinna  begins  to  assume  its  definite  form.  The  tubercles  of  the 
helix  and  antihelix  send  out  processes  which  cross  the  upper  part  of 
the  cleft  and  obliterate  it,  while  the  neighbouring  tubercles  fuse  to  form 


THE  ORGAN  OF  HEARING 


227 


the  definite  parts  of  the  ear.  The  posterior  margin  and  lobule  rise  up  at 
the  same  time  as  a  free  fold.  The  auricular  tubercles  may  not  fuse  com- 
pletely and  thus  leave  fistulae  between  them.  Such  fistulae  are  commonly 
seen  between  the  tragus  and  root  of  the  helix,  or  between  the  antihelix  and 
the  helix.  The  mandibular  part  of  the  auricle  is  supplied,  as  one  would 
expect  from  its  origin,  by  the  third  division  of  the  5th,  while  the  sensory 
fibres  for  the  hyoid  part  come  from  the  2nd  cervical  by  the  great  auricular 
and  small  occipital  nerves. 

Darwin's  Tubercle.— The  human  ear  appears  to  be  derived  from  a 
form  in  which  the  margin  was  pointed  at  the  posterior  superior  angle,  such 
as  is  seen  in  many  of  the  lower  forms  of  apes  and  mammals  generally. 
With  the  retrogression  of  the  j)osterior  border  or  descending  helix  and 
increased  development  of  the  antihelix  in  the  human  ear,  the  posterior 
margin  became  infolded  ;  hence  the  tip  appears  as  a  tubercle  on  the  inturned 


helix 


cms  heli'cfs 


tragus 


position  of  cleft—;   *'-•#- 


Danuin's  point 
ear  margin 
antihelix 

antitragus 


Fig.  232. — Showing  the  part  of  the  Adult  Ear  formed  by  each  Tubercle. 

posterior  margin  or  welt  of  the  human  ear  (Fig.  232).  The  small  size  and 
restricted  mobility  of  the  external  ears  of  higher  primates  result  from  the 
free  manner  in  which  these  animals  can  turn  their  heads  in  the  direction 
of  sounds. 

Muscles  of  the  External  Ear  are  derived  from  the  platysma  sheet 
and  are  supplied  by  the  nerve  of  that  sheet — the  7th  or  facial.  The  part 
of  the  platysma  sheet  which  surrounds  the  external  meatus  and  acts  on  the 
ear  appears  to  have  been  the  first  of  the  facial  muscles  to  be  evolved. 
The  ear  muscles  are  not  so  reduced  in  man  as  in  some  other  primates, 
such  as  the  orang. 

The  Eustachian  Tube. — The  Eustachian  tube  has  usually  been  re- 
garded as  a  derivative  of  the  first  of  the  inner  cleft  recess — a  diverticulum 
of  the  lining  membrane  of  the  primitive  pharynx  between  the  mandibular 
and  hyoid  arches  (Fig.  198,  A).     Recently  Professor  Frazer  ^  has  made  a 

1  Joiirn.  Anat.  1914,  vol.  48,  p.  391. 


228     HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

thorough  enquiry  into  its  development,  and  has  found  that  its  origin  is 
more  complicated  than  was  supposed.  In  Fig.  233,  A,  the  left  half  of  the 
floor  of  the  pharynx  of  a  human  embryo,  five  weeks  old,  is  represented. 
Between  the  1st  and  2nd  and  between  the  2nd  and  3rd  arches  the  lining 
mucous  membrane  of  the  pharynx  is  seen  to  dip  outwards  and  at  first  is 
actually  in  contact ;  as  yet  there  is  no  sign  of  Eustachian  tube  or  of  tym- 
panum. In  Fig.  233,  B,  the  opposite  half  of  the  floor  of  the  pharynx  of  a 
human  embryo  towards  the  end  of  the  2nd  month  of  development  is  shown  ; 
the  basis  of  the  Eustachian  tube  and  tympanum  is  now  apparent  as  a  wide 
recess  between  the  first  and  third  arches,  the  hyoid  arch  being  squeezed 
outwards  on  the  outer  wall  of  the  recess.  The  oblique  fold  forming  the 
roof  and  posterior  wall  of  the  Eustachian  tube  is  formed  by  the  forward 


1"  RECESS  ^^     _ 

M„M, „„,„,„  ,«  >™.x       '.XYMPANUM 


Z"° RECESS 


^3=~-^     EUST:TUBE 


Fig.  233. — Figures  illustrating  tlie  Development  of  the  Eustachian  Tube  and 
Tympanum.     (After  Prof.  Frazer.) 
A. — The  Floor  of  the  Pharynx  of  a  Human  Embryo  Ave  weeks  old. 

The  visceral  arches  are  cut  across  and  the  inner  cleft  recesses  indicated. 
B. — The  Floor  of  the  Pharynx  of  a  Human  Foetus  seven  weeks  old,  showing  the 
origin  of  the  Eustachian  Tube  by  an  Evagination  of  the  Pharynx  opposite 
the  2nd  or  Hyoid  Visceral  Arch.  T,  rudiments  of  tongue  on  floor  of 
pharynx  ;  L,  larynx. 

growth  of  the  substance  of  the  3rd  arch,  which,  it  will  be  remembered,  also 
contributes  to  the  formation  of  the  soft  palate.  The  Eustachian  tube 
retains  through  life  the  ciliated  epithelial  lining  of  the  primitive  pharynx. 
Its  inner  two-thirds  is  bounded  behind  by  a  triangular  plate  of  cartilage, 
which  is  attached  at  its  inner  or  pharyngeal  end  to  the  internal  pterygoid 
plate,  by  its  outer  to  the  tympanic  ring,  both  of  which  are  probably  derived 
from  the  palato-quadrate  bar  (Fig.  173,  p.  172).  The  cartilage  is  developed 
in  the  4th  month  of  foetal  life.  The  tympanic  plate  grows  inwards  and 
forms  the  floor  of  the  outer  third  of  the  tube  (Fig.  235),  while  the  periotic 
capsule  (petro-mastoid)  which  is  developed  above  and  behind  the  1st  cleft, 
grows  forwards  and  forms  the  roof  of  its  outer  third.  The  part  of  the 
petro-mastoid  which  grows  over  it  is  the  tegmen  tympani  (Fig.  229) ; 
it  also  forms  the  roof  of  the  tympanum  and  of  the  antrum  of  the  mastoid. 
The  tensor  tympani  and  tensor  palati  are  developed  on  the  mandibular  side 


THE  OKGAN  OF  HEARING 


229 


of  the  first  cleft  and  are  supplied  from  the  nerve  of  the  mandibular  process 
through  the  otic  ganglion. 

The  Tympanum. — The  tympanum  can  scarcely  be  said  to  exist  until 
the  3rd  month  of  foetal  life.  Until  then,  the  Eustachian  recess  ends  in 
jelly-like  tissue  containing  the  cartilaginous  bases  of  the  malleus  and 
incus.  It  is  directed  outwards  and  backwards  between  the  periotic  capsule 
to  its  posterior  and  inner  side,  and  the  external  cleft  depression  (meatus) 
and  developing  squamosal  to  its  outer  (Fig.  234).  As  the  tympanic  recess, 
in  which  are  represented  both  1st  and  2nd  pharjoigeal  pockets,  extends 
outwards  and  backwards,  the  gelatinous  tissue  is  absorbed,  so  that,  in  the 
later  months  of  development,  the  malleus  and  incus  and  developing  stapes, 


saccus  endofymph, 

semicircular  canal 

«  I  .  periotic  capsule 
otocyst  (utricle) 

vein 

facial  nerve 
antrum 

chorda  tympani 
malleus 
drum 


pharynx 


tympanum 


ext.  aud.  meatus 
tragus 


Fig.  234. — Showing  the  condition  of  the  Auditory  Organs  in  a  7th  week  Human 
Foetus.     (After  Siebenmann.) 

with  the  chorda  tympani,  become  surrounded  by  the  entodermal  lining 
of  the  recess  and  thus  appear  to  lie  within  the  cavity  thus  formed — the 
tympanum.  The  tympanic  plate  forms  the  floor  of  the  tympanum,  the 
membrana  tympani  and  squamosal  its  outer  wall,  while  the  petro-mastoid 
forms  its  inner  wall  and  roof  (Fig.  235).  The  nerve  of  the  2nd  arch — the 
facial — lies  in  its  inner  or  mesial  wall.  That  part  of  the  tympanum  which 
lies  above  the  level  of  the  membrana  tympani  is  named  the  attic,  and 
contains  the  head  of  the  malleus  and  body  of  the  incus  (Fig.  229). 

In  carnivora  and  some  other  mammals  the  floor  of  the  tympanum, 
formed  by  the  tympanic  plate,  is  inflated  into  a  bulla,  the  tympanic  bulla. 
Its  meaning  is  unknown,  but  when  a  bulla  is  developed  the  antrum  of  the 
mastoid  is  small  or  absent. 

Auditory  Ossicles. — In  the  3rd  month  the  auditory  ossicles  become 
clearly  differentiated  in  cartilage  in  the  mesodermal  tissue  between  the 


230 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


meatal  recess  on  their  outer  side  and  the  Eustachian  recess  on  their  inner. 
Concerning  their  development,  the  exact  researches  of  Broman/  of  Ham- 
mar,  and  of  Jenkinson^  give  us  a  very  full  account.  The  malleus  represents 
the  upper  or  articular  end  of  Meckel's  cartilage  (Figs.  237,  132)  ;  the  incus, 
developed  beyond  the  articular  end  of  Meckel's  cartilage,  represents  the 

tegmen  tympani 


int.  carotid 


squam. 


drum 
meatus 


tympanic  plate 


Eustach.  carta. 


Fig.  235.- 


-Showing  the  Cavities  derived  from  the  Eustachian  Recess  of  the  Primi- 
tive Pharnyx. 


cranial  articular  base — the  quadrate  of  lower  vertebrates.  The  stapes  (Fig. 
237)  is  developed  at  the  upper  end  of  the  hyoid  arch,  the  sides  of  the  stirrup 
being  formed  round  the  dorsal  end  of  the  artery  of  the  hyoid  arch.  Even 
in  the  4th  month  of  development  the  cavity  of  the  tympanum  has  only 
reached  the  handle  of  the  malleus  (Fig.  236).     The  upper  part  of  the  drum 


SQUAMOSAL 
MALLEUS 


TYMPANUM 
TYMPANIC     RING 


TYMPANIC   RINfi 


HYOIO    CART: 


Fig.  236. — Section  of  the  External  Auditory  Meatus,  Drum  and  Tympanum  of  a 
Human  Foetus  in  the  4th  month  of  development.  The  meatal  plug  fills  the 
deep  part  of  the  meatus  and  only  the  handle  of  the  hammer  is  in  the  tympanic 
cavity.     (After  Broman.) 

Fig.  237. — The  Auditory  Ossicles  of  the  Left  Side,  seen  on  their  Inner  Aspect,  during 
the  3rd  month  of  development.     (After  Broman.) 

(pars  flaccida)  is  not  yet  differentiated.     The  attic,  antrum,  head  of  the 

hammer,  and  body  of  the  incus  are  still  outside  the  cavity  of  the  tympanum. 

The  Antrum  of  the  Mastoid  represents  the  extreme  outer  or  posterior 

end  of  the  chamber  derived  from  the  extension  of  the  Eustachian  recess 

1  See  Broman's  excellent  Normale  und  abnormale  EntwicHung  des  Menschen,  Wies- 
baden, 1911. 

2  J.  W.  Jenkinson,  Journ.  Anat.  and  Physiol  1911,  vol.  45,  p.  305;    Hugo  Frey, 
Anat.  Hefte,  1911,  vol.  44,  p.  363. 


THE  ORGAN  OF  HEARING 


231 


(Fjgs.  234  and  235).  It  is  formed  during  the  6th  and  7th  months  by  an 
expansion  of  the  tympanic  cavity  upwards  and  backwards  in  the  surround- 
ing mucoid  tissue.  Its  use  is  uncertain,  but  it  has  frequently  to  be  ex- 
posed by  the  surgeon  to  remove  the  effects  of  chronic  middle-ear  disease. 
At  birth  its  outer  wall  is  formed  by  the  thin  post-auditory  part  of  the 
squamosal  (Figs.  238  and  239).  The  squamosal  forming  its  outer  wall  is 
then  only  2  mm.  thick,  but  every  year  until  the  20th,  or  later,  this  plate 
increases  nearly  1  mm.  in  thickness,  so  that  by  the  20th  year  the  antrum  is 
buried  by  a  plate  of  bone  about  20  mm.  thick.  There  is  a  great  individual 
variation,  however,  in  the  thickness  of  its  outer  wall.  The  antrum  lies 
above  and  behind  the  level  of  the  external  auditory  meatus  ;  the  post- 
auditory  spine  and  supra-meatal  triangle  formed  by  the  post-auditory 
part  of  the  squamosal  lie  over  it  and  serve  as  surface  guides  to  it.  The 
antrum  opens  in  front  into  the  attic  of  the  tympanum.  The  tegmen 
tympani  (Fig.  239)  forms  its  roof  and  the  petro-mastoid  its  floor  and  inner 
wall.     The  canal  for  the  Vllth  nerve  runs  down  the  inner  wall  of  its  mouth 


Eustach.  tube 


squam.- 
petro-squam.  fis. 

antrum 
ost-meat  proc. 
petro-mastoid^ 


for.  ovate 

VIL 
antrum 

Eustachian  tube 


tympanum 
tt/mpam'c  ring 


tegmen  iympani 

ext.  semicirc.  canal 
antrum 

petro-mast 

VII 
for.  rotundum 


Fig.  238. — The  Temporal  Bone  at  birth,  showing  the  formation  of  the  Antrum 

between  the  Squamosal  and  Petro-mastoid. 
Fig.  239. — A  Transverse  Section  showing  how  the  Walls  of  the  Antrum  are  formed. 
Fig.  240. — Showing   the   outer   aspect   of   the   Petro-mastoid   at  birth   after  the 

Squamosal  is  removed. 

(Fig.  240),  and  in  its  inner  wall  is  situated  the  external  semicircular  canal. 
The  petro-squamosal  suture  in  its  roof  (Fig.  239)  and  the  masto-squamous 
suture  on  its  outer  wall  (Fig.  181,  p.  179)  become  closed  the  second  year, 
and  thus  the  escape  of  pus  from  it  is  rendered  more  difficult.  The  rudiments 
of  the  mastoid  cells  are  already  present  as  evaginations  or  pits  of  the  antral 
lining  at  birth  (Arthur  Cheatle). 

Petro-Sguamous  Sinus. — We  have  seen  (page  133)  that  the  primitive 
vein  of  the  head,  part  of  which  persists  as  the  cavernous  sinus,  escapes 
from  the  cranial  cavity  just  in  front  of  the  auditory  capsule.  Before 
escaping  from  the  skull  it  receives  a  tributary  from  the  hind-brain — which 
afterwards  occupies  the  petro-squamosal  suture.  This  vein,  frequently 
of  considerable  size,  runs  forwards  from  the  lateral  sinus,  and  commonly 
ends  in  a  tributary  of  the  middle  meningeal  vein.  It  receives  as  it  runs 
along,  venules  from  the  antrum  and  attic  and  may  be  the  means  of  carrying 
infection  from  the  middle  ear  to  the  lateral  sinus  or  to  the  meningeal  veins 
(Cheatle).  The  petro-squamous  sinus  may  open  in  man,  as  it  does  in 
mammals  generally,  at  the  post-glenoid  foramen,  situated  at  the  outer  end 
of  the  Glaserian .fissure,  near  the  base  of  the  zygoma.  The  vein  thus  emerging 
may  represent  the  primitive  vein  of  the  head, 


232      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

The  Membrana  Tympani. — As  may  be  seen  from  Figs.  230,  234,  the 
membrana  tympani  is  of  very  considerable  thickness  until  the  gelatinous 
tissue  in  the  tympanum  is  absorbed.  It  has  an  inner  covering  of  entoderm 
and  an  outer  of  ectoderm.  In  the  mesodermal  tissue  between  the  coverings 
lie  parts  of  the  malleus,  incus  and  chorda  tympani.  As  the  gelatinous 
tissue  round  the  fundus  of  the  Eustachian  recess  is  absorbed  during  the 
later  months  of  foetal  life,  the  tympanic  lining  membrane  expands,  and 
thus  the  handle  of  the  malleus  and  chorda  tympani  come  to  appear  as  if 
they  lie  on  the  membrane,  although  really  within  it  (Fig.  236j.  The 
mucous  lining  of  the  tympanum  covers  them.  The  membrane  is  sup- 
ported by  the  tympanic  ring,  the  age  changes  of  which  have  already  been 
dealt  with  (p.  178).  The  membrane  contains  tissue  derived  from  both 
mandibular  and  hyoid  arches,  and  hence  receives  nerves  and  vessels  from 
both. 

The  Membranous  Labyrinth. — The  various  parts  of  the  membranous 
labyrinth  of  the  internal  ear  are  represented  in  Fig.  241.  It  consists  of 
(1)  the  utricle  ;  (2)  three  semicircular  canals  opening  into  the  utricle  ; 
(3)  the  saccule  ;  (4)  a  canal  uniting  the  utricle  and  saccule — ^from,  or  near 
which,  springs  the  ductus  endolymphaticus.  All  of  these  parts  constitute 
the  vestibular  or  balancing  part  of  the  labyrinth.  (5)  The  cochlear  canal 
- — the  part  connected  with  hearing.  The  labyrinth,  although  a  compli- 
cated structure,  has  a  very  simple  beginning.  The  cells  of  a  certain  area 
of  ectoderm,  situated  above  and  behind  the  first  cleft  and  lying  against  the 


sup.  S.G. 

ductus  endolymph. — [I 


from  primitiue  utricle 
scala  media 

cochlear  canal 

Fig.  241. — Diagram  of  the  Membranous  Labyrinth. 

4th  and  5th  neuromeres  of  the  hind-brain  (Figs.  80,  93,  231)  become  in- 
vaginated  during  the  4th  week.  In  this  manner,  and  at  this  early  date, 
there  is  formed  a  simple  closed  pyriform  sac,  the  otocyst,  which  lies  above 
the  first  visceral  cleft  and  is  soon  surrounded  by  the  mesodermal  tissue 
which  forms  the  primitive  capsule  of  the  cephalic  part  of  the  neural  canal. 
The  sac  contains  a  fluid,  the  endolymph,  and  also  otoliths  are  found  in  it 
later.  The  otocyst  lies  at  first  close  to  the  side  of  the  hind-brain  with  the 
ganglionic  mass  belonging  to  the  7th  and  8th  cranial  nerves  to  its  inner 
and  anterior  side  (Fig.  93).  The  epithelial  cells  lining  it,  all  of  which  are 
originally  columnar,  soon  become  flattened,  except  at  the  maculae  acoustica, 


THE  ORGAN  OF  HEARING  233 

where  they  retain  the  columnar  form  and  develop  hair-like  processes. 
The  hair-like  processes  are  to  serve  as  levers  and  become  capable  of  being 
moved  by  various  means  to  evoke  nerve  stimuli.  Under  the  influence  of 
gravity  otoliths  serve  to  move  them  ;  so  do  the  currents  in  the  semicircular 
canal  as  the  head  is  moved  and,  so  too,  do  the  movements  of  the  stapes. 
The  hair  cells  become  connected  with  the  hind-brain  by  the  auditory  nerve 
fibres  of  the  cochlear  and  vestibular  ganglia.  The  otocyst  clearly  repre- 
sents a  sense  organ  which  was  primarily  situated  in  the  skin  and  through  its 
hair-like  processes  was  sensitive  to  the  position  and  movements  of  the 
body.     Its  auditory  function  arose  at  a  later  stage. 

In  the  lower  vertebrates,  as  in  the  earlier  embryonic  stages  of  the  higher 
mammals,  the  otocyst  is  of  a  saccular  form  with  a  stalk  above — the  ductus 
endolymphaticus  (Fig.  242).  The  simplest  form  of  vertebrate  otocyst  is 
seen  in  the  lamprey  ;    the  superior  and  posterior  semicircular  canals  are 


l-^ductus  endolymphaticus 
-semcircular  canal 


pn'mitiue  utricle 


cochlear  canal 


Fig.  242. — The  Otocyst  in  an  Embryo  of  five  weeks ;   it  shows  a  demarcation  into 
the  various  parts  of  the  Membranous  Labyrinth. 

present,  but,  as  in  the  mammalian  embryo,  the  primitive  cyst  is  undivided 
into  utricle,  saccule  and  cochlear  canal.  The  semicircular  canals  grow  out 
from  the  vesicle  as  fiat,  hollow  plates,  but  only  the  circumferences  of  the 
plates  persist,  the  centres  disappearing. 

The  development  and  differentiation  of  the  human  otocyst  has  been 
closely  studied  by  Prof.  Streeter.^  In  Fig.  243  three  stages  depicted  by 
him  are  represented.  At  the  5th  week  there  are  three  parts  :  (1)  the 
ductus  endolymphaticus,  at  one  time  regarded  as  the  stalk  which  con- 
nected the  cyst  with  the  surface  of  the  head,  but  now  known  to  be  an 
outgrowth  formed  after  the  stalk  is  obliterated  ;  (2)  the  vestibular  pouch 
or  part ;  (3)  the  cochlear  pouch  or  rudiment.  At  the  6th  week  a  higher 
stage  of  differentiation  is  reached  ;  all  the  parts  of  the  adult  labyrinth  are 
indicated — the  ductus  and  saccus  endolymphaticus  (both  of  uncertain 
import),  the  semicircular  canals,  with  their  ampullae ;  the  utricle  and 
saccule.  All  of  these  are  derived  from  the  vestibular  part  of  the  otocyst. 
The  cochlear  rudiment  has  extended  into  a  bent  canal,  and  its  communi- 
cation with  the  saccule  is  constricted  to  form  the  canalis  reuniens.  In 
the  10th  week  all  the  various  parts  are  present,  almost  in  their  adult  form. 

^American  Journal  of  Anatomy,  1907,  vol,  6,  p,  139  ;  1907,  vol.  7,  p.  337  (Develop- 
ment of  Ganglia  of  vii,  viii), 


234 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


The  utricle  and  saccule  are  now  separated  and  only  communicate  by  means 
of  tlie  ductus  endolymphaticus.  The  cochlear  canal  has  assumed  its 
spiral  form. 

The  primitive  utricle  or  vestibular  pouch,  which  represents  the  main 
part  of  the  otocyst,  subdivides  into  the  saccule  and  utricle  (Fig.  243,  C  C, 
G").  The  division  occurs  at  the  entrance  of  the  endolymphatic  canal, 
which  thus  comes  to  open  into  both  saccule  and  utricle.  The  endolym- 
phatic duct  is  enclosed  in  the  petro-mastoid,  its  extremity  appearing  at  the 
hiatus  vestibuli,  where  it  ends  beneath  the  dura  mater  in  a  dilatation. 
The  cochlear  canal  (scala  media),  the  real  auditory  part  of  the  labyrinth, 
although  late  in  point  of  evolution,  is  not  late  in  its  developmental  appear- 
ance. There  is  merely  a  rudiment  of  the  cochlea  in  fishes  and  other 
amphibians.     In  reptiles,  birds  and  monotremes  it  is  a  straight  canal — 


(A) 


(B) 


Fig.  243. — Three  stages  in  the  development  of  the  Human  Membranous  Labyrinth. 
A,  at  the  end  of  the  5th  week ;  B,  at  the  end  of  the  6th  week ;  C,  at  the 
end  of  the  10th  week.     (Streeter.) 

the  Lagena.  Only  in  mammals  is  it  arranged  spirally.  In  it  the  organ 
of  Corti  is  developed. 

Periotic  Capsule. — The  mesoderm  surrounding  the  membranous  laby- 
rinth and  dorsal  aorta  (internal  carotid)  above  the  first  visceral  cleft 
becomes  cartilaginous  at  the  end  of  the  2nd  month  of  foetal  life,  forming 
the  periotic  capsule  (Figs.  228,  234,  139).  There  are  two  centres  of  chon- 
drification,  one  for  the  vestibular  part — surrounding  the  vestibular  division 
of  the  labyrinth,  and  one  for  the  cochlear  part — surrounding  the  cochlea. 
The  course  of  the  facial  nerve  indicates  approximately  their  line  of  union. 
The  cartilage  of  the  cochlear  part  fuses  with  the  parachordal  or  basilar 
cartilage  ;  the  vestibular  part  becomes  continuous  with  the  occipital 
plate  (see  p.  137). 

Perilymph  System. — The  tissue  which  immediately  surrounds  the 
membranous  labyrinth  does  not  undergo  chondrification,  but  becomes 
converted    into    an    open    meshwork   of   cells,    the    intercellular   spaces 


THE  ORGAN  OF  HEARING  235 

containing  perilymph.  The  chief  or  vestibular  cistern  of  the  perilymphatic 
system  is  formed  round  the  saccule  and  utricle.  In  its  tympanic  or  outer 
wall  (Fig.  240)  there  is  an  oval  area  in  which  the  fenestra  ovalis  and  foot 
plate  of  the  stapes  are  formed.  Streeter  ^  found  that  the  vestibular  cistern 
is  the  first  to  form,  commencing  at  the  stapedial  plate  when  the  foetus  is 
50  ram.  in  length  (11  weeks  old) ;  an  extension  grows  out  along  one  side  of 
the  cochlear  canal  to  form  the  scala  vestihuli.  Another  area  of  the  inner 
tympanic  wall  remains  unchondrified,  subsequently  subdivided  to  form 
the  fenestra  rotunda  (Fig.  240)  and  the  aqueductus  cochleae.  In  the  11th 
week  a  second  cistern — the  scala  tympani — begins  to  form  at  the  fenestra 
rotunda,  growing  along  the  side  of  the  cochlear  canal,  opposite  to  the  scala 
vestibuli,  thus  bringing  that  canal  to  lie  between  two  perilymphatic  spaces. 
The  vestibular  and  tympanic  extensions  meet  and  fuse  at  the  lip  of  the 
cochlear  canal,  at  the  end  of  the  3rd  month,  thus  forming  the  helicotrema. 

Ossification  of  the  Petro-mastoid. — About  the  end  of  the  4th  month, 
four  ossific  centres  appear  in  the  periotic  capsule  ;  one,  the  pterotic,  gives 
rise  to  the  tegmen  tympani  which  forms  the  roof  of  the  antrum,  tympanum, 
and  Eustachian  tube  ;  the  petro-squanious  suture  marks  its  outer  edge  ; 
the  hiatus  Fallopii  marks  its  junction  with  a  second  centre — the  opisthotic. 
This  centre  forms  the  posterior  or  vestibular  half  of  the  petrous  bone. 
The  pro-otic  forms  the  anterior  or  cochlear  half  ;  the  mastoid  part,  which 
appears  on  the  surface  of  the  skull,  is  developed  from  the  epiotic  centre. 
While  the  greater  part  of  the  petro-mastoid  is  formed  in  a  cartilaginous 
basis,  the  dense  layers  which  form  the  immediate  bony  capsule  of  the 
labyrinth,  the  modiolus  and  lamina  spiralis  of  the  cochlea,  are  laid  down  by 
the  lining  membrane  of  the  perilymphatic  space. 

The  Mastoid. — The  mastoid  part  of  the  petro-mastoid  is  flat  at  birth  ; 
about  the  2nd  year  the  mastoid  process  appears  as  a  slight  knob,  and  it 
gradually  grows  downwards  to  form  a  cephalic  lever  for  the  sterno-mastoid, 
splenius  and  trachelo-mastoid  muscles.  The  period  of  its  most  active 
growth  is  marked  by  the  eruption  of  the  permanent  teeth.  In  most 
mammals  the  mastoid  grows  out  as  a  flat,  wing-shaped  process  continuous 
with  the  occipital  crest,  and  thus  increases  the  basal  area  of  the  skull  on 
which  the  neck  muscles  are  inserted  (Fig.  148).  The  post-auditory  process 
of  the  squamosal  forms  a  considerable  part  of  the  mastoid  process  ;  it 
reaches  to  the  apex  and  forms  the  anterior  border  (Fig.  181,  C).  As  the 
mastoid  process  grows  the  diploic  sj)aces  within  it  enlarge  into  air  spaces. 
Those  round  the  antrum  come  to  open  into  it,  but  the  more  distal  remain 
closed.  These  spaces  occupy  the  whole  of  the  mastoid  part  of  the  tem- 
poral, but  they  also  extend  forwards  in  the  post-auditory  process  of  the 
squamosal,  and  may  spread  backwards  to  the  occipital.  Three  varieties 
of  mastoids  are  recognized  :  (1)  Dense  processes  in  which  the  air  cells  are 
minute  or  absent  (infantile  type  of  Cheatle)  ;  (2)  a  type  containing  numerous 
large  spaces  (pneumatic)  ;  (3)  an  intermediate  type  with  large  cells  round 
the  antrum,  and  a  few  small  ones  near  the  surface.  The  third  type  is  the 
commonest. 

^  See  Geo.  L.  Streeter,  Contributions  to  Embryology,  1918,  vol.  7,  p.  5. 


236 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


The  Floccular  or  Subarcuate  Fossa. — At  birth  there  is  a  fossa  situated 
on  the  posterior  aspect  of  the  petro-mastoid.  It  is  filled  with  a  process  of 
the  dura  mater  in  the  human  embryo,  but  in  all  except  the  highest  primates 
it  contains  the  paraflocculus  (Fig.  90),  a  part  of  the  cerebellum  which  is 
quite  vestigial  in  man.  The  posterior  semicircular  canal  surrounds  the 
fossa.  This  is  the  condition  in  most  mammals  throughout  life,  but  soon 
after  birth  the  fossa  becomes  closed  in  man,  merely  a  remnant  being  seen 
above  and  internal  to  the  hiatus  vestibuli  in  the  bone  of  the  adult. 

Organ  of  Corti.^ — In  Fig.  244  is  given  a  diagrammatic  section  across 
the  cochlear  canal  to  show  the  manner  in  which  its  ectodermal  lining  is 
modified  to  form  the  organ  of  Corti — ^the  machinery  concerned  in  pro- 
ducing auditory  stimuli.  The  canal  has  become  three-sided — one  side 
lying  against  the  scala  vestibuli  (vestibular  wall),  another  against  the 


STRIAE    VASC. 


SCALA     TYMPfyf^^ 


Fig.  244. — Diagrammatic  section  across  the  Cochlear  Canal  of  a  newly  born  child 
to  show  the  differentiation  of  Ectodermal  Epithelium  to  form  the  Organ  of 
Corti.    (After  Keibel.) 

scala  tympani  (tympanic  wall),  the  third  being  peripheral  or  outer. 
The  ectoderm  on  the  vestibular  wall  atrophies  and  disappears — the  fibrous 
base  forming  Reissner's  membrane.  The  ectoderm  is  modified  to  form  a 
secretory  apparatus — ^the  vascular  body  (striae  vasculares).  On  the 
tympanic  wall  the  ectoderm  is  modified  to  form,  (a)  hair  or  sensory  cells, 
(6)  supporting  or  pillar  cells — comparable  to  neuroglial  cells  in  the  spinal 
cord,  and  fibres  of  Miiller  on  the  retina  ;  (c)  tectorial  cells,  producing  a 
peculiar  cuticular  substance,  which  forms  the  tectorial  membrane — in 
which  the  hair  processes  of  the  sensory  cells  are  embedded.  The  auditory 
nerve  fibres  commence  round  the  hair  cells. 

While  in  the  saccule,  utricle  and  ampullae  of  the  semicircular  canals, 
the  hair  cells  are  planted  on  a  fixed  base,  their  hair-like  processes  being 
moved  by  otoliths  acting  under  the  influence  of  gravity,  or  by  currents 
set  up  in  the  semicircular  canal,  the  hair  cells  of  the  cochlea  are  planted 

^  Tor  differentiation  of  Organ  of  Corti  see  0.  van  der  Stricht,  Contrib,  to  Embryology, 
1920,  vol.  9,  p.  109. 


THE  ORGAN  OF  HEARING 


237 


on  a  movable  base — tbe  basilar  membrane,  wbich  responds  to  every  move- 
ment of  the  stajDes,  because  of  the  displacement  of  perilymph  in  the  ad- 
joining scalae.  The  tectorial  membrane  bends  the  hair-like  processes 
with  every  movement  of  the  basilar  membrane,  because  the  tectorial 
membrane  is  attached  to  a  fixed  base  on  the  spiral  bony  lamina  while  the 
hair  cells  rest  upon  a  movable  one. 

The  Acoustic  Ganglia. — The  origin  of  the  mass  of  nerve  cells  lying 
between  the  otocyst  and  hind-brain  has  already  been  mentioned  (p.  224). 
It  becomes  divided  into  three  parts  :  (1)  the  geniculate  ganglion  of  the 
facial  nerve,  which  is  included  in  the  petro-mastoid,  but  has  no  functional 
relationship  to  the  labyrinth  ;  it  gives  rise  to  the  great  superficial  petrosal 
nerve,  chorda  tympani  and  pars  intermedia  (root  part  of  ganglion)  in 
the  same  manner  as  a  ganglion  of  the  posterior  root  produces  the  sensory 


Fig.  245. — The  differentiation  of  the  Ganglion  of  the  Labyrintli.  (Streeter.) 
A.  The  otocyst  and  ganghon  of  a  human  embryo  in  the  4th  week ;  A^.  In 
the  5tli  week.  The  parts  are  those  of  the  left  side,  and  are  viewed  on  their 
lateral  aspect.  B.  From  a  foetus  in  the  7th  week  (16  mm.  long) ;  C.  From 
a  foetus  in  the  9th  week  (30  mm.  long). 

1.  Branch  from  ampulla  of  superior  canal.  3.  Branch  from  utricle. 

2.  Branch  from  ampulla  of  lateral  canal.  4.  Branch  from  saccule. 

5.  Branch  from  ampulla  of  posterior  canal. 
A.  Cochlear  ganglion.  A^.  Cochlear  nerve. 

fibres  of  a  spinal  nerve  (Dixon)  (Fig.  93)  ;  (2)  the  vestibular  part — applied 
to  the  vestibular  part  of  the  labyrinth  ;  (3)  the  cochlear  part,  which  be- 
comes applied  to  the  cochlear  canal  (scala  media).  The  differentiation  of 
the  vestibular  and  cochlear  ganglionic  masses  proceeds  at  the  same  rate  as 
the  development  of  the  membranous  labyrinth.^ 

In  Fig.  245  four  stages  in  the  differentiation  of  the  nerve  equijjment 
of  the  ear  are  reproduced.  The  figures  are  those  of  Professor  Streeter  ^ 
and  represent  stages  in  the  first,  second  and  third  months  of  develojDment. 
Towards  the  end  of  the  first  month  the  cochlear  part  becomes  apparent 
{A^)  ;  in  the  second  month  this  part  is  undergoing  rajjid  growth  [B) ; 
early  in  the  third  month  ((7)  it  has  assumed  a  spiral  form,  and  lies  within  the 
spiral  lamina  of  the  cochlea,  and  hence  is  often  named  the  spiral  ganglion. 
The  cells  of  the  spiral  ganglion  send  out  two  sets  of  processes — to  the 

*  Cameron  and  Jlilligan,  Journ.  Anat.  and  Physiol.  1910,  vol.  44,  p.  111. 

*  Amer.  Journ.  Anat.  1907,  vol.  6,  p.  139. 


238 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


organ  of  Corti  (periplieral  fibres),  to  ganglia  situated  in  the  hind-brain 
(root  fibres).  The  cochlear  fibres  form  the  lateral  root  of  the  VIITth 
nerve.  The  vestibular  ganglionic  mass  becomes  partially  subdivided  into 
a  dorsal  mass — connected  with  the  areas  of  sensory  cells  in  the  utricle 
and  the  ampullae  of  the  superior  and  external  semicircular  canals  (Fig. 
245,  1,  2,  3)  ;  the  lower  or  ventral  mass,  which  sends  fibres  to  the  saccule 
and  posterior  semicircular  canal.  The  vestibular  ganglion  is  lodged  in  the 
fundus  of  the  internal  auditory  meatus.  Its  ingrowing  or  centripetal 
fibres  form  the  mesial  root  of  the  Vlllth  nerve.  While  the  cochlear  root 
enters  the  floor  of  the  4th  ventricle  superficial  to  the  inferior  peduncle  of 
the  cerebellum,  the  vestibular  or  mesial  root  passes  deep  to  it.  The  lateral 
or  cochlear  root  is  connected  with  hearing,  the  mesial  or  vestibular  with 
balancing. 

Nerve    Centres.    (1)  Cochlear  or  auditory. — By  the  end  of  the  5th 

week  (Fig.  246)  the  ingrowing  root  fibres  of  the  cochlear  ganglion  have 


PRECENTRAU. 


NTRAPARIETAL 


OLF    BULB 

INSULAR    AREA 


OCCIP;  POLE 


AUDITO- SENSORY   AREA 


Fig.  246. — Section  across  on  half  of  the  Wall  of  the  Hind-Brain  of  an  Embryo  at 
the  end  of  the  first  month.  (His.)  A.  Peripheral  cochlear  ganglion; 
A^.  Central  cochlear  ganglion ;  B.  Peripheral  vestibular  ganglion ;  B^. 
Central  vestibular  ganglion ;  C.  Geniculate  ganglion  of  facial ;  C".  Motor 
nucleus  of  facial  nerve  ;  the  motor  nucleus  of  the  Vlth  cranial  nerve  is  shown 
adjacent  to  that  of  the  Vllth. 

Fig.  247. — Lateral  view  of  the  Cerebrum  of  Foetus  in  the  seventh  month  of  develop- 
ment.   (Retzius.)    The  audito-sensory  area  on  Heschl's  gyri  is  stippled. 

reached  a  central  mass  of  nerve  cells  (central  cochlear  mass)  developed  in 
the  alar  lamina  of  the  hind-brain.  The  central  cochlear  ganglion  gives 
rise  to  the  acoustic  tubercle  (situated  on  the  restiform  body)  and  a  lateral 
accessory  nucleus  on  the  outer  aspect  of  the  restiform  body.  By  means 
of  the  striae  acousticae  and  lateral  fillet  the  cochlear  central  ganglia  are 
united  with  the  superior  olive,  inferior  corpus  quadrigeminum  (mid-brain) 
and  internal  geniculate  body  (thalamencephalon)  of  the  opposite  side. 
Projection  fibres  connect  the  geniculate  body  with  the  cortex  of  the  first 
temporal  gyrus  (see  Fig.  113).  Heschl's  gyri  (audito-sensory)  of  the 
first  temporal  convolution  are  already  apparent  at  the  beginning  of  the 
7th  month  (see  Fig.  247).  The  cortex  of  these  gyri,  with  the  neighbouring 
area  of  the  first  temporal,  receives  the  fibres  from  the  internal  geniculate 
nucleus,  and  forms  the  audito-sensory  areas.     It  is  highly  probable  that  the 


THE  OEGAN  OF  HEAEING  239 

cortex  of  the  greater  part  of  the  temporal  lobe  forms  association  areas,  for 
the  interpretation  of  sounds.  The  auditory  centres  are  necessarily  con- 
nected with  the  centres  for  sight,  movement  and  speech,  but  the  develop- 
ment of  these  connections  is  as  yet  imperfectly  known. 

(2)  The  ingrowing  fibres  of  the  vestibular  ganglion  pass  beneath  the 
inferior  peduncle  of  the  cerebellum  to  terminate  in  the  nerve  cells  of  the 
dorsal  nucleus  and  Deiter's  nucleus  in  the  floor  of  the  4th  ventricle  (Fig. 
246).  These  nerve  cells  and  fibres  are  in  no  sense  auditory,  but  concerned 
with  the  balancing  of  the  body.  Through  the  inferior  peduncle  of  the 
cerebellum,  the  nuclei  in  which  the  vestibular  root  ends  are  connected 
with  both  the  vermis  and  lateral  cerebellar  lobes.  The  cerebellum  and 
acoustic  ganglia  arise  from  the  same  part  of  the  hind-brain  ;  there  is  a 
close  developmental  relationship  between  the  origin  of  the  vestibular  or 
balancing  part  of  the  ear  and  the  cerebellum. 

Internal  Auditory  Meatus. — The  internal  auditory  meatus  is  formed 
round  the  8th  nerve,  its  ganglia,  and  the  7th  nerve.  The  falciform  crest 
separates  the  fibres  of  the  dorsal  and  ventral  parts  of  the  vestibular  nerve. 
The  meatus  also  contains  a  prolongation  of  the  arachnoid  and  subarachnoid 
space.  Fractures  of  the  base  of  the  skull  frequently  cross  the  petro-mastoid 
in  the  line  of  the  internal  auditory  meatus,  vestibule  and  membrana  tym- 
pani.  In  such  cases  the  cerebro-spinal  fluid  and  perilymph  may  escape  by 
the  external  auditory  meatus. 

Summary. — A  study  of  the  development  and  evolution  of  the  human 
ear  leads  to  the  following  conclusions  : 

(1)  That  the  otocyst  was  originally  an  external  sense  organ  connected 

with  the  balancing  of  the  body  ;    it  became  encysted  above  the 
first  visceral  cleft,  and  part  of  it  became  sensitive  to  sound  waves. 

(2)  Parts  of  the  dorsal  laminae  of  the  hind-brain  were  connected  with 

it,  and  from  those  were  developed  the  acoustic  ganglia  and  nuclei, 
and  probably  also  the  cerebellum  (see  page  85). 

(3)  The  first  and  part  of  the  second  clefts  were  modified  in  air-breathing 

forms,  to  become  air  passages  for  transmitting  sounds. 

(4)  Parts  of  the  skeletal  bases  of  the  first  and  second  visceral  arches 

became  the  auditory  ossicles. 


CHAPTEE  XVII. 

PHAEYNX   AND  NECK. 

In  previous  chapters  the  origin  of  various  pharyngeal  structures  has 
been  touched  on.  We  have  seen  that  a  forward  prolongation  of  the  arch- 
enteron  during  the  3rd  week  gives  rise  to  the  fore-gut  (Fig.  18),  that  the 
anterior  or  pharyngeal  part  of  the  fore-gut  is  separated  from  the  primitive 
mouth  or  stomodaeum  by  the  oral  plate  (Fig.  102),  that  the  notochord  is 
laid  down  along  the  dorsal  wall  of  the  pharynx  (Fig.  102)  and  that 
the  heart  lies  under  its  floor,  while  the  aortic  arches  encircle  it  (Fig.  80). 
Mention  has  been  made  of  its  cartilaginous  skeleton  (Fig.  150),  of  the 
segmentation  of  its  mesoderm  (Fig.  M9)  and  of  its  nerves  (Fig.  93).  In 
this  chapter  we  have  to  knit  these  isolated  statements  together  by  following 
the  developmental  changes  which  transform  the  simple  fish-like  pharynx 
of  the  embryo  into  the  complex  of  structures  found  in  the  neck  and 
throat  of  the  adult. 

Evolution  of  the  Pharyngeal  Region.— In  the  latter  part  of  the  first 
month  and  opening  part  of  the  second  the  neck  of  the  human  embryo 
undergoes  a  very  remarkable  transformation.  In  the  5th  week,  when  the 
human  embryo  is  about  5  mm.  in  length,  representations  of  four  gill  clefts 
and  five  gill  or  branchial  arches  are  plainly  to  be  seen  in  the  region  of  the 
neck  or  pharynx  (Fig.  248) ;  the  elevation  caused  by  the  heart  reaches 
forwards  almost  to  the  mandibular  arch  ;  properly  speaking,  there  is  no 
neck  at  the  ith  week  ;  as  in  a  fish,  the  head  is  fixed  directly  to  the  body. 
By  the  beginning  of  the  7th  week  (Fig.  43,  p.  46)  all  traces  of  the  branchial 
arches  have  disappeared  ;  the  head  of  the  embryo  is  now  extended  and 
lifted  away  from  the  thoracic  region,  which  now  contains  the  heart.  Before 
the  branchial  arches  have  begun  to  disappear  in  the  6th  week,  a  pouch  has 
grown  out  from  the  floor  of  the  pharynx  to  form  the  larynx,  trachea, 
bronchi  and  lungs  (Fig.  250).  In  the  passage  from  the  6th  to  the  7th  week 
of  development  we  see  the  human  embryo  evolve  from  a  stage  in  which  the 
parts  are  adapted  for  a  branchial  respiration,  as  in  fishes,  to  a  higher  one 
in  which  its  parts  are  fitted  for  breathing  air.  Pharyngeal  glands,  such 
as  the  tonsil,  thyroid  and  thymus,  originally  developed  in  connection  with 
the  visceral  or  gill  arches,  become  modified  in  structure  and  position  to  suit 
the  new  conditions  of  life.  With  the  evolution  of  the  mammalian  method 
of  mastication  and  swallowing,  the  pharynx,  originally  a  respiratory 
structure,  was  further  modified.  The  tongue  became  difierentiated  from 
parts  in  the   floor  of   the   pharynx,  and   muscles,  which   were   at  first 

240 


PHARYNX  AND  NECK 


241 


designed  to  move  the  branchial  arches,  became  converted  into  muscles 
of  deglutition. 

Pharynx   of  the  Embryo. — There  is  very  little  resemblance  between 
the  pharynx  and  neck  of  a  human  embryo  in  the  third  week  and  that  of  the 


cerebral  uesicid 

olfact  pit 
max.  proc. 

stomodaeum 
artery  of  1st  (mandib)  arch 
artery  of  2nd  arch 
rtery  of  3rd  arch 

bulbus  arteriosus 
dorsal  aorta 
ventricle 


sinus  uenosus 


Fig.  248. — Showing  the  Visceral  Arches  and  Cleft-depressions  in  the  Pharyngeal 
Wall  of  a  Human  Embryo  at  the  beginning  of  the  5th  week.  Each  Visceral 
Arch  contains  an  Aortic  Arch.     (After  His.) 

adult.  Indeed,  in  the  4th  week  the  human  pharynx  resembles  closely  that 
of  a  fish  (Figs.  248,  249).  In  both  the  human  embryo  and  fish  the  pharynx 
is  bounded  by  visceral  or  branchial  arches,  which  are  separated  by  depres- 


nasal  pit 


Fig.  249.- 


artery  of  1st  arch 
artery  of  2nd  arch 
gill  cleft 
ventral  aorta 
gill  cleft 
artery  of  4th  arch 

ventral  aorta 
bulbus  arteriosus 

ventricle 

auricle 


-Showing  the  position  of  the  Heart,  Visceral  and  Aortic  Arches  in  a  Fish. 
(Diagrammatic — after  Gegenbaur.) 


sions  (human  embryos)  or  clefts  (fishes)  ;  in  both  the  heart  is  situated  under 
the  pharynx,  and  from  the  ventral  aorta,  aortic  arches  pass  up  on  each  side, 
one  in  each  visceral  arch,  to  terminate  in  tlie  dorsal  aortae.     In  fishes  the 

Q 


242 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


aortic  arches  give  off  vessels  to  the  gills,  in  which  the  blood  is  arterialized. 
In  the  human  embryo  the  blood  passes  directly  through  the  aortic 
arches.  The  walls  of  the  pharynx  were,  therefore,  primarily  respiratory 
in  function. 

A  considerable  part  of  the  human  neck  lying  in  front  of  the  vertebral 
column  and  between  the  mouth  above  and  the  thorax  and  clavicles  below, 
with  the  bounding  walls  of  the  adult  pharynx,  is  formed  from  the 
embryonic  visceral  arches.  A  knowledge  of  the  transformation  of  the 
embryonic  to  the  adult  pharynx  is  of  the  utmost  practical  importance  : 
it    explains    the   occurrence   of  fistulae   and   cysts   found   in   the   neck ; 


dorsum  sellae 


prim,  pharynx 
2nd  deft  recess 
3rd  cleft  recess 


max.  proc. 
^position  of  oral  plate 
stomodeum 


Fig.  250. — Showing  the  Primitive  Pharynx  of  a  5th  week  Embryo  in  Sagittal 
Section,  bounded  by  tlie  Visceral  Arches.    (After  His.) 


it  accounts  for  the  peculiar  courses  taken  by  nerves,  such  as  the  recurrent 
laryngeal  and  phrenic  ;  it  explains  the  peculiar  distribution  of  nerves  to 
the  pharynx  ;  and  throws  light  on  the  nature  and  anomalies  of  the 
thymus,  thyroid  and  tonsil.  As  may  be  seen  from  Fig.  248,  the  floor  of 
the  pharynx  of  the  human  embryo  rests  on  the  dorsal  wall  of  the 
pericardium ;  in  the  adult  the  pharynx  and  pericardium  are  separated  by 
the  whole  length  of  the  neck. 

Visceral  Arches. — The  visceral  arches  bound  and  form  the  whole  thick- 
ness of  the  wall  of  the  primitive  pharynx,  which  is  flattened  dorso-ventrally, 
so  that  its  cavity  forms  a  transverse  cleft  when  seen  in  cross-sections  of  the 
embryo.  Four  arches,  each  bounded  behind  by  a  depression,  are  to  be 
recognized  sujDerficially  on  each  side  of  the  pharynx  of  the  5th  week  human 


PHARYNX  AND  NECK 


243 


embryo  (Fig.  248),  but  behind  the  4tb.  cleft  are  fifth  and  sixth  arches  which, 
however,  never  become  raised  or  superficially  differentiated  from  the  body 
wall  behind  (Fig.  251).  Sagittal  and  coronal  sections  of  the  primitive 
pharynx  (Figs.  250  and  251)  give  a  better  idea  of  the  arrangement  and 
constitution  of  the  visceral  arches  than  can  be  had  from  a  surface  view. 
They  are  developed  round  the  most  anterior  part  of  the  fore-gut,  which 
forms  the  lining  membrane  of  the  primitive  pharynx.  The  pharyngeal 
lining  membrane,  therefore,  is  the  same  as  that  of  the  alimentary  canal  from 
which  spring  all  the  organs  and  glands  of  digestion  and  assimilation. 

Visceral  Clefts. — The  epithelium  or  entoderm,  which  lines  the  primitive 
pharynx,  covers  the  inner  aspects  of  the  arches  and  passes  outwards  in  the 
recesses  between  them  and  there,  for  a  short  time,  comes  in  contact  with 
the  epithelial  covering  of  the  body  (ectoderm)  which  dips  in  to  meet  it 


TONGUE  /Mandib  part) 


HYOID  ARCH 


3KD  —  <^TH 

BR.  ARCHES 


Z"" POUCH 
cIeft  MEMB. 

rURCULA 
3'*°  POUCH 
'f-TH-^TH  POUCHES 


PULM.QROOVe. 
COELOM 


Fig.  251. — Showing  the  Floor  of  the  Pharynx  of  a  5th  week  Human  Embryo.    (After  His.) 


(Fig.  251).  The  membrane  thus  formed  by  the  union  of  the  ectoderm 
and  entoderm  in  the  recesses  between  the  arches,  may  be  named  the  "  cleft 
membrane."  It  is  never  ruptured  nor  disappears  in  the  development  of 
mammals,  but  is  invaded  by  the  mesoderm  of  neighbouring  arches ;  in 
fishes  it  disappears  and  real  clefts  are  formed  between  the  arches.  On  the 
outer  side  of  the  membrane  is  the  "  cleft  depression,"  on  its  inner  side  a 
"  pharyngeal  recess,"  jDresently  developed  into  a  p)ouch.  From  the  ento- 
dermal  Iming  of  the  pharyngeal  pouches  we  shall  see  that  the  tonsil,  thyroid 
and  thymus  arise  :  from  the  external  depressions  are  formed  the  various 
branchial  cysts  and  fistulae,  which  occasionally  occur  in  the  neck  of  the 
adult.  Further,  at  the  upper  end  of  each  cleft  depression  there  develop 
remarkable  sense-organs,  known  as  the  epibranchial  placodes.  In  each 
arch  there  develop  exactly  the  same  elements  as  are  to  be  seen  in  the  gill 
arches  of  fishes,  namely  : 

(a)  A  skeletal  basis  of  cartilage  ;  (b)  an  aortic  or  vascular  arcii ;  (c)  a 
larger  nerve  along  its  anterior  border  and  a  smaller  along  its  posterior  ; 
(d)  a  muscle  element. 


244 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


In  Fig.  149  (p.  155)  a  schematic  transverse  section  of  a  vertebrate  embryo 
lias  already  been  given  to  show  the  relationship  of  the  tissues  of  a  branchial 
arch  or  branchiomere  to  the  segments  of  the  head. 

The  first  visceral  arch  is  known  as  the  mandibular,  the  second  as  the 
hyoid  (Fig.  251).  The  remaining  four  are  branchial  arches,  having  been 
at  one  stage  of  evolution  devoted  solely  to  the  purpose  of  carrying 
gills.  The  hyoid  arch  is  specialized  in  fishes,  to  protect  the  branchial 
arches,  and  assist  in  the  circulation  of  blood  through  the  gills  and 
water  through  the  pharynx.  The  mandibular  arch  bounds  part  of  the 
buccal  cavity  in  all  vertebrates,  and  forms  part  of  the  apparatus  of 
mastication. 

Formation  of  the  Cervical  Sinus.— The  first  arch  especially,  and  also 
the  second,  grow  and  increase  at  a  much  greater  rate  than  the  branchial 


ECTODERM 


TUBO- 

TYMPA 

isr  POCKET 


TONSIL 
Z'^o  POCHCT 


CERVICAL     ^ 

SINUS         ^ 


CERV  VESICLE 


TUBO-TYMP. 
FtECESS 


PARATHYR.TU 


THYMUS  M 
THYROID 


'■rPYRirORM 
ROSSA 


PARATHYR.TV 

ultimo-branch(thyroid) 


CESOPH 


Fig.  252. — The  Lining  Membrane  of  tlie  Pliarynx  of  a  Human  Embryo  at  tlie  end  of 
the  6th  week  of  development  (10  mm.)  viewed  on  its  ventral  aspect.    (Grosser.) 

arches.  The  second  arch  (hyoid)  which  in  fishes  forms  the  operculum 
for  the  gills,  grows  over  and  buries  the  third  and  fourth  in  the  human 
embryo.  Already  at  the  end  of  the  5th  week  (Fig.  251)  there  is  clear 
evidence  of  the  sinking  in  of  the  hinder  arches,  and  it  is  easy  to  see  that 
as  the  hyoid  arch  grows  backwards  over  them,  an  ectodermal  space  will 
become  covered  over  and  form  the  cervical  sinus — representing  the  gill 
cavity  of  fishes.  Its  formation  is  effected  in  the  6th  week.  In  Fig.  252 
a  model  of  the  lining  membrane  of  an  embryo  at  the  end  of  the  6th  week 
of  development  is  depicted  as  seen  on  its  ventral  aspect.  The  enclosed 
pocket  of  ectoderm — the  cervical  sinus — is  shown  to  be  connected  with  the 
3rd  pouch  by  a  vesicular  prolongation — the  cervical  vesicle,  also  by  exten- 
sions to  the  2nd  or  tonsillar  pouch  and  to  the  4th.  The  last  named  con- 
nection is  short  lived  ;  indeed,  before  the  end  of  the  2nd  month  all  traces 
of  the  sinus  should  have  disappeared. 


PHARYNX  AND  NECK 


245 


Although  the  cervical  sinus  usually  disappears,  it  may  remain  and  form 
a  cyst  in  the  neck,  which  opens  on  the  anterior  border  of  the  sterno-mastoid 
a  short  distance  above  the  sterno-clavicular  joint.  It  may  be  drawn  out 
into  a  trumpet-shaped  tube,  which  ends  in  contact  with  the  tonsillar  recess, 
passing  between  the  internal  and  external  carotid  arteries  or  in  contact 
with  the  pharynx  behind  the  hyoid  (Fig.  253),  connections  which  are 
explained  by  the  origin  of  the  sinus  (Fig.  252).  Often  the  cutaneous 
orifice  is  marked  by  a  tag  of  skin  representing  a  rudimentary  external  ear, 
which  encloses  a  piece  of  cartilage.^  If  the  outer  cleft  depression  in  front 
of  or  behind  the  third  arch  persist,  it  must  open  in  the  cervical  sinus. 


TONSIL 

Z'^i    REC 
CAROT; BODY 
THYRGLOS.  DUCT 

CERV:  SINUS 


STALK    of 
THYMUS 


ORIFICE  of 
CERV:  SINUS 


INT:  CAROT; 
EXT: CAROT 

CAROT; BODY 

3'A  RECESS 

MED    THYROID 


STALK  of  LAT! 
THYROID.  4?*?  REC: 


CERV:  SIN: 


-S  V-'--    -  STER  NUM 


THYMUS 


Fig.  253. — Diagram  to  illustrate  the  various  parts  of  the  Visceral  Clefts  which  may 
persist.  The  2n(i  inner  cleft  recess  gives  rise  to  the  tonsil ;  the  recess  may  be 
in  contact  with  an  epithelial  tube  derived  from  the  cervical  sinus.  The  3rd 
inner  recess  gives  rise  to  the  thymus  and  carotid  bodies. 

What  becomes  of  the  Visceral  Clefts.— By  the  end  of  the  second 
month  the  clefts,  or,  to  be  more  exact,  the  representatives  of  clefts  in  the 
human  embryo,  have  disappeared,  except  the  upper  part  of  the  first. 
From  the  external  depression  of  this  part  a  soKd  ingrowth  of  epithelium 
takes  place  which,  ultimately  becoming  canaliculized,  forms  the  external 
auditory  meatus  (Fig.  230).  In  connection  with  the  upper  or  dorsal  parts 
of  the  first  and  second  cleft  depressions  the  Eustachian  tube  and  tympanum 

^  For  an  account  of  the  various  developmental  anomalies  of  the  pharjTigeal  region 
see  Keith,  Brit.  Med.  Journ.  1909,  Aug.  7th,  14th,  21st.  For  a  description  of  its 
development  see  J.  Ernest  Frazer,  Journ.  Anat.  and  Physiol.  1910,  vol.  44,  p.  156  ; 
H.  Fox,  Amer.  Jo^irn.  Anat.  1908,  vol.  8,  p.  187  ;  B.  Kingsbury,  Amer,  .Journ.  A7iat. 
1915,  vol,  18,  p.  329. 


246 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


are  formed,  tlie  membrana  tympani  remaining  approximately  in  the  position 
of  a  cleft  membrane  (p.  232). 

If  traces  of  the  other  clefts  remain  as  fistulae  or  cysts  they  will  occur  in 
the  positions  shown  in  Fig.  254.  Part  of  the  second  cleft  is  marked  in  the 
goat  by  an  opemng  and  auricular  appendage.  As  already  pointed  out, 
superficial  remnants  of  the  second  and  third  clefts  are  rare  ;  they  are 
usually  included  with  the  cervical  sinus  beneath  the  hyoid  operculum. 

Within  the  pharynx  traces  of  inner  cleft  recesses  are  to  be  seen  besides 
the  Eustachian  opening  (see  Figs.  253,  266).  The  tonsil  is  developed  in 
the  second  cleft ;    the  anterior  pillar  of  the  fauces  represents  only  the 


Fig.  254.- 


mandible  (1st  arch) 

yoid  (2nd  &  3rd  arches) 
sterno-mastoid 

ceruical  sinus 
3rd  &  4th  clefts 


-Showing  the  position  of  the  External  Cleft  Depressions  in  the  Adult. 
(For  explanation,  see  text.) 


position  of  the  second  arch.  The  lateral  recess  of  the  pharynx  (fossa  of 
Rosenmiiller),  behind  the  Eustachian  tube,  although  sometimes  regarded 
as  a  derivative  of  the  second  cleft,  is,  as  we  have  seen  (p.  228),  a  secondary 
formation.  The  pyriform  fossa,  at  each  side  of  the  laryngeal  aperture, 
represents  the  position  of  the  fourth  and  fifth  clefts  (see  Fig.  253). 

The  Cartilages  of  the  Arches  (Fig.  255).— The  history  of  the  skeletal 
basis  of  the  first  arch  (Meckel's  cartilage)  has  been  already  traced  (p.  175). 
The  cartilage  of  the  2nd  or  hyoid  arch  forms  (Fig.  255)  : 
(1)  The  tympano-hyal,  embedded  in  the  petro-mastoid,  and  originally 
continuous  with  the  ear  ossicles  (Fig.  237).  (2)  The  stylo-hyal,  which 
ossifies  in  the  early  years  of  life  and  becomes  joined  to  the  tympano-hyal 
to  form  the  styloid  process.     (3)  The  segment  below,  the  epi-hyal,  becomes 


PHARYNX  AND  NECK 


247 


ligamentous,  and  forms  the  stylo-hyoid  ligament,  but  it  also  may  become 
ossified.  (4)  The  lowest  segment,  the  cerato-hyal,  forming  the  small 
horn  of  the  hyoid.  The  epi-hyal  lies  behind  and  outside  the  tonsil,  and 
when  ossified  has  been  excised  under  the  belief  that  it  was  a  foreign 
body.  The  body  of  the  hyoid  (basi-hyal)  represents  the  fused  ventral 
parts  (copulae)  of  the  2nd  and  3rd  cartilages  ;  in  the  floor  of  the  embryonic 
pharynx  (Fig.  251)  the  ventral  ends  of  the  2nd  and  3rd  arches  end  in  a 
common  or  mesobranchial  field.  In  this  area  the  body  of  the  hyoid  develops. 
Prof.  Parsons  ^  has  drawn  attention  to  the  fact  that  there  is  a  ridge  of 
bone  on  the  upper  surface  of  the  body  of  the  hyoid,  which  may  occasionally 
form  an  almost  separate  bar.  It  lies  between  the  lesser  horns,  and  appears 
to  represent  the  copula  or  body  of  the  2nd  arch.     It  may  be  separated 

malleus 


incus 


tympanic 
tympano-hyai 


stylo-hyal. 


1st  arch— Meckel's  cart. 


epi-hyal 
cerato-hyal 
thyro-hyal. 


thyroid 


2nd  arch— stylo-hyoid 
^3rd  arch -thy ro-hyoid 


4th  arch 
5th  arch 


Fig.  255. — Showing  what  becomes  of  the  Cartilages  of  the  Visceral  Arches. 

from  the  body  of  the  hyoid  by  a  foramen  evidently  for  the  passage  of  a 
remnant  of  the  thyro-glossal  duct.  It  will  be  seen  later  that  the  basal 
or  pharyngeal  part  of  the  tongue  arises  from  the  floor  of  the  pharynx  in 
the  field  between  the  2Dd  and  3rd  arches.  The  skeletal  bases  of  their 
ventral  parts  come  to  form  the  bone  of  the  tongue.  The  skeletal  part  of 
the  hyoid  arch  suspends  the  tongue.  There  may  be  a  process  of  bone  from 
the  concavity  of  the  body  representing  the  hyolingual  of  lower  vertebrates 
(Parsons). 

The  great  horn  of  the  hyoid  represents  the  cartilage  of  the  3rd  arch 
(Fig.  255).  In  the  lowest  mammals  the  cartilaginous  bases  of  the  4th  and 
5th  arches  unite  to  form  the  thyroid  cartilage,  but  in  higher  mammals, 
including  man,  this  cartilage  is  made  up  entirely  by  the  4th  arch.^  The 
cartilages  of  the  ultimate  arches  (5th  and  6th)  are  probably  represented  by 
the  cricoid,  arytenoid  and  rings  of  the  trachea  (see  also  Fig.  375,  p.  352). 

1  F.  G.  Parsons,  Jouni.  Anat.  and  Physiol.  1909,  vol.  43,  p.  279. 

2  F.  H.  Edgeworth,  Quart,  Journ.  Mic.  Sc.  1916,  vol.  61,  p.  383. 


248 


HUMAN  EMBRYOLOaY  AND  MORPHOLOGY 


Even  in  mammals  the  cartilages  of  the  three  last  branchial  arches  remain 
subservient  to  the  purposes  of  respiration,  just  as  in  vertebrate  animals 
in  which  these  arches  carry  gills. 

Nerves  of  the  Visceral  Arches  (see  Figs.  256,  257,  93). — The  3rd  division 

of  the  5th  nerve  is,  as  has  been  already  seen,  the  principal  nerve  of  the 
first  or  mandibular  arch.  The  nerve  for  the  second  or  hyoid  arch  is  repre- 
sented by  the  6th  or  facial.  The  nerve  of  the  3rd  arch  is  the  glosso- 
pharyngeal, that  for  the  4th  is  the  superior  laryngeal  branch  of  the  vagus, 
and  for  the  5th  and  6th  the  inferior  laryngeal  (Fig.  257). 

Each  nerve  of  a  visceral  arch  supplies  (1)  the  muscles  of  the  arch,  (2) 
the  pharyngeal  lining  and  cleft  recess  in  front  of  the  arch.     The  chorda 


Fig.  256. — The  Visceral  Arches  and  their  Nerves  and  Ganglia  in  a  Human  Embryo  of 
the  5th  week.     (Professor  Streeter.) 

tympani  and  great  superficial  petrosal  nerves  represent  the  sensory  branches 
of  the  facial  to  the  first  cleft. 

The  relationship  of  the  nerves  to  the  visceral  arches  is  shown  in  Fig. 
256,  in  a  human  embryo  of  five  weeks.  The  position  of  these  nerves  in 
the  adult  is  diagrammatically  represented  in  Fig.  257.  The  Vth  nerve 
and  Gasserian  ganglion  are  seen  to  lie  at  the  base  of  the  mandibular  process. 
The  ganglion  of  the  Vllth  and  Vlllth  nerves  lies  at  the  base  of  the  hyoid 
(second)  arch,  in  front  of  the  otic  vesicle,  the  fibres  of  the  facial  having 
already  entered  the  arch.  The  glosso-pharyngeal  and  its  ganglia  lie  behind 
the  otic  vesicle  and  at  the  base  of  fche  third  arch.  The  large  ganglionic 
mass  of  the  vagus  lies  over  the  bases  of  the  fourth,  fifth  and  sixth  arches 
— or  rather  the  tissue  representing  these  arches.  At  this  stage — the  5th 
week — ^the  ganglion  of  the  vagus  and  its  issuing  fibres  rest  on  the  dorsal 
wall  of  the  pericardium,  the  heart  being  quite  close  to  the  source  of  its 
nerve  fibres. 

Epibranchial  Placodes.— When  the  ganglia  of  the  Vllth,  IXth  and 
Xth  nerves  begin  to  difierentiate  in  the  5th  week,  they  are  in  contact  with 


PHARYNX  AND  NECK 


249 


the  upper  ends  of  their  respective  gill  depressions — the  1st,  2nd  and  3rd. 
An  area  of  ectoderm  at  the  upper  end  of  each  cleft  depression  becomes 
modified  to  form  an  epibranchial  placode  representing  sense  organs  which 
are  now  lost  in  higher  vertebrates.  During  the  5th  week  these  placodes 
are  in  contact  with  the  ganglia  just  mentioned  and  the  ganglion  of  the 
trunk  of  the  vagus  (ganglion  nodosum)  and  of  the  trunk  of  the  glosso- 
pharyngeal (ganglion  petrosum)  receive  additions  from  cells  which  are 
produced  in,  and  migrate  from,  the  placodes. 


0  nasal  processes 


to  max.  proc. 

chorda  tympani 

r-2nd  arch 
3rd  arch 
4th  arch  (sup.  laryngeal) 

5th  arch  (recurrent  laryngeal) 


Fig.  257. — Showing  what  becomes  of  the  Nerves  of  the  Visceral  Arches. 

Aortic  Arches — the  Arteries  of  the  Visceral  Arches.— In  Fig.  248 
is  given  the  foetal  arrangement  of  the  aortic  arches,  and  in  Fig.  258  the 
vessels  in  the  adult  which  are  formed  from  them.  The  primitive  aorta 
in  the  embryo  divides  into  two  trunks,  which  run  forwards  along  the  floor 
of  the  pharynx,  one  on  each  side,  lying  between  the  ventral  ends  of  the 
visceral  arches.  These  may  be  termed  the  right  and  left  ventral  aortic 
stems.  From  these  stems  arteries  (aortic  arches)  pass  upwards,  one  in 
each  visceral  arch,  to  terminate  in  the  right  and  left  dorsal  aortae,  which 
run  backwards  and  become  fused  to  form  the  descending  thoracic  aorta. 
The  aortic  arches  are  formed  at  a  very  early  date.  At  the  beginning  of 
the  4th  week  the  first  or  mandibular  aortic  arch  is  already  present ;  the 
second  (liyoid),  third,  fourth,  fifth  and  sixth  appear  in  succession,  but  by 
the  6th  week,  when  the  6th  or  pulmonary  arch-^  has  appeared,  the  first 

^  For  literature  on  6th  arch  and  5th  cleft  recess  see  Frank  Reagan,  Amer.  Journ, 
Amt.  1911,  vol.  12,  p.  493  ;   J.  Tandler,  Aiiat,  Hefte,  1909,  vol,  38,  p.  393. 


250 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


and  second  are  in  a  process  of  atrophy.  Only  for  a  brief  period  towards 
the  end  of  the  5th  week,  when  the  embryo  is  about  5  mm.  long,  are 
all  the  arches  open,  and  even  then  the  1st  is  atrophic  while  the  6th  or 
pulmonary  is  developing.  The  5th  arch  has  only  a  transient  existence. 
The  aortic  arches  are  formed  by  the  union  of  a  network  of  blood  spaces 
which  are  developed  within  each  visceral  arch. 

The  1st  and  2nd  aortic  arches  disappear  ;    the  3rd  remains  as  the  first 
part  of  the  internal  carotid,  the  4th  forms  the  1st  and  2nd  stages  of  the 


int.  carotid 
1st  arch 
2nd  arcli 
3rd  arch 


right  dorsal 
aorta     y 

4th  arch 


sup.  infercosi. 
right  dorsal  aorta- 


lingual  from  2nd 

■uperior  thyroid 

(from  3rd) 

^uentral  aorta 
(right  trunk) 

ventral  aorta 
(left  trunk) 

left  subclau. 
th  arch,  left 
th  arch,  left 

left  dorsal  aorta 


Fig.  258. — Showing  what  becomes  of  the  Aortic  Arches  in  the  Adult.  Only  the 
shaded  parts  persist.  The  position  of  the  1st  and  2nd  aortic  arches  should 
be  indicated  above  and  below  the  position  of  the  external  auditory  meatus. 

right  subclavian.  On  the  left  side  the  4th  aortic  arch  forms  that  part  of 
the  arch  of  the  aorta  between  the  origin  of  the  left  carotid  and  entrance  of 
the  ductus  arteriosus  (Fig.  258).  The  right  and  left  5th  arch,  or,  to  be 
more  accurate,  the  6th,  for  a  transient  arch  appears  between  it  and  the 
4th,  give  off  vessels  to  the  lungs  which  are  developed  in  close  connection 
with  these  arches.  This  arch  on  the  left  side  persists  as  part  of  the  right 
pulmonary  artery  and  ductus  arteriosus  (Fig.  260).     On  the  right  side 


PHARYNX  AND  NECK 


251 


the  dorsal  part  disappears,  the  remaining  segment  joining  in  the  formation 
of  the  right  puhiionary  artery.  When  it  is  remembered  that  the  6th  or 
puhiionary  arch  lies  at  the  level  of  the  larynx  in  the  5th  week,  and  that, 
owing  to  the  development  of  the  neck,  it  has  almost  reached  its  final 
position  in  the  7th  week,  the  rapid  transformation  of  the  parts  in  the 
region  of  the  pharynx  in  the  second  month  will  be  realized.  It  is  in  this 
period  that  the  hinder  gill  arches  are  buried  and  the  cervical  sinus  formed 
and  obliterated. 

Subclavian  Arteries.— The  visceral  arches  with  their  arteries  are  well 
developed  before  the  limb  buds  appear.  When,  at  the  end  of  the  4th 
week,  these  buds  grow  out  to  form  the  upper  extremities,  the  artery  which 
supplies  each  bud  springs  from  the  dorsal  aorta  and  represents  a  dorsal 
segmental  branch  of  that  vessel.     The  embryonic  or  primitive  subclavian 

inno'jrfinate 
carotid 


vert.—. 

subclau. 

constriction 
right  puL  art 


5th  right  arch 
right  dorsal  aorta 


uert. 
ubciau. 
onstriction 
5th  arch,  left  sidb 

///I/    left  pulmonary  art 


'eft  dorsal  aorta 


Fig.  259.— The  condition  of  the  Right  and  Left  Dorsal  Aortae  in  a  7th  week  Human 
Foetus.  (After  His.)  The  right  arch  and  right  dorsal  aorta  disappear  beyond 
the  origin  of  the  right  subclavian ;  a  constriction  may  appear  at  the  corre- 
sponding point  on  the  left  side. 

is  the  artery  of  the  7th  cervical  segment,  being  situated  at  a  considerable 
distance  behind  the  6th  aortic  arch.  As  the  aortic  arch-system  is  elongated 
to  form  the  great  vessels  of  the  neck  during  the  6th  and  7th  weeks,  the 
origin  of  the  subclavian  comes  to  lie  opposite  the  dth  arch  (Fig.  259). 
This  artery  forms  the  entire  subclavian  on  the  left  side,  but  only  that  part 
beyond  the  origin  of  the  vertebral  on  the  right  side  (Fig.  259). 

Aortic  Arch  on  the  Right  Side.— In  birds  it  is  the  4th  right  arch 
which  forms  the  aortic  arch,  and  this  occasionally  happens  in  man.  In 
amphibians  both  the  right  and  left  4th  arches  persist.  The  two  dorsal 
aortae  in  which  they  end,  unite  together,  as  they  do  in  the  human  embryo, 
to  form  the  descending  thoracic  aorta.  The  primitive  subclavian  arteries 
spring  from  the  dorsal  aortae  above  the  point  where  these  two  vessels  fuse 
together.  In  the  latter  part  of  the  second  month  the  short  part  of  the 
right  dorsal  aorta,  between  the  origin  of  the  right  siibclavian  artery  and 
point  of  aortic  fusion,  disappears,  and  then  the  subclavian  artery  appears 
as  if  it  arose  from  the  4th  right  arch  (Fig.  259).     The  communicating 


252 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


arterial  twig,  which  is  often  seen  uniting  the  superior  intercostal  artery  of 
the  right  side  with  the  artery  of  the  lower  spaces,  is  formed  by  a  secondary 
anastomoses,  and  does  not  represent  the  right  dorsal  aorta. ^ 

Not  unfrequently  the  right  subclavian  arises,  not  from  the  innominate, 
which  represents  the  right  ventral  aortic  stem,  but  as  the  last  of  the  great 
branches  which  arise  from  the  arch  of  the  aorta  (Fig.  260).  In  such  cases 
two  things  have  happened  :  (1)  the  4th  right  aortic  arch  has  been  .obliter- 
ated, (2)  the  right  dorsal  aorta  has  persisted. 

Cases  occur  in  which  the  permanent  aorta  is  very  much  constricted 
at  or  near  the  point  of  entrance  of  the  ductus  arteriosus  (see  Fig.  259). 
It  will  be  noticed  that  the  corresponding  part  of  the  right  dorsal  aorta 
is  obliterated.  Such  a  constriction  on  the  left  side  is  to  be  regarded  as 
corresponding  to  that  on  the  right  side,  and  indicates  an  attempt  to  produce 
a  right  aortic  arch. 

Dorsal  Aortae. — It  will  be  noticed  that  the  parts  of  the  dorsal  aortae 
between  the  3rd  and  ith  arches  disappear  (Fig.  260).  The  ventral  aortae 
persist  as  the  innominate,  the  common  carotid  and  external  carotid  arteries. 
With  the  marked  elongation  of  the  cervical  region  and  the  development  of 
the  lungs  in  the  second  month,  the  primitive  position  of  the  aortic  arches 
is  greatly  disturbed.  The  heart,  being  the  pump  of  the  lungs,  must  accom- 
pany these  organs.     The  ventral  aortae  become  elongated  into  the  common 


ext  carotid 


4th  arch 

right  subclav, 

right  aorta 


■int.  carotid 

4th  arch 

vert,  art 

left  subclau. 

— duct  art 

left  aorta 

pulm.  art. 


Fig.  260. — Diagram  showing  tiie  manner  in  which  tlie  Right  Subclavian  may  arise 
as  the  last  branch  of  the  Arch  of  the  Aorta.  The  parts  of  the  aortic  arch  system 
which  become  obliterated  are  stippled. 

carotid  and  innominate  arteries  (Figs.  258,  260).  The  4th  aortic  arch, 
which  should  lie  opposite  the  upper  part  of  the  thyroid  cartilage,  comes 
to  rest  at  the  level  of  the  1st  rib  on  the  right  side  and  within  the  thorax 
on  the  left,  while  the  last  aortic  arch  dragging  the  nerve  of  its  segment 
in  front  of  it  (the  recurrent  laryngeal)  comes  to  be  situated  within  the 
thorax. 


1  See  E.  Pearce  Gould,  Journ.  Anat.  and  Physiol,  1909,  vol.  43,  p.  329. 


PHARYNX  AND  NECK 


253 


Muscles  of  the  Visceral  Arches.^— Within  each  visceral  arch  a  muscle 
plate  is  formed — recalling  in  mode  of  appearance  the  muscle  plates  which 
develop  in  connection  with  each  vertebral  somite.  The  muscles  arising 
in  each  arch  are  supplied  by  the  nerve  of  that  arch  ;  hence  from  the  nerve 
supply  alone  one  could  infer  the  derivation  of  the  musculature  of  the 
pharyngeal  region.  The  muscles  become  differentiated  in  the  latter  part 
of  the  second  month.  All  the  muscles  supplied  by  the  facial  nerve — the 
platysma,  muscles  of  expression,  the  stapedius,  stylo-hyoid,  posterior  belly 
of  the  digastric,  etc. — are  derived  from  the  muscle  plate  of  the  2nd  or 
hyoid  arch.  The  muscles  of  mastication,  with  the  tensors  of  the  palate 
and  tympanum,  the  anterior  belly  of  the  digastric  and  mylohyoid,  are 
derived  from  the  muscle  segment  of  the  mandibular  arch.  The  stylo - 
pharyngeus  is  derived  from  the  3rd  arch.  The  musculature  of  the  soft 
palate  and  the  constrictors  of  the  pharynx  are  derived  from  the  third  and 
fourth  arches.  The  musculature  of  the  larynx  comes  from  the  fifth  and 
sixth  arches. 


PLATYSMA 


OCC I  PITO- AURICULAR 
GROUP 


Fig.  261. — The  expansion  of  the  Platysma  Sheet  in  a  Human  Foetus  of  7  weelvs. 

(Futamura.) 

The  Platysma  and  Muscles  o£  the  Face  and  Scalp.^ — The  platysma 
myoides,  the  muscles  of  the  face,  scalp  and  external  ear,  are  derived  from 
the  muscle  plate  of  the  second  or  hyoid  arch.  They  are  supplied  by  the 
facial,  the  nerve  of  this  arch.  The  muscle  bud,  from  which  the  whole 
platysma  sheet  is  developed,  is  still  confined  to  the  area  of  the  hyoid  arch 
until  the  7th  week  of  development  when  the  bud  spreads  out  and  forms  a 
continuous  muscular  hood  over  the  head  and  neck.     To  this  hood  or  sheet, 

IF.  H.  Edgeworth,  Journ.  Anat.  1920,  vol.  54,  pp.  79,  124. 

2  R.  Futamura,  Anat.  Hefte,  1907,  vol.  32,  p.  479  ;  1906,  vol.  30,  p.  433. 


254     HUMAN  EMBRYOLOaY  AND  MORPHOLOGY 

whicli  is  composed  of  two  layers,  a  deep  and  superficial,  the  name  of 
platysma  sheet  may  be  given.  It  is  developed  in  the  superficial  fascia. 
During  its  expansion  or  migration  the  platysma  sheet  separates  into  three 
main  divisions — a  part  for  the  neck — platysma  colli ;  for  the  ear  and  occiput 
—the  occipito-auricular  ;  and  the  facial  division — for  mouth,  nose,  orbits 
and  forehead  (Fig.  261).  The  muscles  become  differentiated  during  the 
3rd  month. 

In  man,  the  platysma  sheet  has  undergone  marked  retrograde  changes 
in  the  neck,  scalp  and  external  ear,  but  over  the  face  it  has  become  more 
highly  specialized  and  differentiated  than  in  any  other  animal.  From  this 
sheet  are  derived  the  epicranial  aponeurosis,  the  occipitalis  and  frontalis 
muscles.  On  the  face  the  platysma  sheet  forms  the  muscles  round  the 
orbit,  nose  and  mouth.  The  buccinator  and  levator  anguli  oris  represent 
parts  of  the  deeper  layer  of  the  sheet.  The  transversus  nuchae,  fibres 
occasionally  seen  in  man  passing  from  the  middle  line  of  the  neck  behind, 
towards  the  ear  and  cheek,  represent  fibres  constantly  developed  in  lower 
primates,  and  better  still  in  rodents  and  carnivora  as  the  sphincter  colli 
and  sterno-facialis. 

The  muscles  supplied  by  the  facial  nerve  are  peculiar  in  that  they  are 
the  physical  basis  into  which  many  mental  states  are  reflected  and  in  which 
they  are  realized.  Through  them  mental  conditions  are  manifested.  It 
is  found  that  the  differentiation  of  this  sheet  into  well-marked  and  separate 
muscles  proceeds  pari  passu  with  the  development  of  the  brain.  The 
more  highly  convoluted  the  brain  of  any  primate,  the  more  highly  specialized 
are  its  facial  muscles.  It  is  remarkable  that  the  sheet  should  arise  from 
a  visceral  arch,  which  originally  was  closely  connected  with  the  function 
of  respiration.  To  some  extent  the  platysma  does  come  into  play  during 
forced  respiration  even  in  man. 

The  Neck. — If  the  reader  will  turn  to  Fig.  43  it  will  be  seen  that  the 
head  becomes  demarcated  from  the  trunk  and  a  neck  comes  into  existence 
in  the  human  embryo  during  the  7th  and  8th  weeks  of  development.  It 
is  during  these  weeks  that  the  fish-like  organization  of  the  embryonic 
pharynx  becomes  replaced  by  one  which  is  mammalian.  Although  the 
seven  cervical  somites  are  demarcated  early  in  the  4th  week  of  develop- 
ment, the  head  is  so  flexed  upon  the  trunk  that  the  mandible  is  in  contact 
with  the  pericardium.  The  neck  comes  into  existence  by  the  production 
and  growth  of  tissues  between  the  mandibular  arch  and  pericardium, 
this  growth  in  the  ventral  aspect  of  the  cervical  region  being  accompanied 
by  an  extension  or  elevation  of  the  head.  The  heart  itself  is  anchored  to 
the  roots  of  the  developing  lungs  ;  all  the  tissues — nerves,  vessels,  muscles, 
air  and  food  passages — passing  from  the  head  to  the  region  of  the  thorax 
are  elongated  during  this  movement. 


^ 


CHAPTER  XVIII. 


TONGUE,   THYEOID   AND   STEUCTURES   DEVELOPED   FKOM 
THE   WALLS   OF   THE   PEIMITIVE   PHARYNX. 

The  Tongue  and  its  Development.^ — Two  parts  are  to  be  recognized 
in  the  tongue.  The  buccal  part  (Fig.  262)  is  situated  in  front  of  the  foramen 
caecum  and  the  V-shaped  groove.  It  is  covered  by  papillae,  concerned  in 
mastication  and  liable  to  cancer.  The  second  or  pharyngeal  part,  bounding 
the  buccal  wall  of  the  pharynx  (Fig.  262),  is  covered  by  glandular  and 


PharY: 
party:'.' 

2nd  and  Srjd, 
arches  '.{.Vi 


no.  262. — Showing  the  Buccal  and  Pharyngeal  parts  of  the  Tongue. 

lymphoid  tissue  and  concerned  with  swallowing.     These  two  parts  are  not 
only  different  in  function  but  also  in  origin  and  development. 

The  buccal  part  arises  during  the  4th  week  by  an  upgrowth — the  tuher- 
culum  impar — which  springs  from  the  floor  of  the  pharynx,  in  front  of  the 
united  ventral  ends  of  the  2nd  and  3rd  arches  (see  Figs.  251,  264).  This 
outgrowth  was  at  one  time  believed  to  give  rise  to  the  whole  of  the  buccal 
part  of  the  tongue,  but  researches  made  by  Kallius  and  others  have  clearly 
demonstrated  that  in  the  5th  week  there  arise  from  the  mandibular  arch, 

1  E.  Kallius,  Anat.  Hefte,  1910,  vol.  41,  p.  177,  etc.  ;  J.  L.  Paulet,  Archiv.  f.  mikro. 
Anat.  u.  Entioick.  1911,  vol.  76,  p.  658  ;  also  reference  on  p.  226,  under  Frazer. 

255 


256     HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

on  eacli  side  of  the  tuberculuni  impar,  riglifc  and  left  lingual  buds  which,  fuse 
with  and  bury  the  median  element  (Fig.  251).  It  is  probable  that  this 
fusion  has  already  occurred  in  Fig.  264,  and  that  the  tuberculuni  impar 
already  represents  the  buccal  element.  Hence  the  buccal  part  of  the 
tongue  is  bilateral  in  origin,  and  as  its  nerve  supply  shows,  is  entirely 
derived  from  the  mandibular  arch.  In  the  7th  week  the  tip  of  the  tongue 
is  bifid,  because  the  lateral  buds  are  imperfectly  fused  (Paulet).  The 
bilateral  origin  of  the  tongue  explains  the  occasional  occurrence  of  a  bifid 
tip  and  the  formation  of  cysts  in  the  median  raphe.  Besides  the  lingual 
nerve,  the  chorda-tympani — the  branch  of  the  facial  nerve  which  enters 
the  mandibular  arch — also  supplies  the  buccal  parts  with  sensory  fibres. 
Until  the  7th  week  the  buccal  part  of  the  tongue,  still  separated  from 


LOWER   UIP 


alveolar   ridge 
Glandular  field 


-  BUCCAL  PART! 

FORAMEN   CAECUM 
TONSIL 
HARYNGEAL  PART 


Fig.  263. — Upper  Surface  of  the  Tongue  of  a  Cliild  in  which  the  glandular  tissue, 
which  forms  the  sublingual  and  submaxDlary  glands,  has  been  imperfectly 
separated  from  the  tongue  by  the  down-growt,h  of  the  mandibulo-lingual 
plate  of  epithelium.  A,  lower  lip ;  B,  alveolar  ridge ;  C,  glandular  tissue 
(sublingual) ;  D,  submaxillary  ;  E,  buccal  part  of  tongue  ;  F,  tonsil ;  O, 
pharyngeal  part  of  tongue ;    E,  opening  of  larynx. 

the  pharyngeal  part  by  a  depression  in  the  floor  of  the  pharynx,  from 
which  the  thyroid  bud  has  arisen,  remains  unseparated  from  the  man- 
dibular arch.  There  then  occurs  a  down-growth  of  epithelium  in  the 
form  of  a  horse-shoe  plate,  which  separates  the  lingual  from  the  mandibular 
tissues  ;  in  this  way  the  tongue  becomes  separated  from  the  alveolar  ridge 
of  the  mandible.  In  the  floor  space  between  the  tongue  and  mandible  are 
developed  the  submaxillary  and  sublingual  glands.  Not  unfrequently 
part  of  this  glandular  field  may  be  imperfectly  separated  from  the  tongue, 
and  in  this  manner  various  peculiar  congenital  malformations  of  the 
tongue  are  produced  (see  Fig.  263). 

The  pharyngeal  part  of  the  tongue  is  derived  from  the  fused  ventral 
ends  of  the  2nd  and  3rd  arches,  in  which,  as  we  have  already  seen,  the 
body  of  the  hyoid  is  developed.  The  glosso-pharyngeal,  the  nerve  of  the 
3rd  arch,  or  more  strictly  of  the  2nd  cleft,  supplies  it.     The  V-shaped 


TONGUE,  THYROID  AND  PRIMITIVE  PHARYNX         257 


groove  (sulcus  terminalis)  marks  the  union  of  the  buccal  with  the  basal 
or  pharyngeal  part.  The  foramen  caecum,  at  the  apex  of  the  V-shaped 
fissure,  marks  the  site  from  which  the  thyroid  outgrowth  took  place. 

Musculature  of  the  Tongue.^ — The  muscles  of  the  tongue,  which  make 
up  almost  its  entire  substance,  do  not  arise  within  the  visceral  arches,  but 
are  of  extraneous  origin.  It  has  been  shown  that  the  head  is  probably 
composed  of  nine  segments.  From  the  muscle  plates  of  the  three  posterior 
or  occipital  segments  processes  arise  and  grow  downwards  and  forwards 
until  they  reach  the  mesenchymal  basis  of  the  tongue  derived  from  the 
three  visceral  arches,  carrying  their  nerves  with  them — ^the  hypoglossal 
or  12th  cranial  nerve  (Fig.  257).  Hence,  while  the  sensory  nerves  of  the 
tongue  come  from  the  nerves  of  the  1st,  2nd,  and  3rd  visceral  arches,  its 
motor   fibres   are   derived   from   the   posterior   cephalic   segments.     The 

,buccai  part 


Nerve  of  1st  arcfi 
(3rd  diu.  of  5th) 


cervio.  sinus. 


Ist  arch 


f\ —  2nd  arch 


epiglottis 
foramen  caecum 


:  C\-j.^^^:Jv^pharyng.  part. 

3rd  arch 
4th  arch 


Fig.  264.— Showing  the  Origin  of  the  Tongue  in  the  Floor  of  the  Primitive  Pharynx. 
The  condition  represented  is  from  an  embryo  in  tiie  6tli  week.     (After  His.) 

primitive  muscle  of  the  tongue  is  the  genio-hyoid  ;  the  genio-glossus  is  a 
derivative  of  it,  and  so  is  the  hyo-glossus.  The  lingual  muscles  are  already 
recognizable  in  the  6th  week,  but  the  intrinsic  muscles  of  the  tongue,  which 
have  much  to  do  with  its  fine  movements,  are  later  in  point  of  differentiation 
— appearing  in  the  fourth  month.  The  sense  of  taste  is  present  in  a  child 
born  at  the  8th  month  of  development. 

Lingual  Papillae. — The  filiform  papillae  are  the  first  to  appear,  then  the 
fungiform,  a  few  of  which,  along  the  posterior  border  of  the  buccal  part, 
become  enlarged  and  sink  to  form  circumvallate  papillae,  round  the  bases 
of  which  taste  buds  are  developed.  The  papillae  are  confined  to  the  buccal 
or  masticatory  part  of  the  tongue.  It  will  be  observed  that  the  taste 
papillae  are  situated  at  the  brink  of  the  pharynx  (Fig.  262),  at  which  the 
food  is  seized  and  carried  away  by  the  involuntary  muscles.  At  the  lateral 
margins  of  the  buccal  part  of  the  tongue,  just  in  front  of  the  anterior 
pillars  of  the  fauces,  the  fungiform  papillae  are  arranged  in  a  series  of 
laminae,  recalling  and  corresponding  to  the  papillae  Soliatae  of  low  primates 

1  See  Warren  H.  Lewis,  Keibel  and  Mall's  Manual  of  Human  Embryology,  1910, 
vol.  1,  p.  518. 


258     HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

and  of  rodents.  Between  the  papillae  foliatae  occur  taste  buds.  On  the 
under  surface  of  the  tongue  at  birth,  on  each  side  of  the  sublingual  papillae 
and  over  the  position  of  the  ranine  artery,  are  two  fimbriated  folds  of 
mucous  membrane,  the  plicae  flmbriatae,  structures  which  are  well  developed 
in  lemurs,  serving  as  tooth-combs  ^  (Wood  Jones).  A  remnant  of  the 
plicae  fimbriatae  can  commonly  be  seen  on  the  under  surface  of  the  human 
tongue. 

The  Epiglottis.^— The  origin  of  the  larynx,  trachea,  bronchi  and  lungs 
as  a  depression  and  bud  from  the  floor  of  the  pharynx  will  be  dealt  with 
later  (p.  270) ;  but  the  origin  at  the  4th  week  of  the  furcula  (Fig.  251), 
a  process  from  which  the  epiglottis  is  derived,  may  be  noted  here.  Both 
epiglottis  and  thyroid  cartilage  arise  from  the  4th  visceral  arch.  The 
superior  laryngeal  is  the  nerve  of  the  4th  arch,  hence  it  supplies  the  epi- 
glottis and  upper  part  of  the  larynx. 

The  epiglottis  and  palate  are  peculiar  to  mammals.  They  separate 
the  respiratory  passage  from  the  mouth.  In  all  mammals  the  epiglottis 
lies  within  the  naso-pharynx  in  contact  with  the  soft  palate,  but  with  the 
acquisition  of  speech  in  man  this  relationship  is  lost. 

Origin  of  the  Sahvary  Glands.^— In  the  depression  between  the 
tongue  and  the  mandible,  formed  by  the  opening  out  of  the  linguo-man- 
dibular  plate  of  epithelium,  there  appear  two  linear  furrows  (Fig.  265). 
From  the  inner  or  mesial  of  these  two  furrows  arises  the  submaxillary 
gland  ;  from  the  outer  or  lateral,  at  a  rather  later  date  (7th  week),  grows 
the  sublingual.     While  the  latter  arises  by  a  series  of  buds  from  the  ento- 

UPPER    DENTAL  RIOQE 
BUCCAL   CAV.  / 

■NAS.CAV. 


LOtVER   DENTAL  RIDGE  \    XU        \      SUBLINGUAL    OL . 

LINGUAL  NERVE.  SUBMAXILLARY    GL. 

Fig.  265.— Showing  the  Origin  of  the  Submaxillarv  and  Sublingual  Glands  from 
furrows  between  the  gum  and  tongue  during  the  7th  week.  The  tongue  pro- 
jects between  the  maxillary  folds  into  the  nasal  cavity.    (After  His.) 

dermal  lining  of  the  groove,  the  former — the  submaxillary — is  developed 
by  the  depth  of  the  entodermal  furrow  being  enclosed  in  the  mesoderm 
in  the  shape  of  a  cord,  which  later  becomes  canaHculized  and  opens  as  a 
duct  at  the  sublingual  papilla,  while  the  gland  itself  arises  by  a  process  of 
budding  from  the  distal  end  of  the  enclosed  entodermal  cord.     The  sub- 

1  Prof.  Wood  Jones,  Journ.  Anat.  1918,  vol.  52,  p.  345. 

2  J.  Schaffer,  Anat.  Hefle,  1907,  vol.  33,  p.  455  (Evolution  of  Epiglottis). 

3  See  reference  under  Paulet,  p.  235  ;   also  W.  Rubashkin,  Anat.  Hefte,  1912,  vol.  46, 
p.  343. 


TONGUE,  THYROID  AND  PRIMITIVE  PHARYNX         259 


maxillary  ganglion  is  made  up  of  nerve  cells  carried  out  from  the  geniculate 
ganglion  during  the  outgrowth  of  the  chorda  tympani.  The  parotid  gland, 
which  is  the  first  of  the  salivary  glands  to  be  developed  (6th  week),  springs 
as  a  bud  of  entoderm  from  the  lateral  or  bucco-alveolar  recess  of  the  primi- 
tive mouth  (Fig.  265).  Its  duct  is  formed  first  as  a  groove,  which  later 
becomes  enclosed  to  form  a  canal.  It  grows  backwards  in  the  connective 
tissue  over  the  masseter,  and  at  birth  is  comparatively  superficial  in  posi- 
tion, but  as  the  mandible  and  external  auditory  process  grow,  it  sinks 
inwards  to  surround  the  styloid  process,  pushing  the  deep  cervical  fascia 
beneath  it.  In  this  way  the  stylo-mandibular  ligament  is  formed  from  the 
fascia  pushed  in  front  of  it.     Its  nerves  are  derived  from  the  3rd  division 


epssel's  pocket 
Eustach.  tube 

lateral  recess  of  pharynx 
soft  palate 
plica  semilunaris 
tonsil 
ost  pillar 

epiglottis 


duct 


position  1st  cleft' 
remnant  of  thyro-glosr 
hyoid- 
2nd  cleft—/ 
3rd  cleft-/ 
4th  cleft- 
median  lobe  of  thyroid' 


pyriform  fossa 
oesophagus 
ventricle  of  larynx 


Fig.  266. — Showing  the  position  of  the  Visceral  Clefts  in  the  Adult.  The  lines  only 
indicate  the  approximate  positions  of  the  clefts.  For  instance,  the  soft  palate 
is  made  up  largely  from  the  3rd  arch.     See  also  Fig.  272. 

of  the  fifth   (auriculo-temporal).     Salivary  glands  are  accessory  to  the 
function  of  mastication,  and  hence  are  developed  only  in  mammals. 

Median  Pharyngeal  Recess. — ^In  the  middle  line  of  the  roof  of  the 
pharynx,  just  under  the  basi-occipital,  there  is  a  depression  or  recess  of 
mucous  membrane  which  receives  this  name.  In  Fig.  266  it  is  erroneously 
named  SeesseVs  pocket  which,  as  has  been  mentioned  on  page  107,  dis- 
appears in  the  human  embryo  during  the  development  of  the  pituitary 
gland.  Its  embryological  significance  is  doubtful,  but  the  site  of  its 
appearance  corresponds  to  the  point  at  which  the  notochord  remained 
unseparated  from  the  dorsal  wall  of  the  embryonic  pharynx  (Fig.  131). 
Lymphoid  tissue  is  developed  in  its  walls  immediately  after  birth,  and  in 
the  mucous  membrane  round  it.  It  is  developed  behind  the  oral  plate. 
The  adenoid  tissue  of  the  naso-pharynx  continues  to  increase  in  amount 
until  the  age  of  puberty,  when  it  begins  to  undergo  a  slow  process  of  atrophy 
(Symington). 


260 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


The  Tonsil.^ — The  tonsil  arises  early  in  the  3rd  month  of  foetal  life  from 
that  part  of  the  second  cleft  recess  which  is  left  between  the  soft  palate  and 
the  tongue  (Fig.  270,  B).  In  the  4th  month  eight  or  ten  isolated  buds 
of  entoderm  push  out  from  an  elevation  or  tubercle  situated  in  this  recess 
or  pocket,  and  grow  into  the  mesodermal  tissue  in  the  wall  of  the  pharynx 
(Fig.  268).  The  buds  afterwards  canaliculize  and  form  the  crypts  and 
glandular  tissue  of  the  tonsil.  Follicles  of  lymphoid  tissue — for  the  tonsil 
must  be  regarded  as  a  lymphoid  structure — begin  to  collect  round 
these  glandular  buds  in  the  5th  month  of  foetal  life. 

Concerning  the  origin  of  the  lymphoid  cells,  both  of  the  tonsil  and  the 
thymus,  there  are  two  quite  distinct  theories.  The  more  recent  (Gulland's) 
is  that  the  epithelial  entodermal  cells,  which  form  the  glandular  buds  of 
the  tonsil,  give  rise  to  broods  of  lymphoid  cells  ;  the  older  and  the  better 
founded,  that  these  lymphoid  cells  arise  from  the  blood  or  surrounding 
connective  tissue,  creep  in  and  form  follicles  round  the  glandular  entodermal 
buds. 

A  fold  of  mucous  membrane,  the  plica  triangularis  (Fig.  267),  passes 
from  the  lower  part  of  the  tonsil  to  the  anterior  pillar  of  the  fauces.     It 


HYPOBLAST 


ANT:TONSIL   BUDS 

ELEVATION     IN 
LOORof  2'?«!  CLEFT 


POST    TONSIL  BUDS 


of  PALATE 

T:  PILLAR 

ECESS 

DNSIL 

CA  TRIANGULARIS 
POST:   PILLAR 

EPIGLOTTIS 
TONGUE 

Fig.  267.    The  Tonsil  in  a  Human  Foetus  of  8  months.    (Hett  and  Butterfleld.) 
Fig.  268. — Section  across  the  2nd  cleft  recess  showing  the  Outgrowth  of  the  Ton- 
sillar Buds.    The  elevation  between  the  anterior  and  posterior  groups  forms 
the  lowei  part  of  the  plica  triangularis.    (After  Hammar.) 

represents  the  anterior  part  of  the  elevation  or  tubercle  in  which  the 
glandular  buds  develop.  Although  present  in  the  foetus,  it  commonly 
disappears  in  the  adult.  Its  attachment  to  the  tonsil  marks  a  line  of 
separation  between  an  anterior  and  posterior  group  of  tonsillar  outgrowths 
(Fig.  268).  The  recess  above  the  tonsil,  sometimes  crossed  by  a  fold — 
the  plica  semilunaris — is  a  remnant  of  the  recess  of  the  second  cleft  in 
which  the  tonsil  is  developed  (Fig.  267).  In  many  mammals  the  tonsillar 
recess  assumes  the  form  of  a  funnel-like  process  resembling  the  finger  of  a 
glove,  the  blind  end  reaching  almost  to  the  angle  of  the  jaw. 

The  tonsil  is  part  of  a  great  lymphoid  system  stationed  along  the  ali- 
mentary canal.  It  reaches  its  fullest  growth  in  youth,  as  is  the  case  with 
the  lymphoid  system  generally  ;  when  active  growth  of  the  system  is  over, 
and  especially  in  the  years  of  decay,  it  becomes  markedly  reduced  in  size. 
The  upper  part  of  the  2nd  cleft  recess  is  included  with  the  1st  in  the 

^  For  an  account  of  the  comparative  anatomy  and  development  of  the  tonsil  see 
paper  by  Seccombe  Hett  and  Butterfleld,  Journ.  Anat.  and  Physiol.  1910,  vol.  44,  p.  35. 


TONGUE,  THYKOID  AND  PRIMITIVE  PHARYNX         261 


Eustachian  tube  (Frazer).  The  lower  part  of  the  2nd  recess,  containing 
the  tonsil,  is  separated  from  the  Eustachian  part  by  the  growth  for- 
wards of  tissue  of  the  3rd  arch  to  help  in  the  formation  of  the  palatal  folds 
in  the  latter  part  of  the  second  month.  Occasionally  the  tonsillar  recess 
projects  outwards,  and  comes  in  contact  with  a  tubular  fistula  representing 
the  cervical  sinus  (see  Fig.  253). 

The  Pharyngeal  Recess  and  Pharyngeal  Tonsil.— At  each  side, 
the  roof  of  the  pharynx  is  produced  outwards,  behind  the  Eustachian  tube 
and  levator  muscles  of  the  palate,  to  form  the  lateral  recesses  of  the  pharynx 
(Fig.  266).  In  the  recess,  and  especially  on  the  posterior  wall  of  the 
pharynx  between  the  recesses  and  also  in  and  round  the  median  pocket, 
there  is  developed  a  submucous  carpet  of  lymphoid  tissue,  the  pharyngeal 
tonsil,  which  often  becomes  hypertrophied  to  form  adenoids  in  youth. 

The  Lingual  Tonsil. — That  part  of  the  tongue  (pharyngeal)  produced 
between  the  2nd  and  3rd  arches  is  studded  with  mucous  glands  which  are 
surrounded  by  nodules  of  lymphoid  tissue — the  collective  glandular  mass 
receiving  the  name  of  lingual  tonsil.  It  will  thus  be  seen  that  from  the 
2nd  cleft  and  its  neighbourhood  is  produced  a  circum-pharyngeal  ring  of 
lymphoid  tissue  of  great  physiological  and  pathological  importance. 

The  Thymus.^ — The  thymus  arises  in  the  same  manner  as  the  tonsil, 
only  from  the  3rd  instead  of  the  2nd  cleft  (Fig.  270).     The  3rd  cleft  is 


tuberciilum  impar. 


Jst  recess  (salivary  glands) 

2nd  recess  (tonsil) 

median  thyroid  bud. 

-furcula  (epiglottis) 

3rd  recess  (thymus) 
4th  recess  (lat.  thyroid  bud) 
aryteno.  epi.  fold 
coelom  (pericardium) 
pulmonary  grooue 


stomach 


Fig.  269. — Showing  the  origin  of  the  Tonsil,  Thymus  and  Thyroid  from  the  Internal 
Cleft  Recesses  during  the  5th  week.     (After  His.) 

represented  in  the  adult  by  the  space  in  front,  and  on  each  side,  of  the 
epiglottis.  It  is  crossed  by  the  posterior  pillars  of  the  fauces,  which 
represent  a  continuation  of  the  palatal  processes  (Fig.  266).     In  the  6th 

1  T.  H.  Bryce,  Journ.  Anat.  and  Phjsiol.  1906,  vol.  40,  p.  91  ;  P.  Stoehr,  Anat.  Hefte, 
190o,  vol.  31,  p.  409  ;  J.  A.  Hammar,  Ergebnisse  der  Anat.  1909,  vol.  19,  p.  1  :  Anat. 
Hffte,  1911,  vol.  43,  p.  201  (Thymus)  ;  Fraser  and  Hill,  Phil.  Trans.  1916,  vol.  207  (B) 
p.  1  ;   B.  F.  Kingsbury,  Amer.  Journ.  Anat.  1915,  vol.  IS,  p.  329. 


262 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


week  the  3rd  pharyngeal  pocket  has  assumed  the  form  shown  diagram- 
matically  in  Fig.  270,  B,  where  its  lower  and  hinder  wall  is  represented  as 
extended  in  the  form  of  a  flask-like  process,  lined  by  thickened  entoderm, 
the  embryological  basis  of  the  thymus.  On  the  dorsal  part  of  the  same 
pocket  there  is  another  thickening  representing  the  lower  parathyroid' 
or  epithelial  body,  while  the  original  mouth  of  the  pocket  has  been  drawn 
out  to  form  a  tubular  process  or  duct.  In  Fig.  270,  A,  is  represented 
another  view  of  the  3rd  pocket,  during  the  5th  week  of  development. 
By  the  7th  week  the  ectodermal  covering,  shown  in  Fig.  270,  A,  has  been 
invaginated  to  form  the  cervical  sinus  and  vesicle,  the  latter  being  con- 
tinuous with  the  thymic  outgrowth.     The  neck  of  the  glandular  thymic 


STOMODAEUM 


MANDIB.AHCH 
ECTODERM 


l^t  POUCH 

HYOIO 
ARCH 
DORS.  DIV 
2"cl  POUCH 
VENT.   DIV 
3'''^  ARCH 
3'"'^  POUCH 
■^'^h  POUCH 

ULTIMATE 
5th  POUCH 


MESOBRANCH.AHEA 


J  ?^  POUCH 
TUBO-TYMP. 


TONSIL 
PARATHYROID 


CES 


THYMUS. 


PARATHYROID 
•f  !^  POUCH 
THYMUS 

ultimate  pouch  (5^^) 
(thyroid) 


(A)  5m.m. 


(B) 


Fis.  270,  ^.— The  Lining  Membrane  of  the  Pharynx  of  a  Human  Embryo,  5  mm.  long 
(5  weeks  old),  seen  on  its  ventral  aspect  and  showing  the  external  configuration 
and  relationships  of  the  pharyngeal  pockets.     (After  Grosser.) 

Fig.  270,  B. — A  schematic  representation  of  the  pliaryngeal  pockets  and  the  glandu- 
lar structures  rising  from  them  in  the  6th  week  of  development.    (After  Grosser.) 


pocket  becomes  separated  from  the  pharynx  in  the  7th  week  and  usually 
disappears,  but  a  strand  of  tissue  frequently  persists  and  represents  the 
stalk  of  the  outgrowth  (Fig.  271).  By  a  species  of  secondary  budding  the 
thymic  entodermal  outgrowth  becomes  broken  up  into  islands  or  separated 
acini.  The  epithelial  acini  proliferate  and  give  rise  to  a  meshwork  of  united 
cells  (syncytium),  in  which  broods  of  lymphoid  cells  appear  during  the  3rd 
month.  The  lymphoid  cells — lymphocytes — become  aggregated  into  fol- 
licles, where  the  production  of  lymphocytes  is  continued.  All  trace  of  the 
original  epithelial  cells  disappears.  The  concentric  bodies,  known  as  the 
corpuscles  of  Hassall,^  were  at  one  time  supposed  to  represent  remnants  of 

'^  Dr.  E.  T.  Bell  defends  the  theory  of  their  Epithelial  origin,  Amer.  Journ.  of  Anat. 
1906,  vol.  5,  p.  30. 


TONGUE,  THYROID  AND  PRIMITIVE  PHARYNX         263 


the  epithelium,  but  they  are  now  known  to  be  produced  from  single  ceUs, 
which  divide  without  a  separation  of  the  daughter  cells  thus  formed. 
Hassall's  corpuscles  also  arise  from  capillaries,  some  of  which,  after  invad- 
ing the  thymus,  become  broken  up  into  segments.  The  endothelial  cells 
lining  those  segments  may  proliferate,  occlude  the  lumen,  and  thus  give 
rise  to  a  Hassall's  corpuscle  (Nussbaum)  (see  also  p.  335).  The  surrounding 
mesoderm  supplies  the  connective  tissue  stroma  and  capsule  of  the  thymus. 
The  lateral  lobes  come  together  under  the  ventral  aortae  and  pericardium 
during  the  7th  week,  and  ultimately  assume  a  thoracic  position  along 
with  these  structures.  The  pointed  upper  extremity  of  each  lateral  lobe 
can  be  traced  upwards  in  the  fully  developed  foetus,  under  the  lateral 
lobes  of  the  thyroid  towards  the  thyro-hyoid  membrane  (Figs.  171,  253). 
These  apical  strands  represent  the  stalk  of  the  thymic  buds.     Thymus 

-for.  caec. 
—raphe  of  tongue 
hyoid  (2nd  &  3rd  arches) 
susp.  ligament 


thyr  cart.  (4th  &  5th  arches) 

sup.  parathyr. 

-from  4th  cleft 

inf.  parathyr. 

thymic  strand 

thymus  (3rd  cleft) 


Fig.  271. — Diagram  of  the  Thyroid  and  Thymus.    The  position  of  the  parathyroids 
on  the  posterior  aspect  of  the  lateral  lobes  of  the  thyroid  is  indicated. 

buds  also  arise  from  the  4th  pouch  (Fig.  270),  and  from  the  cervical  sinus, 
but  these  never  proceed  beyond  a  rudimentary  stage  in  the  human  embryo. 

While  Beard  regards  the  thymus  as  the  parent  source  of  all  the  white 
blood  corpuscles  of  the  body,  many  interpret  the  appearances  in  quite 
an  opposite  manner  and  are  of  opinion  that  the  leucocytes  are  brought 
within  the  epithelial  element  of  the  thymus  along  with  the  mesodermal 
invasion.  Professor  Bryce  has  demonstrated  that,  white  blood  corpuscles 
appear  in  vertebrate  embryos  before  any  are  seen  in  the  thymus. 

The  thymus  reaches  its  fullest  growth  in  early  childhood  (3rd  or  4th 
year),  and  continues  large  as  long  as  the  body  is  in  a  state  of  active  growth. 
It  begins  to  shrivel  up  when  maturity  is  reached,  and  only  a  remnant  is 
left  as  a  rule,  less  remaining  in  men  than  in  women.  It  receives  its  blood 
supply  from  the  4th  aortic  arches  through  the  internal  mammary.  In 
manner  of  origin  it  resembles  the  tonsil ;  indeed  it  may  be  regarded  as  a 
buried  tonsil.  There  is  a  profuse  production  of  lymphoid  cells  in  the  gill 
clefts  of  fishes,  many  of  which  wander  out,  and  by  their  phagoc}i;ic  pro- 
perties help  to  keep  the  gill  surfaces  clean.     This  fact  throws  some  light  on 


264      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

the  origin  of  so  mucli  lymphoid  tissue  from  the  second  and  third  cleft 
recesses  in  higher  animals. 

The  Thyroid.^ — The  site  at  which  the  thyroid  gland  arises  is  shown 
in  Fig.  269 — on  the  floor  of  the  pharynx  behind  the  mandibular  arch  and 
'  exactly  in  the  middle  line.  The  entoderm  of  the  retro-mandibular  furrows . 
gives  rise  to  a  saccular  diverticulum  almost  as  soon  as  the  fore-gut  becomes 
differentiated — early  in  the  4th  week  of  development.  Immediately  in 
front  of  the  thyroid  evagination  arises  the  buccal  part  of  the  tongue  ; 
behind  it  the  pharyngeal  part,  the  foramen  caecum  in  the  sulcus  terminalis 
remaining  to  mark  the  site  of  origin.  The  entodermal  vesicle  thus  formed 
grows  downwards  and  backwards  through  the  tissue  in  which  the  body  of 
the  hyoid  will  be  formed  and  as  it  extends,  bifurcates.  The  stalk  of  the 
.  evagination,  at  first  hollow  and  representing  a  duct,  quickly  becomes  solid, 
breaks  up,  and  by  the  6th  week  has  disappeared.  The  epithelium  of  the 
evagination  proliferates,  and  in  the  7th  week  forms  a  transverse  plate 
ventral  to  the  larynx  (Fig.  252).  The  plate  is  invaded  and  broken  up 
into  reticulating  columns  by  the  surrounding  mesoderm.  In  the  3rd 
month  the  epithelial  cells  become  arranged  as  follicles  ;  these  at  a  later 
date  are  converted  into  vesicles.  The  original  plate  assumes  a  bent  or 
horse-shoe  form,  the  middle  part  forming  the  isthmus,  the  side  parts  the 
lateral  lobes  (Fig.  271). 

The  thyroid  is  present  in  all  vertebrates  and,  although  it  arises  in  a 
manner  which  suggests  that  at  one  time  it  was  a  gland  of  the  mouth,  yet 
in  no  animal  does  the  duct  persist.  Its  early  origin  in  the  embryo  and  its 
universal  distribution  in  vertebrates  point  to  the  antiquity  and  importance 
of  its  function.  We  now  know  that  duct  glands  have  a  double  function, 
producing  both  external  and  internal  secretions.  It  is  reasonable  to 
suppose  that,  as  regards  the  thyroid,  its  excretory  function  has  been  lost. 
Thyro-glossal  Duct. — In  the  great  majority  of  subjects  the  thyro- 
glossal  duct  or  stalk  completely  disappears  at  the  commencement  of  the 
2nd  month  of  development ;  the  foramen  caecum  marks  one  extremity, 
while  a  ligament  or  a  pyramid  of  thyroid  tissue  prolonging  the  isthmus 
towards  the  hyoid  bone  often  marks  the  other  extremity  (Fig.  271).  The 
pyramid  of  the  isthmus  may  carry  on  it  a  detached  part  of  the  thyro- 
hyoid muscle — the  levator  glandulae  thyroideae.  The  body  of  the  hyoid 
bone  is  developed  in  the  tract  of  the  thyro-glossal  duct  (Figs.  271,  272) 
and  splits  it  up.  Remnants  of  the  duct  or  of  secondary  detached  acini  of 
the  thyroid  may  persist  and  form  cysts  or  thyroid  tumours  in  the  base  of 
the  tongue  above  the  hyoid,  and  commonly  between  the  genio-glossus 
muscles.  They  may  also  occur  between  the  hyoid  and  thyro-hyoid  mem- 
brane. The  supra-hyoid  or  infra-hyoid  bursae  may  also  become  cystic, 
and  may  be  mistaken  for  thyro-glossal  cysts  (see  Fig.  272). 

In  lower  vertebrates  the  lateral  lobes  of  the  thyroid  are  stationed  under 
the  mandible.  It  is  not  uncommon  to  find  in  the  right  submaxillary 
region  of  man  a  thyroid  tumour  or  cyst,  evidently  arising  from  an  arrest 
in  the  descent  of  a  part  or  of  the  whole  of  a  lateral  lobe.     Aberrant  masses 

^  Edgar  H.  Norris,  Amer.  Journ.  Anat.  1916,  vol.  20,  p.  411;  1918,  vol.  24, 
p.  443. 


TONGUE,  THYROID  AND  PRIMITIVE  PHARYNX         265 


of  thyroid  are  often  met  with  in  the  neck,  and  frequently  become  the  site 
of  cystic  tumours.  Occasionally  the  lumen  may  persist  in  the  median 
thyroid  and  open  as  a  fistula  in  front  of  the  larynx  (Fig.  272). 

Ultimate  Branchial  Bodies. — In  Fig.  270,  B,  is  represented  the  ento- 
dermal  outgrowth  from  the  5th  or  ultimate  pharyngeal  pouch.  At  one 
time  it  was  supposed  that  the  entodermal  outgrowth — the  ultimate  branchial 
body — gave  rise  to  the  greater  part  of  the  lateral  lobes  of  the  thyroid. 
They  do  give  rise  to  tissue  which  is  thyroidal  in  structure,  often  containing 
tube-shaped  vesicles.  The  tissue  so  produced  is  applied  to  the  dorsal 
aspect  of  the  lateral  lobes  of  the  thyroid,  but  forms  a  very  small  part  of 
their  glandular  mass.  Like  the  thymic  buds  they  lose  their  connection  with 
the  embryonic  pharynx  by  the  7th  week.  The  pyriform  fossa,  within  the 
ala  of  the  thyroid  cartilage  marks  their  point  of  origin  (Fig.  272).     The 


EUST  :  TUBE 
LAT:   RECESS 


PLICA   TRIANGULARIS 
—  TONSIL 

ORIFICE     a'?'*  CLEFT 

TKYRO-GLOS:  OUCT 
^\^  &  5^   CLEFTS 

_INFRA-HVOlD    PART 
O-f  THYROID 


SYMPH; 


HYOID 


THYROID 


FISTULA 


Fig.  272. — Section  of  the  Pharynx  to  show  the  Track  of  the  Median  Thyroid 
Outgrowth.  In  rare  cases  there  is  a  flstvila  connected  with  the  thyroid,  which 
opens  in  front  of  the  larynx.  The  point  of  origin  of  the  thymus  outgrowth 
from  the  3rd  cleft  may  be  marked  by  a  recess  containing  lymphoid  tissue 
as  is  represented  in  the  figure.  The  pyriform  fossa  occurs  at  the  site  of  the 
4th  and  5th  clefts.  The  group  of  mucous  glands  in  front  of  the  epiglottis 
may  give  rise  to  cystic  tumours. 

blood  supply  suggests  a  double  origin  for  the  thyroid  gland,  for  while  the 
superior  arteries  supply  the  area  formerly  assigned  to  the  median  out- 
growth, the  dorsal  parts  of  the  lateral  lobes  are  nourished  by  the  inferior 
thyroid  branches  of  the  4th  aortic  arch. 

Para-thyroids.^ — There  are  usually  two  para-thyroid  or  epithelial  bodies 
on  each  side,  an  upper  and  a  lower  (Fig.  271).     Both  are  usually  applied 

^  For  a  full  account  of  the  comparative  anatomy  of  the  para-thyroids  see  Dr.  Forsyth's 
Memoir  in  Journ.  Anat.  and  Physiol.  1908,  vol.  42,  pp.  141,  302.  He  found  that  the 
para-thyroids  are  irregular  in  number  and  often  aberrant  in  position,  and  that  it  is 
very  difficult  to  distinguish  microscopically  between  embryonic  thyroid  tissue  and 
adult  para-thyroid  tissue.  The  para-thyroids  were  discovered  by  Sandstrom  in  1880. 
See  also  F.  D.  Thompson,  Phil.  Trans.  1911,  vol.  201,  Ser.  B,  p.  91. 


266      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

to  tlie  deep  or  posterior  aspect  of  the  thyroid  body,  the  upper  being  situated 
amongst  the  terminal  branches  of  the  superior  thyroid  artery,  the  lower 
amongst  the  branches  of  the  inferior.  They  are  flattened  bodies,  about 
6  to  8  mm.  in  diameter,  yellowish  in  colour  when  contrasted  with  the 
substance  of  the  thyroid,  but  they  cannot  be  recognized  with  certainty 
except  by  their  microscopical  structure.  Their  origin  is  shown  in  Fig. 
270  ;  the  lower  bodies  arise  from  the  dorsal  recess  of  the  3rd  pair  of  pouches  ; 
they  are  drawn  into  a  low  position  by  their  attachment  to  the  stalk  of  the 
thymus  (see  Fig.  270).  The  upper  para-thjo-oids  arise  from  the  4th  pair  of 
pouches  (Fig.  270),  and  become  more  or  less  united  to  the  ultimate  bran- 
chial bodies,  and  with  these  are  applied  to  the  dorsal  aspect  of  the  lateral 
masses  of  the  thyroid.  In  structure  they  are  made  up  of  reticulating 
columns  of  cells,  with  vessels  arranged  between  the  columns,  thus  resem- 
bling in  structure  the  carotid  body,  and  probably  also  in  nature  and  origin 
the  medullary  part  of  the  supra-renal.  Their  presence  is  essential  to  the 
function  of  the  thyroid  body. 

Carotid  Bodies. — The  carotid  body  lies  at  the  inner  side  of  the  fork 
between  the  internal  and  external  carotid  arteries.  The  commencement 
of  the  internal  carotid  represents  the  artery  of  the  3rd  arch  ;  that  of  the 
external  carotid,  the  ventral  aortic  trunk.  The  body  is  developed  near  the 
3rd  pharyngeal  pouch  with  the  thymus  (Fig.  253).  In  the  carotid  fork 
nerve  cells  assemble  which  are  derived  from  the  superior  cervical  ganglion  ; 
the  body  is  linked  to  the  superior  cervical  ganglion  by  numerous  nerve 
fibrils.  It  is  essentially  parasympathetic  in  nature,  being  made  up  of 
chromaffin  cells,  similar  to  those  of  the  medulla  of  the  adrenal  bodies. 


CHAPTER  XIX. 


OKGANS   OF    DIGESTION. 


Divisions  of  the  Alimentary  Tract.^ — It  is  always  advantageous  to 
approach  the  development  of  every  system  of  the  body  by  a  recapitulation 
of  the  various  evolutionary  stages,  so  far  as  these  stages  are  known  to  us. 
As  regards  the  evolution  of  the  various  parts  of  the  alimentary  system, 
comparative  anatomy  does  not  help  us  greatly,  because  in  even  the  lowest 
forms  of  vertebrates  the  main  parts  are  already  present — the  mouth. 


■fore-gut         mid-gut 


hind  gut 


cord 
allantois 
uinb.  art. 


position  of  oral  plate     J 


art.  of  ijolli  sac(sup.  mesent.) 


yolli  sac 


Fig.  273. — The  Form  of  the  Alimentary  Canal  in  a  Human  Embryo  of  the  4th  week. 

oesophagus,  stomach,  liver  and  intestine.  In  tracing  the  development 
of  the  earliest  digestive  cavity  (archenteron)  of  the  human  embryo  (p.  38) 
we  saw  that  its  origin  was  similar  to  that  of  the  lower  invertebrates  and  that 
its  first  mouth  apparently  became  converted  into  the  blastopore,  primitive 
streak  and  cloacal  membrane.  A  new  mouth  is  formed  by  the  breaking 
down  of  the  bucco-pharyngeal  membrane  (oral  membrane.  Fig.  273)  early 
in  the  third  week  ;  we  shall  see  that  a  new  kind  of  vent  or  anus  is  formed  at 

1  For  literature  on  development  of  alimentar}'  s^ystem  see  A.  Oppel,  Ergebnisse  der 
Anat.  1905,  vol.  15,  p.  207  ;  1906,  vol.  16,  p.  216  ;  6.  Grosser,  Verhand.  Aimt.  Gesellsch. 
1911,  p.  173  ;   Keibel  and  Mall's  Manual  of  Human  Embryology,  1912,  vol.  2,  p.  291. 

267 


268      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

a  later  stage  in  the  development  of  the  human  embryo — namely,  at  the 
end  of  the  2nd  month  of  development.  There  are  other  reasons  why  com- 
parative anatomy  does  not  help  us  to  understand  the  early  stages  in  the 
development  of  the  alimentary  system.  They  will  be  understood  by  a 
reference  to  Fig.  273.  In  the  human  embryo  a  large  part  of  the  alimentary 
cavity  has  been  specialized  and  precociously  developed  to  form  the  yolk 
sac  for  the  nourishment  of  the  embryonic  tissues  ;  the  embryonic  adapta- 
tions mask  and  obliterate  the  ancestral  stages  (see  page  35). 

With  the  development  of  the  cephalic  and  caudal  evaginations  of  the 
embryonic  plate  the  archenteron  becomes  differentiated  into  three  parts 
(Fig.  274) — the  Mid-gut,  which  represents  the  body  and  chief  part  of  the 
primitive  cavity  ;  the  Fore-gut  and  Hind-gut.  There  can  be  no  doubt 
these  represent  three  functional  divisions.  The  mid-gut  is  supplied  by 
the  superior  mesenteric  artery  and  serves  for  one  kind  of  digestion  and 
absorption  ;  the  hind-gut,  supplied  by  the  inferior  mesenteric  artery,  is 
mainly  excretory  in  nature  ;  the  fore-gut,  separated  by  the  outgrowth 
of  the  liver  from  the  mid-gut,  is  supplied  mainly  by  the  coeliac  axis  and 
serves  the  preparatory  purposes  of  digestion.  The  pharynx,  respiratory 
tract,  oesophagus,  stomach,  liver  and  pancreas  represent  parts  of  the 
fore-gut.  The  hind-gut  gives  rise  to  the  colon  from  the  splenic  flexure 
to  the  anus  ;  the  allantois,  bladder  and  urethra  are  also  separated  from  its 
hinder  end — the  cloaca. 

Differentiation  of  Parts. — How  rapidly  the  various  parts  of  the  ali- 
mentary system  are  differentiated  during  the  4th  week  of  development 
will  be  seen  by  comparing  Figs.  274  and  275.  Fig.  274,  which  represents 
the  alimentary  tract  of  a  human  embryo  near  the  beginning  of  the  4th 
week,  shows  the  pharynx  large,  the  lung  bud  beginning  to  evaginate  from 
the  floor  of  the  fore-gut  just  behind  the  pharynx  and  at  this  date  lying 
directly  under  the  occipital  part  of  the  head  ;  the  oesophagus  and  stomach 
and  first  part  of  the  duodenum  scarcely  marked  off  from  one  another,  all 
of  them  lying  on  the  dorsal  wall  of  the  pericardium  and  lying  under  the 
cervical  segments  of  the  embryo.  The  evagination  to  form  the  liver 
indicates  the  junction  of  the  fore-gut  with  the  mid-gut.  The  latter  division 
is  in  wide  communication  with  the  yolk  sac.  The  various  parts  of  the 
hind-gut  are  already  indicated.  The  condition  towards  the  end  of  the  4th 
week  is  shown  in  Fig.  275.  The  oral  membrane  is  gone  ;  the  pharynx  is 
relatively  smaller  ;  the  outgrowth  of  the  pulmonary  system  is  now  very 
apparent,  the  oesophagus  and  stomach  are  longer  and  narrower  ;  the 
liver  outgrowth  has  become  massive  ;  the  mid-gut  is  tubular,  and  the  neck 
of  the  yolk  sac  reduced  to  a  duct  (vitello-intestinal  duct).  The  parts  of  the 
hind-gut  have  assumed  a  more  definite  shape. 

Primitive  Mesentery  and  Coelom.^ — It  will  be  remembered  that  almost 
as  soon  as  it  appears,  the  mesoderm  is  cleft  into  two  layers — an  outer  applied 
to  the  ectoderm  to  form  the  somatopleure  or  body  wall,  and  an  inner  to 
the  entoderm  or  archenteron  to  form  the  visceral  wall  or  splanchnopleure. 
The  cavity  formed  by  the  cleavage  of  the  mesoderm  is  the  coelom  (Fig.  39). 
Originally  the  cavity  was  designed  for  the  purposes  of  excretion  ;  its  wall 
1  Broman,  Ergebnisse  der  Anat.  1905,  vol.  15,  p.  332. 


ORGANS  OF  DIGESTION 


269 


also  served  as  the  nidus  for  the  reproductive  cells.  In  vertebrates  the 
coelom  came  to  serve  the  purposes  of  a  large  bursa,  in  order  that  the 
muscular  movements  of  the  digestive  canal,  lungs  and  heart  might  proceed 
without  undue  friction.  Hence  the  alimentary  canal  is  developed  within 
the  cavity  of  the  coelom,  being  situated  within  the  median  partition,  which 
separates  the  right  coelomic  space  from  the  left.  The  median  partition 
suspends  the  alimentary  canal  to  the  dorsal  or  vertebral  wall  of  the  body 
cavity,  and  forms  the  primitive  dorsal  mesentery  ;  the  part  of  the  median 
partition  which  fixes  the  tract  to  anterior  or  ventral  wall  of  the  body  cavity 


ORAL  PLATE 


HARYNX- '      "yJX         ^ 


Fig.  274. — The  Alimentary  System  of  a  Human  Embryo  2'5  mm.  long,  and 
near  the  commencement  of  the  4th  week  of  development.  (Professor  Peter 
Thompson.) 

Fig.  275. — The  Alimentary  System  of  a  Human  Embryo,  although  only  3  mm.  long, 
is  in  the  stage  of  development  reached  at  the  end  of  the  4th  week.  (After 
Professor  Broman.) 

forms  the  primitive  ventral  mesentery  which,  however,  is  formed  only  in 
connection  with  the  fore-gut  and  the  cloacal  segment  of  the  hind-gut,  all 
the  rest  being  destitute  of  a  ventral  mesentery  from  the  beginning.  Hence 
the  right  and  left  coelomic  spaces  in  the  abdomen  are  thrown  into  one, 
and  form  the  peritoneal  cavity.  The  only  parts  of  the  alimentary  canal 
which  never  come  to  lie  within  the  coelom  are  the  anterior  part  or  pharynx 
and  the  most  posterior  part  of  the  cloaca.  The  anterior  part  of  the  coelomic 
space  forms  the  cavity  of  the  pericardium,  which  lies  beneath  the  pharynx 
(Fig.  274)  ;  it  is  separated  from  the  peritoneal  space  by  a  transverse  parti- 
tion— the  septum  transversum,^  already  well  marked  at  the  beginning  of 
the  4th  week.  The  primitive  oesophagus  crosses  the  upper  or  dorsal  border 
of  the  septum  transversum  (Fig.  279).     At  each  side  of  it  is  situated  a 

^  P.  Thompson,  Journ.  Anat.  and  Physiol.  1908,  vol.  42,  p.  170. 


270 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


communication  between  the  pericardial  and  peritoneal  spaces — tlie  pleuro- 
peritoneal  passages.  These  two  passages  are  separated  not  only  by  the 
primitive  oesophagus,  but  also  by  the  primitive  median  mesentery,  which 
encloses  the  oesophagus  (Fig.  279). 

Oesophagus. — In  the  4th  week  the  oesophagus  of  the  human  embryo 
resembles  that  of  a  fish  ;  it  is  merely  a  sphincter  or  constricted  part  be- 
tween the  pharynx  and  stomach  (Fig.  274).  During  the  6th  and  7th 
weeks,  when  the  neck  is  being  difierentiated,  and  the  pharynx  and  head 
separated  from  the  heart  and  thorax,  the  oesophagus  undergoes  a  rapid 


oes:(A) 


OES:   (B) 


PANCREAS 


Fig.  276. — Fore-gut  of  an  Embryo  in  the  4th  week  of  development.     (Broman.) 
Fig.  277. — Fore-gut  of  an  Embryo  at  the  end  of  the  5th  week  of  development.    A, 

hepatic  stalk  ;  B,  ventral  pancreatic  bud.     (Broman.) 
Fig.  278. — Irregular   Separation   of   the    Trachea   and    Oesophagus.      The   upper 

or  pharyngeal  part  of  the   oesophagus  forms   a  blind   sac ;    the   lower  part 

passes  from  the  trachea  to  the  stomach.     The  normal  trachea — oesophageal 

septum — is  marked  *  ;  the  abnormal  septum  **. 

elongation.  The  chief  cause  of  the  elongation  of  the  oesophagus  is  to  be 
sought  for  in  the  development  of  the  lungs  and  pleural  cavities  (Fig.  277), 
by  which  the  stomach  is  forced  backwards  in  the  body  cavity.  The  oeso- 
phagus is  of  double  origin  ;  the  upper  or  paratracheal  part  is  derived  with 
the  trachea  from  the  retropharyngeal  segment  of  the  fore-gut ;  the  lower 
or  retrotracheal  part  arises  from  the  pregastric  segment  of  the  fore-gut. 
In  the  5th  week  the  jDulmonary  bud  and  tracheal  groove  are  being  separated 
from  the  oesophagus,  the  lateral  septa  which  effect  the  separation,  beginning 
behind  and  spreading  forwards  (Figs.  276,  277).  Children  are  sometimes 
born  in  which  the  process  of  separation  has  taken  place  in  an  irregular 


ORGANS  OF  DIGESTION 


271 


manner  (Fig.  278).  The  paratracheal  part  ends  blindly,  and  is  surrounded 
by  striated  pharyngeal  musculature  ;  the  retrotracheal  part  opens  from 
the  trachea,  and  is  covered  by  non-striated  muscle.^  The  oesophagus  is  at 
first  lined  by  columnar  epithelium,  but  in  the  2nd  month,  as  it  elongates 
the  epithelium  proliferates,  forming  several  irregular  layers,  which  almost 
occlude  the  lumen  of  the  tube  for  a  time.  In  the  5th  month  glands  are 
formed  in  the  submucous  tissue.  In  the  6th  week  the  oesophagus  is  only 
2  mm.  long  ;  at  birth  it  measures  100  mm.  (4  inches).  Its  commencement 
is  surrounded  by  a  sphincter  formed  by  part  of  the  inferior  constrictor 
of  the  pharynx  ;  above  this  sphincter,  in  later  life,  a  pouch  (retropharyngeal 
diverticulum)  may  arise  ;    such  pouches  are  never  congenital  in  origin. 


transverse  sinus 


dorsal  aorta 


1st  aortic  arch 


art.  meso-card.A 


left  duct  of  Ouuier 
lung  bud  in  mesentery 
dorsal  mesogast. 
stom. 


con.  artcr 
pericard. 


—vitelline  vein 
~~-  liver  bud 
yolk  sac. 

ventral  mesentery 


septum  transu. 


Fig.  279. — The  Mesentery  of  the  Fore-gut  and  its  Contents,  viewed  from  the  left 
side  (schematic). 

At  the  lower  end  the  oesophagus  is  also  closed  by  a  sphincter.  The  muscle 
coats  are  difierentiated  in  the  7th  week,  the  circular  first,  the  longitudinal 
later. 

Development  of  the  Liver.^— Before  proceeding  to  describe  the  de- 
velopment of  the  stomach,  it  is  convenient  to  deal  first  with  the  Hver, 
because  the  manner  in  which  this  viscus  arises  gives  the  key  to  the 
complicated  developmental  changes  of  the  abdominal  viscera.  The 
human  liver  in  its  development  repeats  broadly  the  forms  met  with  in 
ascending  the  animal  scale.  In  amphioxus  the  liver  is  merely  a  caecal 
diverticulum  of  the  digestive  canal  ;  in  amphibians  it  is  a  modified  tubular 

1  See  Keith,  Brit.  Med.  Journ.  1910,  vol.  1,  p.  301.  For  development  of  mucous 
membrane  see  F.  P.  Johnson,  Amer.  Journ.  Anat.  1910,  vol.  10,  p.  521. 

2  0.  Charnock  Bradley,  Journ.  Anat.  and  Physiol.  1909,  vol.  43,  p.  1  ;  F.  P.  Mall, 
Amer.  Journ.  Anat.  1906,  vol.  5,  p.  227  ;  F.  T.  Lewis,  Keibel  and  Hall's  Manual  of 
Human  Embryology,  1912,  vol.  2,  p.  403  ;  Prof.  P.  Thompson,  Journ.  Anat,  1914, 
vol.  48,  p.  222.  See  also  references  to  Bamiville  (p.  47)  and  Waterston  (p.  18)  ;  Prof. 
Frazer,  Journ.  Anat.  1919,  vol.  54,  p.  116. 


272 


HUMAN  EMBEYOLOGY  AND  MOKPHOLOGY 


gland — the  hepatic  cells  being  arranged  in  cylinders  around  the  bile  ducts. 
In  mammals  the  tubular  arrangement  is  lost  and  a  lobular  form  substituted. 
In  every  case  it  is  so  placed  that  the  blood,  laden  with  the  products  of 
absorption  from  the  alimentary  tract  or  from  the  placenta,  must  come  into 
intimate  relationship  with  the  hepatic  tissue  before  passing  into  the  general 
circulation  of  the  body. 

To  understand  the  development  of  the  liver,  the  condition  of  parts  at 
the  commencement  of  the  4th  week  must  be  studied.  At  this  time,  the 
anterior  wall  of  the  yolk  sac  and  that  part  of  the  fore-gut  which  becomes 
the  stomach,  lie  in  the  dorsal  wall  of  the  septum  transversum  (Fig.  274),  or 
to  be  more  accurate,  in  the  substance  of  the  dorsal  and  ventral  mesentery 

PERICARD. 

TRUNC  ART. 

FOREGUT 
DUCT  or  CUVIER  PERICARD.  PAS? 


PERICARD. 
PASSAGES 


SIN    VEN 
NT.CARD.V 


PERICARD. 
YOLK  SAC 

Cut-  SEPT.  TRANS. 


DUODENUM 
DORSAL  MES 

LEET  VI T.  VEIN 

PERIT.CAV 


BODY  WALL 
0MB.  VEIN 


Fig.  280. — Dissection  of  the  Septum  TransversTim  of  a  Human  Embryo  early  in  the 
4th  week  of  development.  The  right  half  is  cut  away  to  expose  the  yolk  sac. 
(After  Low.) 

Fig.  281. — Coronal  Section  of  the  Septum  Transversum  of  a  Human  Embryo  in  the 
5th  week  of  development,  showing  the  liver  trabeculae  invading  the  terminal  parts 
of  the  vitelline  veins.     (After  His.) 


which  have  not  yet  been  differentiated  from  the  septum  transversum 
(Fig.  279).  Two  other  views  of  the  septum  transversum  are  given  in 
Figs.  280,  281,  which  will  assist  the  reader  to  understand  the  early  relation- 
ship of  the  liver.  When  the  liver  bud  grows  out,  it  springs  from  the 
junction  of  the  fore-gut  and  yolk  sac  (Fig.  279)  ;  and  spreads  into  the  tissue 
which  becomes  the  ventral  mesentery  of  the  fore-gut.  The  part  of  the 
gut  from  which  it  arises  afterwards  becomes  the  second  stage  of  the  duo- 
denum. The  hepatic  bud  is  at  first  a  hollow,  a  fold-like  diverticulum  of 
the  fore- gut,  lined  with  entoderm  ;  from  the  upper  or  cranial  end  of  the 
diverticulum  arises  the  outgrowth  of  liver  tissue  ;  its  lower  or  caudal  end 
becomes  the  gall  bladder  and  main  bile  ducts  (Fig.  276).  The  diverticulum 
is  surrounded  in  the  mesogastrium  by  a  mass  of  mesodermal  cells  which 


ORGANS  OF  DIGESTION  273 

form  the  vessels,  capsule  and  connective  tissue  of  the  liver.  From  the 
hollow  hepatic  diverticulum  arise  right  and  left  solid  processes  of  ento- 
dermal  cells,  which  invade  and  form  masses  round  the  right  and  left  veins 
from  the  yolk  sac — the  vitelline  veins  (Figs.  279,  281).  Professor  Bradley  ^ 
has  pointed  out  that  the  right  and  left  masses  do  not  correspond  to  the 
right  and  left  lobes  of  the  fully  formed  liver  ;  the  separation  between  the 
right  and  left  lobes  is  formed  late,  and  has  no  functional  significance. 
A  line  from  the  fundus  of  the  gall  bladder  to  the  caval  impression 
divides  the  liver  into  embryonic  and  functional  right  and  left  halves 
(Cantlie). 

The  hepatic  buds  are  developed  just  behind  the  sinus  venosus  and 
between  both  the  vitelline  and  umbilical  veins  which  are  also  situated  in 
the  ventral  mesentery  (Figs.  279,  281,  282).  The  veins  are  broken  up 
by  the  ingrowth  ;  from  them  starts  an  invasion  of  sinus-like  capillaries 
which,  with  the  surrounding  mesoderm,  penetrates  the  liver  bud  and 
breaks  the  solid  entodermal  processes  into  reticulating  cylinders.  Ac- 
cording to  F.  T.  Lewis  the  hepatic  processes  perforate  and  proliferate 
within  the  lumina  of  the  vitelline  veins,  the  venous  capillaries  thus  arising 
directly  from  venous  spaces.  Secondary  processes  arise  from  the  primary 
hepatic  reticulating  cylinders  and  form  smaller  and  smaller  meshes  of 
hepatic  cells.  The  hepatic  cells,  first  grouped  in  trabeculae,  become 
arranged  in  lobular  units  ;  round  the  periphery  of  the  units  are  the  ter- 
minal portal  venules  ;  in  the  centre  of  each  unit  is  the  beginning  of  a 
tributary  of  the  hepatic  vein  ;  the  portal  or  placental  blood  as  it  passes 
from  the  periphery  to  the  centre  of  each  lobule  is  exposed  to  the  action  of 
the  liver  cells.  Growth  takes  place  by  successive  division  or  dichotomy 
of  the  lobules,  the  chief  areas  of  proliferation  being  always  at  the  surface 
of  the  organ  or  subcapsular.  Growth  is  particularly  rapid  during  the 
2nd  and  3rd  months,  the  liver  reaching  its  largest  relative  size  at  this  time. 
Up  to  the  10th  week,  when  the  foetus  is  42  mm.  long,  the  right  and  left 
halves  have  grown  symmetrically,  but  then  occurs  the  retraction  of  the 
bowel  from  the  umbihcal  cord  and  the  enlargement  of  the  stomach,  leading 
to  an  atrophy  of  part  of  the  left  lobe.  The  ducts  of  the  liver,  unlike  those 
of  any  other  gland,  arise  by  a  secondary  process.  Undifferentiated  tissue 
lying  along  the  distribution  of  the  portal  vein  in  the  liver  group  themselves 
in  cords,  develop  lumina,  become  covered  by  mesodermal  tissue  and  thus 
form  the  intra-hepatic  bile  ducts. 

Veins  of  the  Liver. — Within  the  liver  the  two  vitelline  veins  become 
divided  so  as  to  form  two  sets  of  vessels — afferent  or  distributing  and 
efferent  or  collecting  veins  (Fig.  282).  In  the  5th  week  a  number  of  re- 
markable changes  occur  :  (1)  The  left  umbilical  vein,  which  opens  at 
first  in  the  left  duct  of  Cuvier,  establishes  a  communication  with  the  portal 
sinus  in  the  transverse  fissure  of  the  liver  (Figs.  281,  282,  283)  ;  (2)  the 
right  umbilical  vein  disappears  ;  (3)  a  new  channel — the  ductus  venosus — 
is  opened  between  the  portal  sinus  and  the  inferior  vena  cava  ;  (4)  the 
right  vitelline  vein,  all  except  its  terminal  part,  becomes  obliterated 
(Fig.  283). 

1  Journ.  Anat.  and  Physiol.  1909,  vol.  43,  p.  1. 
s 


274 


HUMAN  EMBKYOLOGY  AND  MORPHOLOGY 


Gall  Bladder  and  Bile  Duets.' — The  hepatic  diverticulum,  from 
which  the  liver  buds  arise,  may  be  regarded  as  a  direct  extension  of  the 
wall   of   the   fore-gut.     From  its   hinder   part  (Fig.  276)  are  developed 


R''SUP;VEN:CAV: 


-    L*;    SUP;VEN;  CAV: 
SINUS   VENOSU5 
L^  DUCT   CUVIER 


Lt   UMB:VEIN 
PORTAL  SINUS 
L*  VIT:  VEIN 


Rt  UMB:  VEIN 


TRANS:  UNION 
"GUT 
R?  VIT:  VEIN 


Fia.  282. — The  Liver  Mass  invading  the  Vitelline  Veins  during  the  4th  week  of 
development.     (Professor  Mall.) 

the  common  bile  duct,  the  gall  bladder,  and  the  cystic  duct  formed  at  the 
junction  of  the  gall  bladder  and  common  bile  duct.  The  hepatic  ducts 
arise  within  the  stalks  of  the  solid  hepatic  buds.     At  first  the  gall  bladder 


R?SUP:VEN:CAV: 


SEPT:TRANSV: 


DUCT: VENOS: 


OBLIT:   RiaHT 
UMB:  VEIN 


#  -L^  SUB.VENrCAV 


OBLIT:  LEFT 
VIT:   VEIN 


OBLIT :  LEFT 
UMB:  VEIN 


LEFT   UNB:VE1N 


RIGHT  VITELLINE  VEIN 


LEFT  VITELLINE   VEIN  (poRTAl) 


DUODENUM 


!FiG.  283.^ — Diagram  to  show  the  Transformation  in  the  Veins  round  the  Liver  at 
the  end  of  the  5th  week  of  development.     (After  Professor  Mall.) 

lies  in  the  ventral  mesentery  (gastro-hepatic  omentum) — a  position  which 
is  permanent  in  some  vertebrates  and  may  occur  as  a  rare  anomaly  in 
man.  In  the  second  month  it  becomes  embedded  in  the  hepatic  tissue, 
its  fundus  appearing  on  the  diaphragmatic  surface  ;    at  a  later  date  it 

^  A.  Pensa,  Aiiat.  Anz.  1912,  vol.  41,  p.  155. 


OEGANS  OF  DIGESTION 


275 


assumes  its  superficial  position.  The  lumen  of  the  ducts  is  occluded  by 
an  epithelial  proliferation  until  the  3rd  month  ;  bile  enters  the  gall  bladder 
in  the  6th  month.  Originally  its  veins  end  in  the  adjoining  hepatic  tissue. 
Occasionally  the  bud  for  the  gall  bladder  divideS;  giving  rise  to  a  bifid  or 
double  gall  bladder.  Eound  the  termination  of  the  common  bile  duct  a 
sphincter  is  developed  from  the  musculature  of  the  duodenum.  The 
manner  in  which  the  common  bile  duct,  hepatic  artery  and  portal  vein 
come  to  occupy  the  free  edge  of  the  ventral  mesogastrium  will  be  described 
in  another  paragraph. 

Separation  of  the  Liver  from  the  Septum   Transversum.— As   the 
liver  develops,  the  dorsal  and  ventral  mesenteries  of  the  fore-gut,  in  the 


per/card. 


ductus  uenosus 
cava 


coronary  lig. 


posit,  of  liuer 

falciform  lig. 
umb.  vein 


cord. 


stom. 


mesogastrium 
gastro-.hep.  oment 
portal  vein 


post,  border 


Fig.  284. — The  origin  of  the  Peritoneal  Ligaments  connected  with  the  Liver. 
Diagram  to  show  the  foetal  relationship  of  the  ventral  mesentery  to  veins  and 
the  stomach,  the  liver  being  removed. 

substance  of  which  the  liver  and  stomach  are  formed,  become  differentiated 
from  the  tissues  of  the  septum  transversum.  The  typical  arrangement  of 
these  membranes,  as  seen  in  reptiles,  is  shown  in  Fig.  284.  In  the  dorsal 
mesentery  (mesogastrium)  lie  the  inferior  vena  cava  and  arteries  of  the 
fore-gut ;  in  the  ventral  mesentery  (gastro-hepatic  omentum)  are  contained 
the  terminal  parts  of  three  veins — the  umbilical,  portal  and  inferior  vena 
cava,  the  last  vessel  reaching  the  ventral  mesentery  by  passing  to  the  right 
of  the  oesophagus.  The  liver  develops  within  both  ventral  and  dorsal 
mesenteries,  but  that  part  of  the  mesentery  in  which  it  and  the  inferior 
vena  cava  lie — the  mesohepar — becomes  separated  from  the  part  which 
is  occupied  by  the  bile  ducts,  portal  vein  and  the  stomach.  Broman 
found  that  this  separation,  which  occurs  in  all  higher  vertebrates,  takes 
place  towards  the  end  of  the  4th  week  in  the  human  embryo,  by  the  de- 
velopment of  a  recess  in  the  mesentery — the  Mesenteric  Recess — which 


276 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


BURSA    INF-RACARD. 

INF  VENA.CAV. 

MESOHEPAT. 


SPia.LOBE 


SPIGELEAN  RECESS 
CAUO. LOBE 


GALL  BLADDER 


Fig.  285. — The  Liver  viewed  from  beliind  to  show  its  relationship  to  the  Gastro- 

hepatic  Omentum,  part  of  the  Ventral  Mesentery. 
Fig.  286.— The  Visceral  Surface  of  the  Liver  of  a  Foetus,  16  mm.,  in  the  7th  week  of 
development.     (P.  Thompson.) 

commences  to  the  right  side  of  the  duodenum,  and  extends  forwards  (see 
Fig.  287).  The  mesenteric  recess  ^  (bursa  omentalis,  Broman)  forms  the 
vestibular  or  hepatic  part  of  the  lesser  sac  of  the  peritoneum,  and  extends 

inf.  uen.  cau.  ^  ,    ,. 
yfah.  lig. 


diaph.. 


\\ 


gastrd^hep.  om. 

oesoph.^ 
mesogastr 


niesohepatj 
mesent  recHi 

inf.  uen.  cau s  ^^"^P-  ^'^■ 

WU  ^liidney 


for.  o/WnsiP(^m^'''"^"^^ 


-mesocolon. 


Juterusy 
\blad^^ 


Fig.  287. — Diagram  of  the  Primitive  Attachments  of  the  Visceral  Mesenteries  to 
the  Posterior  Wall  of  the  Abdomen  as .  seen  in  a  Low  Primate  (Lemur 
Coronatus).  The  condition  illustrates  the  earlier  developmental  phases  of 
the  human  foetus. 

from  the  foramen  of  Winslow  to  behind  the  Spigelian  lobe  of  the  liver 

(see  Figs.  287,  288  and  286).     When  the  liver  and  stomach  are  removed 

1  See  F.  T.  Lewis,  Anat.  Rec.  1916,  vol.  10,  p.  220. 


ORGANS  OF  DIGESTION 


277 


in  the  course  of  dissection,  the  attachment  of  the  mesohepar  will  be  seen 
to  bound  the  Spigelian  part  of  the  lesser  sac  on  the  right,  while  on  its  left 
side,  the  dorsal  mesogastrium  has  been  evaginated  to  form  the  main  body 
of  the  lesser  sac  (Fig.  288).  Thus  it  will  be  seen  that  the  dorsal  and  ventral 
mesenteries  of  the  fore-gut  are  split  into  a  right  lamina — the  mesohepar, 
and  a  left  lamina — the  mesogastrium — by  the  development  of  a  recess 
which  forms  the  earliest  and  first  part  of  the  lesser  sac.  The  mesenteric 
recess  at  first  extends  forwards  in  the  mesentery  of  the  oesophagus  almost 
to  the  right  lung  bud — a  condition  which  is  constant  in  reptiles.  When 
the  lungs  expand  and  the  diaphragm  is  being  formed  during  the  7th  week, 


inf.  uen.  cau.        /^/^-  %     /eft  lat  lig. 


mesohepaticum 
rt.  lat  lig 
for.  of  Winsl. 

gast  hep.  om.-^ 
duoderi: 
asc.  mesocol 


gast  hep.  cm. 
hep.  part  les.  sao. 
mesogastr. 
for  spleen 

oment  part  les.  sac. 
costocol.  lig. 
duoden. 
<ie,  mesocol. 


Fig.  288. — Diagram  of  the  Attachments  of  the  Visceral  Mesenteries  to  the  Pos- 
terior Abdominal  Wall  of  an  Adult.  The  three  chief  modifications  seen,  when 
compared  with  Fig.  246,  are  (1)  the  extensive  adhesion  of  the  mesogastrium, 
(2)  of  the  mesocolon,  and  (3)  mesentery  of  small  intestine,  to  the  posterior 
wall  of  the  abdomen. 

the  apical  part  of  the  mesenteric  recess  is  cut  ofi  and  left  within  the  thorax 
— ^to  the  right  of  the  oesophagus,  and  just  above  the  diaphragm.  To 
this  detached  part,  Broman  has  given  the  name  of  infra-cardiac  bursa 
(Fig.  286).  It  usually  disappears  at  the  end  of  foetal  life,  but  a  remnant 
can  often  be  found  in  adults  if  careful  search  is  made. 

The  Ligaments  of  the  Liver. — When  the  liver  separates  from  the 
septum  transversum  towards  the  end  of  the  2nd  month  of  development, 
it  is  attached  to  the  walls  of  the  abdomen  by  peritoneal  ligaments  derived 
from  the  dorsal  and  ventral  mesenteries  of  the  fore-gut  (Figs.  284,  285). 
These  are  the  following  : 

1.  The  gastro-hepatic  omentum  is  that  part  of  the  ventral  mesentery 
which  passes  from  (1)  the  oesophagus,   (2)  lesser  curvature  or  ventral 


278 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


border  of  stomacli,  and  (3)  first  stage  of  duodenum  to  (1)  the  diaphragm, 
(2)  the  posterior  part  of  the  longitudinal  fissure  of  the  liver,  the  ductus 
venosus  lying  within  its  hepatic  attachment,  and  (3)  the  transverse  fissure 
of  the  liver  (Fig.  285).  The  portal  and  umbilical  veins  lie  in  the  ventral 
mesentery  (Fig.  284) ;  the  hepatic  artery  passes  by  it  to  the  liver.  The 
right  or  free  border  of  the  gastro-hepatic  omentum,  with  the  falcifoT-m 
ligament  containing  the  remnant  of  the  umbilical  vein,  represents  the 
posterior  border  of  the  primitive  ventral  mesentery  (Fig.  284). 

2.  The  falciform  ligament,  containing  the  umbilical  vein,  also  represents 
part  of  the  ventral  mesentery  (Fig.  284).  At  an  early  stage  the  umbilical 
veins  reached  the  sinus  venosus  by  passing  through  the  septum  trans- 
versum.  The  terminal  parts  of  both  veins  become  obliterated  (Fig.  283) ; 
the  new  terminal  channel  for  the  left  vein  is  formed  in  the  ventral  mesentery. 

3.  The  coronary,  the  right  and  left  lateral  ligaments,  and  the  attachments 
to  the  vena  cava  and  diaphragm. — These  ligaments,  which  are  the  chief 


Spigelian 


V.  cau. 


caudate  lobe 


right  lat. 
lobe 


left  lat.  lobe 
lig.  teres 


gl  IjI^       caudate  fis. 
middle  lobe 


Fig.  289. — Diagram  of  a  Mammalian  Liver  viewed  from  behind  and  below. 

hepatic  bonds,  are  derived  from  the  mesohepar  in  the  later  part  of  the 
2nd  month,  when  the  liver  is  being  separated  from  the  diaphragm  by 
invading  pockets  or  recesses  of  peritoneum.  It  would  be  extremely  con- 
venient to  retain  the  term  mesohepar  to  designate  the  bonds  between  the 
liver  and  diaphragm  in  the  adult,  looking  on  the  right  and  left  lateral 
ligaments  as  merely  processes  of  the  mesohepar. 

Morphology  o£  the  Liver.^ — The  liver  of  orthograde  (upright)  animals 
(man,  anthropoids)  differs  widely  in  form  and  lobulation  from  that  of 
mammals  generally,  but  Professor  Arthur  Thomson  has  shown  that  traces 
of  the  fissures  and  lobes  of  the  typical  mammalian  liver  can  be  seen  in  the 
human  organ.  The  liver  of  a  dog  or  dog-like  ape  consists  of  three  main 
lobes — right,  middle  and  left — and  two  accessory  lobes — the  caudate  and 
Spigelian  (Fig.  289).  In  man  the  right  and  middle  lobes  have  fused,  but 
traces  of  the  fissure  which  separates  them  (the  right  lateral  fissure)  are 

1 1  have  dealt  ■with  some  of  the  factors  which  determine  the  shape  of  the  liver  in 
lectures  on  enteroptosis  ;  see  Lancet,  1903,  March  7th  and  14th.  For  cases  of  mal- 
formation of  liver  see  E.  Barclay-Smith,  Journ.  Anat.  and  Physiol.  1909,  vol.  43, 
p.  346  ;    Prof.  P.  Thompson,  Journ.  Anat.  1914,  vol.  48,  p.  222. 


ORGANS  OF  DIGESTION  279 

frequently  to  be  seen  in  the  liver  of  the  newly  born  child  (Fig.  290).  The 
caudate  lobe  has  been  reduced  in  man  to  a  vestige,  but  in  the  third  month 
foetus  it  is  of  considerable  size  (Figs.  286,  290).  It  projects  from  the 
liver  at  the  upper  boundary  of  the  foramen  of  Winslow  ;  in  many  animals 
it  rivals  the  right  lobe  in  size.  The  caudate  fissure  separates  the  caudate 
from  the  right  lobe,  and  a  trace  of  this  fissure  is  very  frequently  to  be  seen 
in  the  human  liver  (Fig.  286).  Irregular  lobulation  of  the  liver  is  not 
uncommon  ;  the  condition  seen  in  the  6th  week,  when  the  gall  bladder  and 
umbilical  vein  occupy  a  common  fissure,  may  be  retained.  The  quadrate 
lobe  arises  in  the  7th  week  (Fig.  286)  from  the  left  lobe  and  grows  across 
the  fissure  occupied  by  the  umbilical  vein  to  occupy  the  space  between  the 
vein  and  gall  bladder  (P.  Thompson). 

Changes   in   the   Liver   after  Birth. — During  foetal  life  the  liver  in- 
creases rapidly  in  size  in  comparison  with  the  other  abdominal  organs. 


Spigelian       ^^„^_  /^^^ 


caud.  fis. 
-right  iat  lobe 

left  Iat  lobe  lj/1     \  ^right  Iat  fissure 
lig.  teresj        gl.  bladder 
middle  lobe 

Fig.  290. — The  Lower  Surface  of  the  Liver  of  a  Human  Foetus  during  the  Srd 
month,  showing  Vestiges  of  Fissures  and  Lobes  of  the  typical  Mammalian 
Liver. 

At  birth  it  occupies  nearly  half  of  the  abdominal  space,  and  measures 
Y^^th  of  its  final  volume.  The  left  lobe  may  still  reach,  and  even  overlap, 
the  spleen.  Up  to  the  time  of  birth  nucleated  red  blood  corpuscles  multiply 
within  it  (page  334).  After  birth  two  factors  come  into  operation  which 
lead  to  a  diminution  in  size  and  change  of  shape.  It  is  supplied  before 
birth  by  placental  instead  of  portal  blood  ;  at  birth,  its  blood-forming 
function  ceases  ;  its  rate  of  growth  becomes  proportionately  less  than 
that  of  other  abdominal  organs.  The  stomach,  formerly  empty,  is  now 
filled,  and  presses  the  liver  towards  the  right  side,  causing  a  change  in 
shape  and  partial  atrophy  of  the  left  lobe.  Riedel's  lobe  is  a  linguiform 
prolongation  of  the  right  lobe  below  the  10th  right  costal  cartilage  caused 
by  compression.     It  is  never  present  at  birth. 

The  Stomach. — The  stomach  is  developed  out  of  that  part  of  the  fore- 
gut  which  lies  between  the  oesophagus  and  pharynx  in  front,  and  the 
yolk  sac,  duodenum  and  liver  bud  behind.  In  the  ith  week  (Fig.  274)  it 
lies  in  the  neck,  with  the  cervical  §omite§  dorsal  to  it,  the  pericardium 


280 


HUMAN  EMBEYOLOGY  AND  MORPHOLOGY 


ventral  to  it,  while  on  each  side  is  the  coelomic  passage  which  leads  from 
the  pericardial  to  the  peritoneal  spaces  (Fig.  279).  At  this  time  heart, 
lungs  and  stomach  lie  near  the  exit  of  the  vagal  fibres  from  the  central 
nervous  system.  During  the  6th  and  7th  weeks,  as  we  have  already  seen, 
the  growth  of  the  lung  buds  leads  to  an  elongation  of  the  oesophagus  and 
a  backward  migration  of  the  stomach  which,  from  being  a  cervical  structure 
comes  to  lie  level  with  the  lower  thoracic  segments  (Figs.  276,  277).  At 
first  its  dorsal  and  ventral  mesenteries  are  undifferentiated  from  the  septum 
transversum.  In  the  5th  week  the  gastric  part  of  the  fore-gut  shows  a 
dorsal  bulging — the  greater  curvature  (Fig.  277).  As  the  liver  and  gut  are 
developed,  the  stomach  separates  itself  from  the  septum  transversum  and 
conies  to  be  suspended  from  the  dorsal  wall  of  the  coelom  by  the  dorsal 
mesogastrium  (Fig.   284).     The  gastro-hepatic  omentum  is  part  of  the 


A  ^ 

Fig.  291,  .4.— Stomach  of  a  Human  Foetus  about  the  end  of  the  Srd  month, 
showing  the  outgrowth  of  the  Fundus  of  the  Stomach.  (Wood 
Jones.) 
B. — Section  across  the  Fundus  (the  line  of  section  is  indicated  in  A), 
showing  the  differentiation  of  the  four  coats  of  the  Stomach. 
(Wood  Jones.) 

ventral  mesogastrium.  The  oesophageal  end  of  the  stomach  lies  between 
the  spinal  fibres  of  the  diaphragm  which  develop  in  its  mesentery  ;  the 
outgrowth  of  the  liver  bud  fixes  its  pyloric  end  in  the  ventral  mesogastrium. 
Three  changes  quickly  ensue  during  the  5th  and  6th  weeks,  the  one  being 
partly  dependent  on  the  other  : 

(1)  The  dorsal  border  of  the  stomach,  to  which  the  dorsal  mesogastrium 
is  attached,  grows  more  rapidly  than  the  ventral  border  to  which  the 
ventral  mesogastrium  is  attached.  The  greater  and  lesser  curvatures 
are  thus  produced. 

(2)  The  fundus  of  the  stomach  is  produced  as  an  outgrowth  from  the 
dorsal  border,  its  origin  being  similar  to  that  of  the  caecum  from  the  small 
intestine  (Fig.  291,  A). 

(3)  While  the  ventral  mesogastrium  attached  to  the  lesser  curvature 
undergoes  a  relatively  slow  growth,  the  dorsal  mesogastrium  is  affected 
by  a  very  rapid  expansion.     Because  of  the  discrepancy  in  the  growth 


ORGANS  OF  DIGESTION 


281 


of  these  two  membranes,  the  greater  curvature  of  the  stomach  becomes 
freely  movable,  while  the  lesser  curvature  remains  relatively  fixed. 

The  three  factors  just  enumerated  lead  to  a  rotation  of  the  stomach, 
the  greater  curvature  moving  to  the  left,  while  the  surfaces,  formerly 
right  and  left,  carrying  the  corresponding  vagus  nerves,  become  posterior 
and  anterior.  The  rotation  is  already  evident  at  the  end  of  the  first  month 
of  development  (Broman).  All  of  these  changes  are  adaptations  to  allow 
the  stomach  to  expand  when  filled  and  contract  when  emptied.  As  the 
stomach  fills,  it  is  the  greater  curvature  which  expands  ;  the  lesser  curva- 
ture remains  relatively  fixed.  By  the  commencement  of  the  4th  month 
the  stomach  is  demarcated  into  a  wide,  vertical,  cardiac  part,  and  a  narrower 

gastro-hep.  oment 
gastro-spl.  oment. 
spleen 

dorsal  mesogastrium 
stomach 
great  oment. 


ventral  mesent 
(falc.  fig.) 


umb.  vein 


uit.  duct — 


■pancreas 


mesentery 


rectum 


Fig.  292. — The   Relationship   of  the   Spleen,   Pancreas   and   Liver   to   the   Meso- 
gastrium in  the  Embryo. 

horizontal  or  pyloric  part.  The  pyloric  sphincter  becomes  difierentiated 
towards  the  end  of  the  2nd  month,  and  it  is  then  possible  to  see  a  distinction 
between  pylorus  and  duodenum. 

Differentiation  of  the  Coats  of  the  Stomach  ^  (Fig.  291,  B). — A 
section  of  the  wall  of  the  stomach  at  the  end  of  the  3rd  month  of  foetal 
life  shows  (1)  an  entodermal  lining  everywhere  showing  depressions  or  pits 
— the  primary  gastric  pits — from  which  gastric  glands  will  be  produced 
during  the  4th  month,  (2)  an  extremely  thick  submucous  layer,  (3)  a  circular 
muscle  coat,  with  nerve  fibres  and  ganglion  cells  applied  to  its  outer  surface  ; 
while  the  circular  coat  appears  during  the  7th  week,  the  outer  longitudinal 
coat  is  not  differentiated  until  the  4th  month,  and  (4)  peritoneal  coat. 
From  the  primary  gastric  pits  solid  processes  grow  within  the  submucous 
coat,  thus  forming  the  epithelial  bases  of  the  gastric  glands.     Even  as  late 

^  See  reference  under  Johnson,  p.  271. 


282 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


as  the  5tli  month  of  foetal  life  the  mucous  membrane  in  the  pyloric  region 
has  a  villous  appearance  owing  to  upgrowths  between  the  mouths  of  the 
primary  gastric  pits.  True  villi,  however,  commence  at  the  distal  border 
of  the  pylorus. 

The  Spleen. — The  spleen  is  formed  in  the  dorsal  mesogastrium  above 
the  cardiac  end  of  the  stomach  (Fig.  292)  and  grows  out  of  the  left  surface 
of  the  mesogastrium  (Fig.  293).  It  appears  at  the  beginning  of  the  6th 
week  by  a  localized  growth  of  the  mesoderm  in  the  mesogastrium.  The 
thickening  becomes  vascularized.  The  coelomic  mesothelium,  which 
covers  this  thickening  on  the  left  aspect  of  the  dorsal  mesogastrium, 
rapidly  proliferates,  the  deeper  cells  invading  the  vascular  basis  of  the 
spleen.     The  tail  of  the  pancreas  (Fig.  292)  reaches  its  point  of  origin.     The 


left  kidney 
spleen 


gastro-spl. 
oment 

stom. 


gastro-hep 
oment. 


right  kidney 

peritoneal  cavity 

aorta 
lieno-renai  lig. 

splenic  artery 
liver 


-falciform  lig. {vent  mesent) 


Fig.  293.- 


-A  Diagrammatic  Transverse  Section  of  the  Mesogastrium  viewed  from 
behind. 


splenic  artery  is  one  of  the  vessels  of  the  mesogastrium  (Fig.  293) ;  its 
branches  end  in  the  developing  tissues  of  the  spleen  and  greater  curvature 
of  the  stomach.  The  splenic  blood  spaces  are  formed  during  the  earlier 
part  of  the  3rd  month  by  a  dilatation  of  the  capillaries,  and  perhaps  also 
from  veins  which,  in  the  developing  spleen,  are  lined  by  columnar  cells. 
The  trabecular  and  muscular  tissues,  and  the  capsule,  are  derived  from  the 
mesoderm  of  the  dorsal  mesogastrium.  Small  masses  of  splenic  tissue 
(accessory  spleens)  are  occasionally  formed  in  the  dorsal  mesogastrium 
near  the  hilum  of  the  spleen.  In  lower  mammals  the  splenic  formation 
spreads  backwards  until  it  forms  a  colic  lobe  lying  in  the  dorsal  mesentery 
of  the  hind-gut.^  In  the  3rd  month  the  surface  of  the  spleen  is  nodular 
and  deeply  incised  ;  about  the  middle  of  foetal  life  the  fissure  begins  to 
disappear  ;  only  on  the  anterior  or  gastric  border  do  they  persist.  The 
spleen  differs  from  a  lymph  gland  in  that  its  spaces  are  formed  by  dilatations 
of  blood  vessels  in  place  of  lymph  vessels.     Lymphoid  nodules  appear  in 

^  W.  Colin  Mackenzie,  Journ.  Anat.  1917,  vol  51,  p.  1. 


ORGANS  OF  DIGESTION 


283 


the  spleen  about  the  6th  month.  The  development  of  the  spleen  in  the 
mesogastrium  and  the  termination  of  its  blood  in  the  portal  circulation 
suggest  that  the  spleen  is  concerned  in  some  way  with  digestion. 

The  gastro-splenic  omentum  is  that  part  of  the  dorsal  mesogastrium 
which  unites  the  spleen  to  the  stomach  (Figs.  292  and  293).  It  becomes 
elongated  and  stretched  as  the  stomach  rotates,  and  as  its  greater  curvature 
is  developed.  The  spleen  comes  to  lie  against  the  posterior  (right)  surface 
of  the  cardiac  end  of  the  stomach.  The  dorsal  part  of  the  mesogastrium 
between  the  roof  of  the  coelom  and  the  spleen  becomes  the  lieno-renal 
ligament.  The  rotation  of  the  stomach  also  leads  to  the  spleen  being 
thrust  towards  the  left  side  ;  the  dorsal  or  renal  surface  of  the  spleen 
becomes  applied  to  the  peritoneum  covering  the  anterior  surface  of  the 
left  kidney  and  supra-renal  body  (Fig.  293).  The  part  of  the  mesogastrium 
between  the  spleen  and  oesophagus  adheres  to  the  diaphragm  and  forms 
the  lieno-phrenic  ligament.     The  manner  in  which  the  dorsal  mesogastrium 


liuer. 
right  hep.  duct  -ti-^-li^/ 

common  bile  duct 


stomach 


hrsal  pancr.  bud 


ventral  pancr.  bud 
'duodenum 


Fig.  294. — The  Pancreatic  and  Hepatic  Processes  of  a  4th  week  Human  Embryo. 
(After  Kollmann.) 

becomes  applied  and  adherent  to  the  posterior  wall  of  the  abdomen  during 
the  2nd  and  3rd  months  will  be  described  in  connection  with  the  secondary 
attachments  of  the  peritoneum  and  mesenteries. 

The  Pancreas.^ — The  Pancreas  appears  during  the  4th  week  as  two 
processes  from  that  part  of  the  gut  which  afterwards  becomes  the  second 
stage  of  the  duodenum  (Fig.  294).  The  pancreatic  buds  develop  within 
the  ventral  as  well  as  within  the  dorsal  mesentery  for,  at  their  points  of 
origin  from  the  duodenum,  these  two  mesenteries  are  continuous  (Fig.  292). 
Of  the  two  buds,  one  is  a  minor  process  which  springs  from  the  ventral 
aspect  of  the  duodenum  in  common  with  the  hepatic  diverticulum.  This 
ventral  bud  forms  only  the  lower  part  of  the  head  of  the  pancreas  (Fig. 
295).  The  greater  part  is  formed  from  a  process  which  springs  from  the 
dorsal  border  of  the  duodenum,  nearer  the  stomach  than  the  ventral 
process,  and  grows  into  the  dorsal  mesogastrium  above  the  stomach  until 

^  For  development  of  pancreas  :  F.  W.  Thyng,  Amer.  Journ.  Anat.  1907-8,  vol.  7, 
p.  489  ;  F.  T.  Lewis,  Keibel  and  Mall's  Manual  of  Embryology,  1912,  vol.  2, 
p.  429  ;  Margaret  Tribe,  Phil.  Trans.  1918,  vol.  20S  (B),  p.  307  ;  Geo.  W.  Comer,  Amer. 
Journ.  Anat.  1914,  vol.  16,  p.  207. 


284 


HUMAN  EMBEYOLOGY  AND  MORPHOLOGY 


it  readies  the  spleen  (Figs.  294,  295,  296).  A  developmental  rotation  in 
the  wall  of  the  duodenum,  brings  the  bile  duct  and  ventral  pancreatic  bud 
in  contact  with  the  right  or  dorsal  aspect  of  the  dorsal  pancreatic  out- 
growth. In  many  animals  there  are  two  ventral  pancreatic  buds,  one  of 
which  sends  a  process  within  the  gastrohepatic  omentum,  round  the  bile 
duct,  almost  to  the  transverse  fissure  of  the  liver.  A  representative  of 
this  omental  lobe  is  occasionally  present  in  man  (Fig.  295).  The  ducts 
of  both  processes  may  persist,  the  duct  of  the  dorsal  bud  (duct  of  Santorini) 
opening  half  an  inch  above  the  opening  of  the  bile  duct ;  the  duct  of  the 
ventral  bud  (Wirsung's)  terminates  with  the  common  bile  duct  (Fig.  295). 
The  terminal  part  of  the  duct  of  Santorini  commonly  becomes  obliterated 
and  the  secretion  of  the  dorsal  pancreatic  outgrowth  finds  a  new  exit 
through  an  anastomosis  between  its  duct  system  and  that  of  the  ventral 


gast  hep.  om. 


com.  bile  d. 
duct  of  Santo^ 

duoden. 


dors,  mesent 


bile  duct 


mesentery 

dors,  pancreas 
ue'nt.  pancreas 
duct  of  l^. 


Fig.  295. — Diagram  of  the  Pancreas  showing  (1)  its  Primary  Relationship  to  the 
Dorsal  and  Ventral  Mesenteries  ;  (2)  the  parts  formed  from  the  Ventral  and 
Dorsal  Outgrowths  ;  (3)  the  Formation  of  the  Duct  of  Wlrsung  (Duct  of  W.) 
by  a  union  between  the  Ducts  of  Dorsal  and  Ventral  Buds. 

bud,  which  is  effected  in  the  3rd  month.  Even  if  the  duct  of  Santorini 
persist,  the  secretion  from  the  dorsal  bud  reaches  the  duodenum  mostly 
through  the  duct  of  the  ventral  bud — the  duct  of  Wirsung.  Occasionally 
the  duct  of  Wirsung  does  not  join  the  common  bile  duct,  but  opens 
separately  in  the  duodenum. 

The  developing  pancreatic  process  is  at  first  hollow,  like  the  primary 
liver  process,  but  the  secondary  outgrowths  are  solid  and  cylindrical. 
They  divide  and  re-divide,  acquire  lumina,  and  form  an  acino-tubular 
gland.  About  the  end  of  the  3rd  month  some  of  the  acini,  particularly  in 
the  tail  of  the  pancreas,  already  distinguished  by  the  staining  reaction  of 
their  cells,  become  partially  or  entirely  separated  from  the  duct-system  and 
form  the  islands  of  Langerhans.^  Rennie,  from  a  study  of  these  in  fishes, 
concludes  they  are  permanent  bodies,  while  the  investigations  of  Dale  led 
him  to  regard  them  as  temporary  in  nature,  representing  resting  acini. 

IK.  A.  Heiberg,  Ergebnisse  der  Anat,  1909,  vol.  19,  p.  948. 


ORGANS  OF  DIGESTION  285 

The  semi-isolated  acini,  of  whicli  there  arc  several  hundreds,  are  found 
in  all  parts  of  the  pancreas,  and  represent  for  us  the  first  stage  in  the  separa- 
tion of  an  ordinary  duct  gland  into  two  elements — one  connected  with  an 
external  secretion,  the  other  with  a  highly  important  internal  secretion. 
We  see  from  the  example  of  the  pancreas  how  ductless  glands  like  the 
thyroid  and  pituitary  may  have  arisen  from  duct  glands  by  atrophy  of  the 
excretory  part.  The  capsule  and  connective  tissue  of  the  pancreas  are 
derived  from  the  mesoderm  of  the  dorsal  mesentery. 

Relationship  of  the  Pancreas  to  the  Peritoneum  and  Vessels.  1.  In 
the  Embryo. — The  pancreas  develops  between  the  layers  of  the  dorsal 
mesogastrium,  just  where  this  structure  is  being  expanded  to  form  the 
wall  of  the  omental  sac.  From  the  first  it  is  completely  surrounded  by 
peritoneum,  and  it  lies  with  its  tail  directed  forwards  against  the  spleen 
and  its  head  on  the  dorsal  bend  of  the  duodenal  loop  (Fig.  296).  It  comes  to 
lie  parallel  to  the  great  curvature  (dorsal  border)  of  the  stomach.  In 
Fig.  296  a  schematic  drawing  is  given  of  the  essential  relationship  of  the 
pancreas  to  the  dorsal  mesogastrium  in  lower  vertebrate  animals ;  it 
also  represents  the  condition  seen  in  a  human  embryo  in  the  5th  week  of 
development,  when  the  dorsal  mesentery  is  swollen  with  young  tissue 
(Fig.  281)  and  attached  along  the  mid-dorsal  line.  The  coeliac  axis  (Fig. 
296)  is  the  artery  of  the  mesogastrium  and  of  the  structures  which  it 
contains.  It  supplies  the  fore-gut  and  its  derivatives,  between  the  septum 
transversum  in  front  and  yolk  sac  behind.  The  coronary  artery  passes 
direct  to  the  cardiac  end  of  the  stomach  ;  the  splenic  is  a  short  vessel 
ending  on  the  cardiac  dilatation  of  the  stomach  and  supplying  the  spleen  ; 
the  hepatic  passes  on  the  right  side  of  the  pancreas  to  the  duodenum  and 
pyloric  end  of  the  stomach,  and  ends  in  the  liver  by  passing  through  the 
ventral  mesentery.  As  the  stomach  migrates  backwards  during  the  6th 
and  7th  weeks,  the  origin  of  the  coeliac  axis  moves  also. 

2.  In  the  Adult. — The  development  of  the  great  omentum  and  the 
rotation  of  the  stomach  to  the  left,  lead  to  the  pancreas  being  pressed 
against  the  left  side  of  the  posterior  wall  of  the  abdomen.  That  part  of 
the  dorsal  mesogastrium  which  lies  between  the  stomach  and  pancreas 
becomes  elongated  enormously,  during  the  3rd  and  4th  months,  to  form  the 
great  omentum,  and  hence  the  two  anterior  layers  of  the  great  omentum 
are  attached  to  the  great  curvature  of  the  stomach  and  to  the  gastro- 
splenic  omentum  (Fig.  296).  The  two  posterior  layers  of  the  omentum 
end  on  the  lower  (formerly  ventral)  border  of  the  pancreas.  The  great 
omentum  is  well  developed  in  all  mammals,  its  origin  being  probably 
related  to  that  of  the  diaphragm.  Its  exact  function  is  unknown,  but  it 
is  connected  with  the  absorption,  and  perhaps  also  with  the  secretion,  of 
peritoneal  fluids  ;  it  is  a  great  phagocytic  mechanism.  The  duodenal 
loop,  with  the  head  of  the  pancreas  in  its  concavity,  is  also  pressed  against 
the  posterior  abdominal  wall.  During  all  the  changes  which  take  place 
in  the  position  of  the  pancreas  and  spleen,  owing  to  the  rotation  of  the 
stomach  and  intestine,  one  structure  remains  fixed,  and  that  is  the  coeliac 
axis.  The  part  of  the  mesogastriimi  in  which  the  spleen  and  tail  of  the 
pancreas    are    situated    becomes    greatly    drawn    out.     Both    structures, 


286 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


instead  of  being  situated  near  the  middle  line  dorsal  to  the  stomach,  come 
to  occupy  a  situation  in  front  of  the  left  kidney,  the  pancreas  thus  coming 
to  lie  across,  instead  of  along,  the  abdominal  cavity.  The  mesogastrium 
is  ballooned  out  towards  the  left  side  to  form  the  lesser  sac  of  the  peri- 
toneum, and  as  the  splenic  artery  lies  in  the  mesogastrium  it  also  is  drawn 
towards  the  left,  circumventing  the  lesser  sac  of  the  peritoneum  (Fig.  297). 
Up  to  the  6th  week  of  embryonic  life  the  pancreas  lies  between  the  layers 
of  the  dorsal  mesogastrium  and  the  extension  from  these  layers  which 
forms  the  mesentery  of  the  duodenal  loop  (Figs.  295,  296)  ;  thus  right 
and  left  surfaces  are  covered  by  peritoneum.  The  left  surface,  which 
becomes  anterior,  retains  its  covering,  but  during  the  6th  week  the  right 


pleen 
coron.  art. 


ab.  aorta 
splenic  art. 
coeliac  axis 


great  omentum 
sup.  mesent  art 

hepatic  art. 
hepatic 

ventral  mesentery 

Fig.  296. — Schematic  representation  of  the  Dorsal  Mesogastrium  and  its  contents. 

aspect  of  the  pancreas  and  duodenal  loop  become  applied  to  the  posterior 
abdominal  wall  in  front  of  the  aorta,  crura  of  the  diaphragm  and  left 
kidney  (Fig.  297).  The  peritoneal  covering  on  the  right  aspect  gradually 
disappears,  and  thus  in  the  adult  the  pancreas  comes  to  appear  as  if  it  lay 
behind  and  outside  the  cavity  of  the  peritoneum.  The  complete  applica- 
tion and  fixation  of  the  pancreas  and  duodenum  to  the  posterior  abdominal 
wall  only  occur  in  animals  adapted  to  the  upright  posture  (see  Figs.  287, 
288,  297). 

The  part  of  the  dorsal  mesogastrium  between  the  pancreas  and  aorta 
(Fig.  297)  is  also  applied  to  the  posterior  abdominal  wall,  and  forms  the 
posterior  lining  of  the  lesser  sac. 

The  Lesser  Sac  '^  is  composed  of  two  parts,  a  vestibular  or  hepatic  part 
formed  from  the  recessus  mesentericus  (Figs.  287,  288)  and  an  omental 
or  gastric  part  formed  by  the  evagination  of  the  dorsal  mesogastrium. 

1  For  fuller  details  see  P.  T.  Crymble,  Journ.  Anat.  1913,  vol.  47,  p.  207. 


OEGANS  OF  DIGESTION 


287 


These  two  parts  communicate  at  an  isthmus  or  constriction  caused  by  the 
coronary  and  hepatic  arteries  (Fig.  297).  The  hepatic  recess  or  pocket 
separates  the  Spigelian  lobe  of  the  liver  from  the  right  crus,  and  permits 
the  liver  to  glide  during  the  respiratory  movements  of  the  diaphragm 
(Figs.  286,  288).  The  gastric  part  isolates  the  stomach,  allows  it  to  con- 
tract, expand  and  move  during  digestion  and  respiration.  In  the  anterior 
wall  of  the  lesser  sac  are  situated  (Fig.  297)  :  (1)  the  gastro-hepatic  omentum 
or  ventral  mesentery,  which  is  at  first  vertical  and  median  ;  (2)  the  stom- 
ach ;  (3)  the  gastro-splenic  omentum,  a  part  of  the  dorsal  mesentery  ; 
(4)  the  two  anterior  layers  of  the  great  omentum,  also  parts  of  the  dorsal 
mesentery.     In  its  posterior  wall  are  situated  :   (1)  the  lieno-renal  ligament 

nesogastnum  becomes 
adherent 


splenic  art. 


ifiesogastn'um 
(gastro-spl.  oment ) 


lesser  sac 


coeliac  axis 
for.  ofW'mslow, 

hep.  art. 
gastro-hep.  am.. 

port,  vein 

liuer-^ 


falciform 


Fig.  297. — Diagram  to  show  the  Formation  of  the  Lesser  Sac  of  the  Peritoneum 
from  the  Dorsal  Mesogastrium.  The  arrow  lies  in  the  isthmus  between  the  vesti- 
bular and  omental  parts. 


(dorsal  mesentery)  ;  (2)  the  dorsal  mesentery  of  pancreas  ;  (3)  two  posterior 
layers  of  the  great  omentum. 

Process  of  Peritoneal  Fixation.^ — We  have  seen  that  certain  develop- 
mental processes,  such  as  the  obliteration  of  the  embryonic  clefts  of  the 
lip  and  of  the  palate,  or  the  union  of  the  medullary  folds  to  enclose  the 
neural  tube,  are  akin  to  the  processes  which  lead  to  the  union  of  the  lips  of 
a  wound  made  by  a  surgeon's  knife.  In  the  peritoneal  cavity  we  are  to  see 
examples  of  another  process  with  which  surgeons  are  familiar — the  forma- 
tion of  adhesions  which  follow  inflammatory  disturbances  of  the  periton- 
eum. The  passages  which  lead  from  the  pericardium  to  the  pleura,  from 
the  pleura  to  the  peritoneum  and  from  the  peritoneal  cavity  to  the  tunica 
vaginalis  of  the  testes,  are  closed  by  the  formation  of  developmental 
adhesions.  The  peritoneal  adhesions  with  which  surgeons  are  familiar 
follow  inflammation,  but  the  developmental  process — the  process  of  zygosis 
— which  leads  to  the  adhesion  of  the  mesentery  of  the  duodenum  and 
part  of  the  mesogastrium  to  the  dorsal  wall  of  the  abdomen  in  the  latter 
part  of  the  2nd  month  of  embryonic  life,  are  not  preceded  by  inflammatory 

1  See  Keith,  Lancet,  1914,  vol.  2,  p.  362. 


288     HUMAN  EMBRYOLOGY  AND  MOEPHOLOGY 

changes,  but  are  the  result  of  growth  impulses  arising  under  an  unknown 
stimulus.  The  process  of  zygosis  is  active  not  only  in  foetal  life  but  is 
also  to  be  seen  at  work  at,  and  even  after,  birth.  The  applied  peritoneal 
surfaces  become  adherent  by  the  proliferation  and  union  of  lining  cells  of 
the  opposed  layers  of  peritoneum.  The  adhesions  as  they  form,  contract 
and  thus  draw  the  various  parts  of  the  alimentary  canal  to  their  final 
position,  much  in  the  same  way  as  the  testes  come  to  be  lodged  in  the 
scrotum.  We  are  here  dealing  with  growth  manifestations  utilized  for  a 
mechanical  purpose.  The  secondary  adhesion  of  the  mesenteries  of  the 
abdominal  viscera  are  apparently  related  to  posture  ;  the  degree  of  ad- 
hesion is  much  more  extensive  in  man  than  any  other  animal,  with  the 
exception  of  the  great  anthropoid  apes.  Man  and  the  anthropoids  are 
distinguished  from  all  other  animal  forms  by  the  upright  posture  of  their 
bodies.  The  peritoneal  adhesions  which  occur  from  the  middle  of  the  2nd 
month  onwards  must  be  regarded  as  adaptations  to  the  upright  posture. 
The  suspensory  ligament  of  the  spleen,  the  right  and  left  costo-colic  liga- 
ments, the  peritoneal  bands  passing  from  gall  bladder  to  the  colon  or 
omentum  are  of  the  same  nature,  and  are  formed  by  secondary  adhesions 
of  the  peritoneum  in  the  later  months  of  foetal  life.^ 

The  Mid-gut,  Yolk  Sac  and  Meckel's  Diverticulum.^The  yolk  sac 
reaches  its  maximum  size  in  the  earlier  part  of  the  ith  week,  when  its  neck, 
filling  the  embryonic  umbilicus,  extends  from  the  septum  transversum 
in  front  to  the  allantois  behind  (Fig.  274).  In  the  5th  week  (Fig.  275) 
the  mid-gut  has  become  a  V-shaped  tube  ;  the  yolk  sac,  now  lying  in 
the  umbilical  cord,  just  beginning  to  be  differentiated, is  joined  to  the  apex  of 
the  mid-gut  by  a  stalk  or  neck.  The  condition  reached  in  the  6th  week  is 
shown  diagrammatically  in  Fig.  298.     The  following  points  are  to  be  noted: 

(1)  The  production  of  the  mid-gut  as  a  U-shaped  loop.  (2)  The  forma- 
tion within  the  umbilical  cord  of  a  long  neck  to  the  yolk  sac — the  vitello- 
intestinal  duct ;  Meckel's  diverticulum  is  formed  by  a  persistence  of  the 
intra-abdominal  part  of  the  canal.  Normally  the  duct  becomes  occluded, 
and  shrivels  up  during  the  6th  week  ;  this  is  the  case  in  all  mammals,  but 
in  birds  the  yolk  sac  is  large  at  the  time  of  hatching,  and  part  of  it  always 
persists  as  an  intestinal  diverticulum.  (3)  The  yolk  sac,  by  the  constriction 
of  the  umbilical  orifice  and  formation  of  the  cord,  comes  to  lie  on  the  plac- 
enta where  a  remnant  of  it  may  be  found  at  birth  near  the  implantation 
of  the  cord  (Fig.  298). 

Vessels  of  the  Yolk  Sac. — Although  at  first  the  yolk  sac  receives  a 
series  of  branches  from  the  aorta,  by  the  time  of  its  separation  from  the 
mid-gut  the  number  has  been  reduced  to  one — ^the  superior  mesenteric, 
which  becomes  the  artery  of  the  U-shaped  loop  (Fig.  298).  Its  vein, 
however,  the  left  vitelline,  has  no  connection  with  the  superior  mesenteric 
vein  but,  when  the  U-shaped  loop  is  formed  continues  its  original  course 
and  ends  in  the  portal  vein  at  the  lower  border  of  the  pylorus  (Fig.  301). 
When  the  vitello-intestinal  duct  atrophies  in  the  6th  week,  the  same  fate 
overtakes  the  vessels  of  the  yolk  sac,  but  they,  too,  may  persist  as  cords. 

^  For  many  details  connected  with  the  formation  of  these  adhesions  see  papers  by 
Dr.  Douglas  G.  Reid,  Journ.  of  Anat.  and  Physiol,  vols.  1911-1915. 


ORGANS  OF  DIGESTION 


289 


The   Umbilical   Coelom   and   Intestinal   Loop.— At  first  the  coelom 
extends  into  the  ]:)roximal  segment  of  the  umbilical  cord  and  it  is  within 

.    left  colic 
coeliac  axis    ,  .  . 

I     sup.mes.     'f^^''- 

/       (  /   /     sup.  haem. 


intest.  loop 


^,     placenta 
"b*- — yolh  sac 


Fig.  298. — Schematic  representation  of  the  Alimentary  Canal,  and  of  its  Mesenteries  and 
Arteries  during  the  Cth  week  of  development. 

this  umbilical  recess  of  the  peritoneal  cavity  that  the  U-shaped  loop — 
the  mid-gut — undergoes  its  earlier  developmental  changes.     The  structural 

ADHESION^ 
DUOOENUM 


COLIC  ANGLE 


7-5  m  m 


COILS  IN  CORD 


2.7  m  m 


Fig.  299. — The  Intestinal  Loop,  seen  from  the  right  side,  in  an  embryo  in  the  Ctli 

week  of  development.     (Prof.  Frazer.) 

Fig.  300.— The  Intestinal  Loop,  with  the  Umbilical  Coelom,  of  a  foetus  in  the  9th] 

week,  seen  from  the  left  side.     (Prof.  Bardeen.) 

features  of  the  loop  are  shown  in  Fig.  298  ;  it  is  made  up  of  a  proximal  or 
jejunal  limb  and  of  a  distal  or  caecal  limb,  for  already  in  the  6th  week, 


290     HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

when  the  embryo  is  little  more  than  5  mm.  in  length,  the  caecal  diverticulum 
is  apparent.  In  Fig.  299  a  dissection  of  the  intestinal  loop  is  shown,  from 
an  embryo  in  the  6th  week  of  development.  Already  the  process  of 
rotation  has  commenced — the  jejunal  limb  coming  to  lie  to  the  right  and 
dorsal  to  the  caecal  limb.  The  mesoduodenum  is  becoming  adherent  to 
the  dorsal  wall  (Fig.  299)  while,  as  Professor  Frazer  has  shown,  certain 
"  traction  bands  "  are  forming  within  the  common  mesentery  and  thus 
guiding  and  regulating  the  movement  and  fixation  of  the  loop.  The  con- 
dition in  the  9th  week  is  shown  in  Fig.  300  ;  within  the  umbilical  coelom 
coils  of  small  intestine  have  been  produced  from  the  jejunal  and  ileal 
parts  of  the  loop  ;  also  a  jejunal  coil  within  the  abdomen  from  the  proximal 
limb.  The  duodeno-jejunal  flexure  has  become  closely  bound  to  the  dorsal 
wall  by  traction  bands — part  of  which  become  muscular  and  form  the 
Muscle  of  Treitz.  Then,  suddenly,  in  the  10th  week,  when  the  foetus  is 
about  42  mm.  long,  the  loop  is  retracted  within  the  abdomen  and  the 
umbilical  recess  becomes  closed.  We  must  regard  the  withdrawal  as 
due  to  the  development  of  "  contraction "  or  "  retraction "  bands  in 
the  mesentery.  During  the  weeks  spent  by  the  intestinal  coils  in  the 
umbilical  recess,  the  lung  buds  are  expanding  and  the  pleural  cavities 
and  diaphragm  are  being  formed,  and  the  safe-guarding  of  these  pro- 
cesses may  be  the  reason  for  an  extra-abdominal  development  of  the 
intestinal  loop.^ 

Persistence  of  Certain  Embryonic  Structures.^ — Many  of  the  structural 
features  seen  in  the  human  embryo  at  the  stage  of  development  reached 
during  the  fifth  or  sixth  weeks  may  persist. 

(1)  The  most  common  structure  to  remain  is  the  intestinal  end  of  the 
neck  of  the  yolk  sac — Meckel's  diverticulum.  It  occurs  in  2  per  cent, 
of  subjects,  and  commonly  forms  a  finger-like  sac  on  the  free  border  of  the 
ileum  from  two  to  four  feet  above  the  ileo-caecal  orifice.  Hence  we  know 
that  this  part  of  the  ileum  forms  the  apex  of  the  U-shaped  loop  of  intestine. 
The  point  on  the  ileum  at  which  the  canal  of  the  yolk  sac  was  attached  is 
frequently  the  seat  of  a  narrowing,  which  may  be  more  or  less  marked. 
This  forms  a  favourable  site  at  which  intussusception  of  the  bowel  occurs. 
The  diverticulum  varies  in  length  and  shape  ;  its  blind  end  is  frequently 
bulbous  and  the  site  of  secondary  diverticula.  Occasionally  pancreatic 
masses  are  developed  at  its  extremity.  It  is  lined  by  a  glandular  epi- 
thelium similar  to  that  of  the  ileum.  Frequently  a  fold  of  the  mesentery 
descends  to  it  (Fig.  302).  In  the  mesenteric  fold  there  is  usually  to  be  found 
a  vestige  of  the  artery  of  the  yolk  sac  (Fig.  298).  The  attached  base  of  the 
mesenteric  fold  may  atrophy,  while  the  free  margin  forms  a  cord,  under 
which  a  loop  of  bowel  may  become  strangulated  (Fig.  302). 

(2)  The  vitello-intestinal  duct  may  remain  patent,  and,  when  the  cord 
is  cut  at  birth,  form  a  fistulous  opening  at  the  umbilicus,  by  which  the 

^For  further  details  see  articles  by  Frazer  and  Robbins,  Journ.  Anal.  1916,  vol. 
50,  p.  75;  C.  R.  Bardeen,  Amer.  Journ.  Anat.  1914,  vol.  16,  p.  427. 

2  For  an  account  of  the  structure  of  the  yolk-sac  see  papers  by  Dr.  H.  E.  Jordan, 
Anat.  Anzeiger,  1907,  vol.  31,  p.  291  ;  1910,  vol.  37,  p.  56.  For  an  account  of  Meckel's 
diverticulum  and  of  malformations  of  the  bowel  see  Keith,  Brit.  Med.  Journ.  1910, 
vol.  1,  p.  301  ;   Ivar  Broman,  Ergebnisse  Anat.  Entw.  1913,  vol.  21,  p.  99. 


ORGANS  OF  DIGESTION 


291 


contents  of  the  ileum  escape.     Or  part  may  become  grafted  on  the  um- 
bilicus and  give  rise  to  a  "  weeping  navel  "  (Stiles). 

(3)  The  artery  of  the  yolk  sac,  the  terminal  part  of  the  superior  mesen- 
teric, may  persist  as  a  fibrous  band  which  stretches  from  the  mesentery  at 
the  situation  of  a  Meckel's  diverticulum  to  the  umbilicus.  Over  it  the 
gut  may  become  strangulated.  The  young  of  all  carnivora  are  born  with 
thread-like  remains  of  both  artery  and  vein,  stretching  from  the  umbilicus 
to  the  mesentery  (Fig.  301).  A  remnant  of  the  vein  is  rarely  seen  in  the 
human  subject.  The  vitello-intestinal  duct  may  also  be  reduced  to  a 
fibrous  structure,  over  which  a  loop  of  intestine  may  fall  and  thus  become 
strangulated. 

(4)  The  U-shaped  loop,  instead  of  retreating  within  the  abdomen  at 
the  beginning  of  the  third  month,  may  remain  within  the  umbilical  recess. 
This  gives  rise  to  a  congenital  umbilical  hernia.  Such  herniae  occur  in  all 
degrees  ;  they  may  contain  a  piece  of  intestine,  or  almost  the  whole  of  the 


DUOD 


MESENT 
X-  STOM;  ILEUM 


MESENTERIC  FOLD 
MECK:  DIVERTlC; 

Fig.  301.— Fibrous  Remnants  of  the  Artery  (a)  and  Vein  (6)  of  the  Yolk  Sac  in  a 

Kitten. 
Fig.  302. — Meckel's  Diverticulum  provided  with  a  Mesentery.    The  arrow  marks 
the  site  at  which  an  aperture  may  be  formed  in  the  mesenteric  fold. 

abdominal  contents.  In  such  cases  the  somatopleure,  or  belly  wall,  which 
forms  the  covering  of  the  hernia,  is  commonly  thin  and  transparent. 

Congenital  Diverticula.^ — During  the  third  month  numerous  out- 
growths of  intestinal  epithelium  are  formed,  which  perforate  the  muscular 
coat.  They  usually  disappear,  but  may  give  rise  to  diverticula,  a  common 
site  being  the  ileo-caecal  junction  where  a  diverticulum  may  develop 
into  a  large  cyst.  Frequently  masses  of  pancreatic  tissue  are  attached  to 
intestinal  diverticula  (Lewis  and  Thyng). 

Congenital  Occlusion  o£  the  Duodenum.-— The  part  of  the  duodenum 
just  above  the  opening  of  the  bile  ducts  may  be  partially  or  completely 
closed — a  rare  occurrence  (Fig.  303).  After  the  liver  and  pancreatic  buds 
grow  out,  this  part  of  the  duodenum  becomes  occluded  by  the  proliferation 
of  the  epithelium  lining  the  gut  (Tandler).     We  have  seen  that  a  rotatory 

1  For  literature  on  congenital  diverticula  see  F.  T.  Lewis  and  F.  W.  Thyng,  Amer. 
Journ.  Anat.  1907-8,  vol.  7,  p.  505. 

2  For  congenital  occlusions  see  H.  Forssner,  Anat.  Hefle,  1907,  vol.  34,  p.  1  ;  C.  P, 
G.  Wakeley,  Journ.  Anat.  1917,  vol.  51,  p.  65;  R.  J.  Gladstone,  Journ.  Anat.  1914, 
vol.  48,  p.  47. 


292 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


movement  occurs  at  this  site  (p.  273).  The  proliferation  of  the  intestinal 
epithelium — in  the  second  month — is  not  confined  to  the  duodenum  ; 
hence  congenital  occlusions  may  occur  at  any  part  of  the  intestine. 

Duodeno-jejunal  Loop  and  Junction.— The  junction  between  the 
duodenal  and  U-shaped  loops  becomes  the  most  fixed  point  in  the  whole 
intestinal  tract  (Fig.  298).  Within  its  dorsal  mesentery  a  band  of  non- 
striated  fibres  is  developed  which  binds  the  junction  to  the  right  crus  of  the 
diaphragm.  The  suspensory  band  ^  is  generally  known  as  the  muscle  of 
Treitz.  The  functional  meaning  of  the  duodeno-jejunal  loop  and  its 
muscular  band  is  unknown,  but  they  are  found  in  all  the  higher  verte- 
brates (see  p.  290). 

Villi  of  the  Intestine.^ — As  early  as  the  7th  week  circular  muscle  fibres 
appear  in  the  coat  of  the  duodenum  and  by  the  10th  week  the  process  has 


Fig.  303. — Congenital  Occlusion  of  the  Duodenum. 

spread  downwards  to  the  ileo-caecal  junction.  The  longitudinal  coat 
appears  in  the  12th  week  and  a  little  later  meconium  is  being  propelled 
towards  the  great  intestine.  A  germinal  zone  is  formed  between  the 
circular  and  longitudinal  coats,  in  which  Auerbach's  plexus  become 
developed.  Villi  begin  to  form  at  the  end  of  the  second  month  while  the 
glands  of  Lieberklihn  appear  in  the  3rd  month,  both  structures  being 
developed  in  the  proximal  part  first  and  spreading  downwards.  The  villi 
arise  by  subdivision  of  the  ridges  (Berry).  Lymphoid  follicles  make  an  ap- 
pearance in  the  4th  month  and  Peyer's  patches  begin  to  form  in  the  7th 
month,  and  are  apparent  to  the  naked  eye  in  the  1st  month  after  birth. 
The  valvulae  eonniventes  arise  as  folds  of  the  mucous  membrane  in  the  8th 
month,  thus  increasing  the  surface  for  absorption.  They  are  formed 
first  in  the  duodenum  ;  their  development  gradually  ceases  at  the  upper 
part  of  the  ileum. 

1  A.  Low,  Journ.  Anat.  and  Physiol.  1908,  vol.  42,  p.  93  ;  P.  T.  Crymble,  Brit.  Med. 
Journ.  1910,  vol.  2,  p.  1156. 

2W.  A.  Hilton,  Amer.  Journ.  Anat.  1901-2,  vol.  1,  p.  459  (Dev.  of  Villi  and 
Valvulae  Conniventes). 


ORGANS  OF  DIGESTION 


293 


DERIVATES  OF  THE  HIND-GUT. 

At  the  beginning  of  the  2nd  month  the  hind-gut  is  almost  equal  in  length 
to  the  mid-gut,  but  its  calibre  is  less.  Indeed,  it  is  not  until  the  5th  month 
that  the  hind-gut  is  marked  off  from  the  mid-gut  by  its  greater  diameter. 
By  the  end  of  the  2nd  month,  as  we  have  just  seen,  the  anterior  (jejunal) 
limb  of  the  intestinal  loop  has  grown  very  rapidly,  and  become  thrown  into 
a  number  of  distinct  loops.  At  birth  the  small  intestine  is  six  times  the 
length  of  the  large  bowel. 

The  Rectum  is  formed  out  of  the  posterior  end  of  the  hind-gut.  The 
manner  in  which  the  rectum  is  separated  from  the  cloaca,  the  anal  canal 
formed,  and  the  permanent  anus  produced,  will  be  described  in  connection 


spl.  flex,  colon 


left  col.  art 
'nf.  mes. 
sigmoid 


mid.  colic 


duoden 


uasa  mtest.  ten. 
pre-art.  mesent. 


ileo-col.  art 

post,  art  mesent. 


f^Heckel's  diuert 


Fig.  304. — The  Mesentery  of  the  Hind-gut.    The  position  assumed  by  the  colon 
after  the  rotation  of  the  gut  has  taken  place. 

with  the  perineum  and  urogenital  passages,  for  their  history  is  closely 
associated  with  the  development  of  these  structures  (see  p.  381). 

The  Descending  Iliac  and  Pelvic  Segments  of  the  Colon  are  also  formed 
out  of  the  hind-gut.  The  artery  of  the  hind-gut  is  the  inferior  mesenteric 
(Fig.  304).  Hence  it  supplies  the  rectum,  sigmoid  and  descending  colon. 
In  the  6th  week  the  hind-gut  is  suspended  from  the  front  of  the  aorta  and 
spine  by  the  dorsal  mesentery  of  the  hind-gut  (Figs.  298,  299).  This 
becomes  transformed  into  the  meso-rectum,  meso-sigmoid  and  descending 
meso-colon.  The  angle  between  the  hind-gut  and  U-shaped  loop  becomes 
the  splenic  flexure  (Figs.  299,  304).  At  the  commencement  of  the  third 
month,  when  the  intestine  takes  up  its  permanent  position  within  the 
abdomen,  the  U-shaped  loop  has  become  twisted  round  on  the  axis  of  the 
superior  mesenteric  artery  (Fig.  304),  so  that  the  part  of  the  hind-gut  which 
forms  the  splenic  flexure  is  turned  forwards  and  to  the  left  until  it  touches 


294 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


the  spleen  (Fig.  310).     It  carries  its  artery,  tlie  left  colic,  with  it.     At 
this  time  the  anterior  limb  of  the  U-loop  elongates  much  more  rapidly 

than  the  posterior  limb,  and  is  pro- 


left  kidney 


'periton. 


ab.  aorta 


Fig.  305. — Diagram  to  show  how  the  descending 
Meso-colon  becomes  applied  to  the  Parietal 
Peritoneum  of  the  left  Lumbar  Region. 


duced  into  coils  of  small  intestine — 
the  jejunum  and  ileum — which 
press  the  descending  meso-colon 
against  the  kidney  and  the  parietal 
peritoneum  covering  the  left  kid- 
ney (Fig.  305).  The  left  surface 
and  layer  of  the  meso-colon  adheres 
by  the  process  of  zygosis  to  the 
pre-renal  layer  of  the  peritoneum, 
both  layers  subsequently  being 
absorbed.  Thus  the  descending  meso-colon,  originally  situated  in  the 
middle  line,  comes  to  be  attached  in  the  left  lumbar  region. 

The  Intersigmoid  Fossa. — The  sigmoid  flexure,  which  is  made  up  of 
the  pelvic  colon  and  part  of  the  iliac  segment,  after  the  rotation  of  the  gut, 
forms  a  loop,  with  its  convexity  directed  towards  the  liver.  The  meso- 
sigmoid  is  originally  attached  in  the  middle  line,  but  the  pressure  of  the 
developing  loop  of  small  bowel  presses  it  against  the  posterior  abdominal 
wall  and  left  iliac  fossa.  It  may  become  completely  adherent  like  the 
descending  meso-colon,  or  only  partially.  When  the  sigmoid  is  lifted 
up  a  recess  or  fossa  may  be  apparent  beneath  the  meso-sigmoid,  to  the 
outer  side  of  the  left  common  iliac  artery,  which  is  due  to  a  failure  of 
adhesion  between  the  meso-sigmoid  and  parietal  peritoneum.  It  occurs 
opposite  the  convexity  of  the  sigmoid  loop  (Fig.  288).  At  birth  the  meso- 
sigmoid  is  relatively  extensive  ;  the  sigmoid  loop  lies  with  its  convexity 
towards  the  right  side  of  the  abdomen,  and  well  above  the  pelvis.  During 
adolescence  the  sigmoid  grows  more  slowly  than  the  rest  of  the  colon. 
It  sinks  within  the  pelvis,  and  forms  the  greater  part  of  the  pelvic  colon. 

Morphology  of  the  Ileo-coUc  Part  of  the  Bowel.^ — In  all  verte- 
brates, from  fishes  upwards,  the  junction  of  the  small  with  the  great 
intestine  is  demarcated  by  the  ileo-colic  sphincter,  developed  from  the 
circular  coat  of  the  bowel.^  As  a  rare  abnormality  the  caecum  may  be 
absent  in  man,  the  only  external  indication  of  the  ileo-colic  junction  being 
the  presence  of  the  ileo-colic  sphincter.  This  is  the  normal  condition  in 
the  frog,  and  in  several  mammals  such  as  the  racoon.  The  sphincter  marks 
the  junction  of  two  difierent  functional  segments  of  the  alimentary  tract. 
Villi,  which  are  originally  developed  in  the  great  bowel,  disappear  in  the 

^For  literature  on  shape  and  development  of  caecum  and  appendix  see  R.  J.  A.  Berry 
and  L.  A.  H.  Lack,  Journ.  Anat.  and  Physiol.  1906,  vol.  40,  p.  247  (Nature  of  Appendix)  ; 
F.  G.  Parsons,  Journ.  Anat.  and  Physiol.  1908,  vol.  42,  p.  30  (Age  Changes  in  Shape  of 
Caecum)  ;  R.  J.  A.  Berry,  "  Intercolon,"  Med.  Journ.  Australia,  1907,  June  20  (Nature 
of  Appendix) ;  G.  S.  Huntingdon,  The  Anatomy  of  the  Human  Peritoneum  and  Abdo- 
minal Cavity,  1903  ;  H.  A.  Kelly  and  E.  Hurdon,  The  Vermiform  Appendix  and  its 
Diseases,  1905. 

"  Keith,  "  Anatomical  Evidence  as  to  the  Nature  of  the  Caecum  and  Appendix," 
Proc.  Anat.  Soc.  Nov,  1903.  See  also  Prof.  T.  B.  Johnston,  Journ.  Anat.  1920,  vol.  54, 
p.  67. 


ORGANS  OF  DIGESTION 


295 


later  months  of  foetal  life.  The  proximal  part  of  the  colon  from  which 
the  caecum  is  developed  forms  the  caecal  colon  (Fig.  306)  ;  it  is  frequently 
demarcated  from  the  ascending  colon  by  a  thickening  of  the  circular 
muscular  coat — the  caeco-colic  sphincter  (Fig.  306),  c) — which  can  com- 
monly be  recognized  in  the  bowel  of  man.  The  caecum  is  developed  as  a 
diverticulum  of  the  caecal  colon.  In  all  vertebrates  its  submucous  coat  is 
rich  in  lymphocytes,  which  in  mammals  collect  in  the  form  of  solitary 
follicles  more  or  less  closely  crowded  together.  R.  J.  Berry  found  that  in 
the  primates  there  is  a  tendency  for  the  lymphoid  tissue  to  be  aggregated 
in  the  apex  of  the  caecum.  In  man,  in  anthropoids,  and  a  few  other  forms, 
the  lymphoid  tissue  becomes  richly  developed  in  the  distal  part  of  the 
caecum,  which  has  a  narrow  lumen,  strong  muscular  coat,  and  is  of  great 
functional  activity  during  digestion.     This  highly  specialized  part  of  the 


caecal  colon 
a. 

ileum 


caecum 


caecal  apex 


Fig.  306. — Diagram  to  show  the  parts  of  a  typical  Mammalian  Caecum.  Five 
parts  are  shown  in  the  iigure  :  (1)  the  termination  of  the  ileum ;  (2)  the 
caecal  colon  in  which  the  ileum  ends  ;  (3)  the  caecum  which  opens  from  the 
caecal  colon ;  (4)  the  apex  of  the  caecum ;  (5)  the  commencement  of  the 
ascending  colon.  At  three  points  the  circular  muscular  fibres  are  thickened 
to  form  sphincters  :  (a)  ileo-colic  junction  ;  (b)  at  the  junction  of  caecum 
and  caecal  colon  (in  man  a  and  b  are  combined  in  the  ileo-caecal  orifice  and 
its  retinacula) ;  (c)  in  the  first  part  of  the  ascending  colon. 

caecum  is  the  appendix  ;  it  is  well  developed  in  man,  and  is  certainly  not  a 
vestigial  structure.  The  lymphoid  tissue  undergoes  a  great  reduction  in 
size  and  growth  when  the  period  of  adolescence  is  past.  Thus  there  are 
five  structures  to  be  observed  in  the  ileo-colic  region  of  a  typical  mammal 
(Fig.  306)  :  (1)  an  ileo-colic  sphincter,  (2)  a  caeco-colic  sphincter,  (3)  a 
caecal  segment  of  the  colon,  (4)  a  caecum,  the  distal  part  of  which  may  be 
specialized  to  form  (5)  an  appendix.  Further,  a  study  of  the  comparative 
anatomy  of  this  region  shows  that  the  caecum  is  largest  in  vegetable- 
feeding  animals,  and  that  there  is  a  correlationship  between  the  development 
of  the  stomach  and  caecum.  In  the  horse,  for  instance,  the  caecum  and 
caecal  colon  are  comphcated,  the  stomach  simple  ;  in  the  ruminants  the 
stomach  is  complex,  the  caecum  comparatively  simple.  In  animals  which 
live  on  a  flesh  diet  the  caecum  is  small. 

Development  o£  the  Colon  and  Caecum. — Early  in  the  6th  week  of 
development  an  elevation  appears  on  the   free  border  of  the  posterior 


296 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


limb  of  the  U-sliaped  loop  (Figs.  298,  299).  The  elevation  contains  a 
diverticulum  of  the  caecal  colon,  which  forms  the  caecum  and  appendix. 
It  continues  to  grow  outwards  and  forwards  in  close  contact  with  the  free 
border  of  the  ileum.  At  first  the  colic  part  of  the  intestinal  loop  and  the 
caecal  process  are  not  of  larger  calibre  than  the  small  intestine,  but  in  the 
fifth  month  the  colon  and  caecum  undergo  an  enlargement,  but  the  terminal 
or  apical  part  of  the  caecum  retains  its  foetal  dimensions,  and  forms  the 
appendix.  As  in  the  small  bowel  the  circular  coat  appears  long  before  the 
longitudinal,  but  whereas  the  muscle  appears  first  at  the  proximal  end  of 
the  small  bowel  and  spreads  distally,  the  muscle  of  the  colon  appears  first 
at  the  rectal  end — where  the  sacral  visceral  nerves  enter — and  spreads 
towards  the  ileo-caecal  junction.  The  longitudinal  coat  appears  in  the 
3rd   month   along   the   mesenteric   border — representing   the   mesenteric 


asc.  col. 


/position  of\ 
inglit  diu.  of] 
\       caeo    ' 


bloodless  fold 

ileum 
ileo-caec.  pouch 


'posit,  of  left  divert 
mesent. 
art.  of  appen. 

appendix 

Fia.  307. — Diagram  of  the  Apex  of  the  Caecum  at  the  time  of  Birth  and  the  Diver- 
ticula which  may  be  produced  later  in  the  Fundus  of  the  Caecum. 

taenia  ;  the  remaining  two  in  the  4th  month.     The  evaginations  or  haustra 
are  distinct  in  the  7th  month  of  foetal  life.^ 

As  the  superior  mesenteric  (vitelline)  artery  descends  in  the  intestinal 
loop,  it  gives  off  three  branches  to  the  posterior  limb — the  middle  colic, 
right  colic  and  ileo-colic  arteries  (Figs.  304,  308).  The  mesentery  of  the 
U-shaped  loop  may  be  divided  into  two  parts,  the  fate  of  the  two  parts 
being  different : 

1.  The  mesentery  of  the  anterior  limb  in  front  of  the  superior  mesenteric 
artery — forms  the  pre-arterial  part.  This  gives  rise  to  the  greater  part  of 
the  mesentery  of  the  small  bowel. 

2.  The  mesentery  of  the  posterior  limb,  behind  the  artery — is  the  post- 
arterial  part.  It  forms  the  mesentery  of  the  ascending  and  transverse 
colon,  and  also  the  lower  part  of  the  mesentery  of  the  small  bowel. 


1  See  Th.  Thaysen,  Anat.  Hefte,  1916,  vol.  54,  p.  321  ;    P.  E.  Lineback,  Anat.  Rec. 
1919,  vol.  16,  p.  155  ;   E.  J.  Carey,  Anat,  Rec,  1920,  vol.  18,  p.  224, 


ORGANS  OF  DIGESTION  297 

When  the  rotation  of  the  intestinal  loop  takes  place  (p.  289)  the  splenic 
flexure  of  the  colon  comes  against  the  spleen,  while  the  transverse  meso- 
colon, containing  the  middle  colic  artery,  is  brought  into  apposition  with 
that  part  of  the  mesogastrium  which  forms  the  great  omentum  (Figs. 
300,  310).  These  two  layers  adhere  ;  thus  the  transverse  colon  is  formed 
by  the  fusion  of  a  part  of  the  dorsal  mesogastrium  with  the  mesentery 
of  the  posterior  limb  of  the  U-shaped  loop  (Fig.  288).  The  rotation  places 
that  part  of  the  loop  mesentery  which  forms  the  mesentery  of  the  caecum 
and  ascending  colon  against  the  duodenum,  and  at  the  same  time  the 
duodenal  loop  becomes  fixed  in  its  permanent  position  in  front  of  the 
right  kidney  and  inferior  vena  cava.  The  caecum  thus  comes  to  be  situated 
in  the  majority  of  foetuses  in  front  of  the  right  kidney,  near  the  gall- 
bladder, and  there  it  remains  until  about  the  time  of  birth,  when  the 
ascending  colon  elongates  and  the  caecum  thus  moves  towards  the  right 
iliac  fossa.  An  iliac  position  of  the  caecum  is  a  feature  which  occurs  only 
in  animals  adapted  to  the  upright  posture.  Thus  the  attachment  of  the 
ascending  meso-colon  is  effected  by  secondary  adhesions  which  are  formed 
during  the  migration  of  the  colon  and  caecum.  The  appendix,  during  the 
migration,  may  be  caught  behind  the  colon,  thus  assuming  a  retro-colic 
position  ;  it  is  then  lodged  and  fixed  in  the  ascending  meso-colon.  The 
peritoneal  adhesions,  which  are  formed  in  the  4th  and  5th  months  of 
foetal  life,  between  the  transverse  meso-colon  and  great  omentum,  and 
especially  the  adhesions  which  the  ascending  colon  forms  just  before  and 
after  birth,  as  the  caecum  assumes  its  position  in  the  iliac  fossa,  are  subject 
to  a  great  range  of  variations,  and  many  peritoneal  folds  and  recesses  may 
be  formed.  The  object  of  all  of  them  is  to  give  a  fixation  of  the  viscera  to 
the  abdominal  wall — a  fixation  which  occurs  only  in  orthograde  primates.^ 

The  Appendix. — At  first,  and  until  the  fifth  month,  the  caecal  diverti- 
culum is  of  the  same  calibre  throughout,  but  from  that  month  onwards. 


jejunum 


colon 

ileo-col.  an. 
mesentery 
artery  of  appendix 

caecum 

^appendix 
post,  aspect.) 


'leum 


uit.  intest.  canal 


Fig.  308. — The  Appendix  and  Peritoneal  Folds  at  the  end  of  the  2nd  montli  of  Foetal 
Life.     The  intestinal  loop  is  ^iewed  on  its  left,  later  its  dorsal,  aspect. 

the  appendix  remains  small  while  the  caecum  grows,  keeping  pace  in 
diameter  with  the  colon.  At  birth  the  appendix  is  still  the  tapered  apex 
of  the  caecal  diverticulum  (Fig.  307),  but  during  cliildhood,  an  outer,  or  an 

^  Dr.  Douglas  Reid  has  described  the  various  forms  of  foetal  adhesions  in  Journal 
of  Anatomy  and  Physiology,  vols.  1911-1915. 


298      HUMAN  EMBRYOLOaY  AND  MORPHOLOGY 

inner  sacculation,  or  botli  together,  arise  in  the  fundus  of  the  caecum  and 
thrust  the  appendix  backwards  and  to  the  left  into  an  asymmetrical  posi- 
tion.^ Villi  are  formed  in  the  mucous  coat  in  the  early  part  of  the  4th 
month  ;  Lieberkiihn's  glands  appear  a  little  later.  Lymphoid  follicles  make 
their  appearance  in  the  5th  month.     The  villi  disappear  in  the  8th  month. 

Although  a  distinctly  marked  appendix  is  only  seen  in  man,  the  anthro- 
poids, lemur,  opossum  and  certain  rodents,  still  a  corresponding  lymphoid 
structure  is  present  generally  in  mammals.  The  appendix  is  a  lymphoid 
diverticulum  of  the  caecal  apex  (R.  J.  Berry).  It  must  be  regarded  as  a 
lymphoid  structure,  and  although  it  can  be  dispensed  with,  is  not  therefore 
to  be  regarded  as  vestigial  in  nature  any  more  than  is  the  tonsil.  In  30  % 
of  adults  both  muscular  and  mucous  coats  have  undergone  a  partial 
degeneration  under  modern  conditions  of  diet,  and  the  appendix  does  tend 
to  become  a  useless  structure. 

Heo-caecal  Valves. — At  the  ileo-colic  junction,  the  development  of 
villi  ends.  In  the  higher  primates  the  junction  is  invaginated  within  the 
caecum,  in  the  form  of  two  lips  or  valves.  The  invagination  becomes 
apparent  in  the  human  foetus  of  the  3rd  month.  Within  these  folds  are 
(1)  the  ileo-colic  sphincter  ;  (2)  muscular  bands  developed  in  the  retinacula 
from  the  circular  musculature  of  the  caecum  and  representing  the  mid- 
caecal  sphincter  of  the  typical  caecum  (Fig.  306).  The  retinacular  muscula- 
ture assists  in  the  emptying  and  filling  of  the  caecum.  To  a  very  slight 
extent  the  ileo-colic  lips  can  serve  as  mechanical  valves  in  the  living  subject ; 
they  assume  a  valvular  form  only  when  dead  and  dried. 

Ileo-caecal  Fossae. — When  the  caecal  diverticulum  grows  out  from  the 
hinder  limb  of  the  U-shaped  loop  it  carries  with  it  three  folds  (see  Fig.  309)  : 

1.  The  ileo-colic  fold,  a  process  from  the  right  side  of  the  mesentery 
containing  the  anterior  caecal  artery ;  in  a  small  proportion  of  cases  this 
fold  forms  the  mesentery  of  the  appendix  ;  ^ 

2.  The  bloodless  or  ileo-caecal  £old,  a  process  from  the  coat  of  the  ileum; 

3.  The  mesentery  of  the  appendix,  a  process  from  the  left  side  of  the 
mesentery,  containing  the  artery  to  the  appendix  (Fig.  308). 

These  three  folds  give  rise  to  three  fossae  (Fig.  309) : 

1.  The  ileo-colic,  between  the  termination  of  ileum  and  ileo-colic  fold; 
2.  The  ileo-caecal,  between  the  bloodless  fold  and  mesentery  of  the  ap- 
pendix ;   3.  The  retro-caecal,  between  the  mesentery  of  the  appendix  and 
commencement  of  the  ascending  meso-colon. 

The  caecum  and  appendix  are  made  up  of  bilateral  halves ;  there  are 
right  (anterior  caecal  fold)  and  left  (mesentery  of  appendix)  mesenteries. 
In  birds  the  appendix  is  divided ;  it  is  occasionally  double  in  malformed 
human  infants. ^  There  is  no  reason  to  suppose,  however,  that  the  appen- 
dix was  ever  a  double  structure  in  the  stem  from  which  man  has  descended. 

The  duodeno-jejunal  fossa  is  formed  to  the  left  of  the  duodeno-jejunal 
flexure  after  the  transverse  colon  and  caecum  have  rotated  to  the  right 

1  See  F.  G.  Parsons,  Journ.  Anat.  and  Physiol.  1908,  vol.  42,  p.  30. 

2  See  Dr.  Geo.  M.  Smith,  Anat.  Record,  1911,  vol.  5,  p.  549  ;  A.  Forster,  Anat.  Hefte, 
1918,  vol.  56,  p.  5. 

'  Dr.  F.  Wood  Jones,  Journ.  Anat.  and  Physiol.  1912,  vol.  46,  p.  193. 


ORGANS  OF  DIGESTION 


299 


hypocliondrium  and  when  the  transverse  meso-colon  has  fused  with  the 
omental  layers  of  the  lesser  sac  (Fig.  310).  The  fossa  is  occupied  by  a 
bend  of  intestine  at  the  duodeno-jejunal  junction  and  serves  as  a  bursa 


ileo-col. 


ftsc.  meso-col. 


ileo-col. 
^'//  fossa 

._.     ileum 

retro-caec.  fossa 

caec.  fossa 
bloodless  fold  \:^^  ^mesent 

appendix 

Fig.  309. — Peritoneal  Fossae  in  the  Ileo-caecal  Region. 

for  this  knuckle  of  gut.  Its  origin  is  connected  with  (1)  the  traction  bands 
developed  at  this  junction  (see  p.  290),  the  passage  of  the  inferior  mesen- 
teric vein  in  or  near  its  left  border.  It  lies  in  the  axis  at  which  the 
mesenteric  rotation  takes  place  (Fig.  310),  and  when  the  plastic  nature  of 


stom 

duoden 

caec. 
post-art  pt.  mesent 

pre-art  part  mes. 


great  oment 
spl.  flex, 
duodeno-jej.  fossa 

sup.  mes.  art. 

■left  colic  artery 


Fig.  310.- 


inf.  mesent. 
hter-sig.  fossa 


sigmoid  artery 


-To  show  the  Rotation  of  the  Intestinal  Loop  and  Formation  of  the 
Duodeno-jejunal  Fossa. 


the  peritoneal  tissue  is  remembered,  it  is  easy  to  realize  how  this  and  other 
recesses  may  be  formed  near  the  termination  of  the  duodenum. 

The  mesentery  of  the  small  gut  is  formed  out  of  the  primitive  mesentery 
of  the  U-shaped  intestinal  loop,  chiefly  from  that  part  of  it  (the  pre-arterial) 
which  lies  between  the  superior  mesenteric  artery  and  the  anterior  limb 
of  the  loop  (Fig.  304).  After  the  rotation,  the  aspect  of  the  mesentery, 
which  was  directed  towards  the  right,  becomes  left  and  anterior  (compare 


300      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

Figs.  298,  310).  During  the  rotation  of  tlie  gut  tlie  superior  mesenteric 
artery  comes  to  lie  in  front  of  the  third  stage  of  the  duodenum.  At  first  the 
mesentery  is  attached  in  front  of  the  spine  only  at  the  origin  of  the  superior 
mesenteric  artery  (see  Figs.  287,  288).  Its  oblique  attachment  to  the 
posterior  abdominal  wall,  from  the  duodenum  to  the  right  iliac  fossa,  is 
effected  by  secondary  adhesions  which  are  formed  after  the  rotation  of  the 
gut  and  during  the  4th  and  5th  months,  and  this  extensive  attachment  is 
found  only  in  animals  adapted  to  the  upright  posture.  The  last  part  of 
the  mesentery  to  become  adherent  to  the  posterior  wall  of  the  abdomen 
is  the  angular  area  between  the  ileum  and  ascending  colon.  Not  unfre- 
quently  this  part  remains  free,  and  it  is  then  possible  for  a  volvulus  to 
form  by  a  rotation  of  the  ileo-colic  loop. 

By  the  rotation  of  the  U-shaped  loop,  the  small  intestine  becomes 
confined  in  a  bursa  or  peritoneal  compartment  formed  by  the  mesentery 
of  the  large  bowel  (Fig.  310). 

Abnormal  Fixation  of  the  Mesentery. — The  rotation  of  the  bowel 
is  subject  to  three  forms  of  disturbance,  giving  rise  to  three  varieties  in 
the  fixation  of  the  mesentery,  which  are  of  importance  to  medical  men. 
(1)  The  bowel  may  undergo  its  normal  rotation,  but  the  process  of  adhesion 
may  fail ;  the  bowel  is  thus  suspended  by  a  free  fan-shaped  mesentery. 
During  life  it  may  become  twisted  round  its  stalk,  formed  by  the  superior 
mesenteric  artery,  and  thus  give  rise  to  obstruction  of  the  bowel  (complete 
volvulus).  (2)  It  may  not  undergo  a  rotation  ;  the  caecum  then  lies  on  the 
left  side  of  the  abdomen,  and  the  colon — ascending  and  descending — are 
situated  behind  and  to  the  left  of  the  small  bowel.  (3)  The  rotation  may 
occur  in  a  direction  opposite  to  the  normal — the  duodenum  and  mesentery 
coming  to  lie  in  front  of  the  transverse  colon  in  place  of  being  situated 
behind  it.  Several  cases  of  this  nature  have  been  recorded  of  late  by 
surgeons  and  anatomists. 

Meconium. — At  birth,  the  great  intestine  and  the  ileum  are  distended 
by  meconium,  a  black,  semi-fluid  substance  secreted  by  the  liver  and 
mucous  membrane  of  the  bowel.  Dr.  A.  Low  found  that  the  meconium 
reaches  the  ileo-colic  junction  in  the  4th  month,  the  rectum  in  the  5th. 
The  meconium  passes  quickly  along  the  jejunum.  At  birth  the  lower 
part  of  the  ileum  and  whole  of  the  great  intestine  are  distended  with  it. 
By  the  3rd  or  4th  day  after  birth  all  the  meconium  has  been  passed,  a 
fact  which  may  be  utilized  to  prove  that  a  child  had  lived  for  a  certain 
time  after  birth. 


CHAPTER  XX. 


CIRCULATORY  SYSTEM. 

Early  Stages  in  the  Evolution  of  the  Heart.— In  Ammocoetes, 
the  larval  form  of  the  lamprey,  is  represented  the  most  primitive  form  of 
heart  in  vertebrate  animals.  Even  in  this  early  type  the  heart  consists 
of  four  chambers  (Fig.  311)  :  (1)  Sinug  venosus,  receiving  the  portal  blood 
through  the  liver  ;  (2)  auricle  ;  (3)  ventricle  ;  (4)  bulbus  cordis,  from 
which  the  primitive  ventral  aorta  passes  out  to  distribute  the  blood  in  the 
branchial   chamber.     The   primitive   heart   is   thus   a   respiratory   pump 


PERICARD; CAV 


PE.RlTON.CA/ 


Fig.  311. 


LIVER 

S1N03   VE.NOSUS 
KlCI-t. 

VENTRAL  AORTA 

-The  Heart  of  AmmoccEtes  seen  in  a  Median  Section.     (After  Vialleton.) 


which  forces  the  portal  blood  through  a  branchial  system.  It  is  clear, 
then,  that  the  early  evolutionary  stages  of  the  heart  must  be  sought  for 
amongst  invertebrate  forms,  but  these  stages  are  as  yet  unknown.  When 
the  heart  appears  in  the  human  embryo  towards  the  end  of  the  3rd  week, 
it  is  double — consisting  of  a  right  and  left  cardiac  tube.  We  therefore 
suppose  that  originally  there  were  right  and  left  hearts,  which  arose  as 
modifications  of  the  vessels  which  convey  the  blood  from  the  alimentary 
to  the  respiratory  systems.  In  Fig.  312  the  left  side  of  such  a  primitive 
circulation  is  represented.  The  left  heart  forces  the  blood  along  a  primitive 
dorsal  aorta  to  the  capillary  system  of  the  archenteron.  An  afferent 
(primitive  portal)  vessel  conveys  the  blood  back  to  the  heart.  When 
the  head  and  tail  folds  are  produced  in  the  embryonic  plate  at  the  beginning 
of  the  4th  week  (see  Fig.  312),  the  right  and  left  cardiac  tubes  are  thrust 
under  the  fore-gut,  where  they   speedily  become  fused  into  a   median 

301 


302 


HUMAN  EMBRYOLOGY  AND  MORPHOLOaY 


heart. 1  In  its  origin  the  heart  is  thus  made  up  of  symmetrical  halves 
derived  from  the  corresponding  sides  of  the  body.  When  formed,  the  heart 
is  suspended  within  the  anterior  part  of  the  coelomic  space — which  becomes 
the  cavity  of  the  pericardium.  In  Ammocoetes  the  pericardial  and  peri- 
toneal cavities  are  continuous  (Fig.  311). 

Angioblastic  Tissue. — That  the  cardiac  tube  has  arisen  by  the  modi- 
fication of  a  blood  vessel  is  apparent  by  the  way  it  commences  to  form  in 
the  human  embryo.  Late  in  the  3rd  week  certain  cells  become  grouped 
under  the  fore-gut  to  form  the  lining  membrane  of  the  heart.  At  the 
same  date  similar  cells  in  the  chorionic  villi,  in  the  wall  of  the  yolk  sac  and 
along  the  tracks  of  the  future  aortae,  are  grouping  themselves  in  an 
identical  manner  to  form  the  lumina  of  blood  channels.  The  mesodermal 
cells  which  have  this  vessel-forming  power  pervade  the  whole  embryonic 


head  fold^ 
foregut 
aortic  arch 

heart 
coelom 
yolk  sac 


embryonic  plate 

dorsal Morta 
y^ -r~tail  fold 
mnd  gut 


somatopleure 
splanchnopleure 


Fig.  312. — Diagram  showing  the  Relationship  of  the  Heart  to  the  Archenteron  of 
the  Developing  Ovum.  The  outgrowth  of  the  head  fold  is  indicated  carrying 
a  process  (fore-gut)  of  the  Archenteron  and  also  the  Aorta  and  Heart.  The 
outgrowth  of  the  tail  fold  and  hind-gut  is  also  shown.     (After  A.  Robinson.) 

mass  and  are  known  as  angioblastic  tissue.  One  group  of  angioblasts 
unites  with  neighbouring  groups,  thus  forming  a  network.  Further, 
angioblasts  not  only  form  the  lining  cells  and  lumina  of  blood  vessels  but 
also  produce  the  blood  cells  and  plasma  which  fill  them.  A  "  blood 
island  "  is  a  group  of  angioblasts  surrounding  a  brood  of  nucleated  red 
corpuscles.  When  neighbouring  islands  unite  the  essential  part  of  the 
circulatory  system  has  come  into  existence.  The  lining  of  the  heart  arises 
in  the  same  manner  as  a  simple  capillary.^ 

Later  Stages  in  the  Evolution  of  the  Heart. — So  long  as  the  heart 
is  merely  a  pump  for  the  gills,  it  retains  the  simple  structure  seen  in 
Ammocoetes — but  with  the  origin  of  a  pulmonary  system  a  series  of  most 
remarkable  changes  occur.     The  pulmonary  system  in  the  human  embryo 

1  Recent  papers  on  fusion  of  cardiac  tubes  :  Chung-Ching  Wang,  Journ.  Anat.  1918, 
vol.  52,  p.  107  ;  H.  W.  Schulte,  Amer.  Journ.  Anat.  1916,  vol.  20,  p.  45  ;  Henry  A. 
Murray,  Amer.  Journ.  Anat.  1919-20,  vol.  26,  p.  29. 

^  For  recent  papers  on  angioblasts  :  J.  L.  Bremer,  Amer.  Journ.  Anat.  1914,  vol.  16, 
p.  447  ;  Florence  R.  Sabin,  Contrib.  to  Embryology,  1920,  vol.  9,  p.  213. 


CIRCULATORY  SYSTEM 


303 


takes  on  its  definite  form  during  the  second  montli ;  at  the  same  time  the 
heart  is  undergoing  a  series  of  changes,  which  converts  it  into  a  double 
pump,  one  for  the  lungs,  another  for  the  body.  We  know  that  these 
evolutionary  changes  occurred  slowly,  for  in  amphibia  the  heart  has 
reached  that  point  in  evolution  where  a  single  ventricle  can  serve  both  the 
respiratory  and  systemic  circulations.  The  evolution  of  a  pulmonary 
system  also  led  to  a  series  of  changes  in  the  arrangements  of  veins. 
Amongst  the  most  remarkable  of  these  is  the  formation  of  a  new  passage, 
by  which  the  blood  of  the  abdomen  can  pass  direct  to  the  heart — the 
inferior  vena  cava.  In  the  human  embryo  of  the  5th  week  the  heart  and 
great  veins  are  arranged  as  in  a  fish  ;  in  the  7th  week  they  take  on  the 
definite  mammalian  form. 

Fixation  of  the  Heart.— At  the  beginning  of  the  4th  week  (Fig.  279) 
the  heart  lies  free  within  the  pericardium,  with  its  two  extremities  fixed 
to  the  wall  of  that  cavity  (Figs.  330,  333).     Its  anterior  or  arterial  extremity 


V- -int.  jug. 


rt  subclau. 


vena 
azyg.  maj.^ 

from  card.-^ 


prim.  jug. 


from  prim.  jug. 
pericardium 
from  rt.  d.  of  Cuuier 


rt  aur. 


pericar. 
\    i    aur 

'sin.  uen. 


cardin.  ueini 


\'  /eft  duct 

^pleura  of  Cuuier 

K  rt.  duct 
\of  Cuuier 


Fig.  313. — The  Superior  Vena  Cava  of  the  Adult. 

Fig.  314. — The  Embryonic  Venous  Trunks  out  of  which  the  Superior  Vena  Cava 

is  formed.      The  arrow  is  in  the  communication  between  the  pericardial  and 

pleuro-peritoneal  cavities.    (See  Fig.  315.) 

perforates  the  dorsal  wall  of  the  pericardium  to  give  of!  the  aortic  arches 
in  the  floor  of  the  pharynx  (Fig.  248).  The  venous  or  posterior  end  is 
fixed  to  the  septum  transversum,  the  embryonic  partition,  which  is  formed 
between  the  pericardial  and  peritoneal  cavities  (Fig.  279).  The  fate  of 
the  aortic  arches,  which  convey  the  blood  from  the  ventral  to  the  dorsal 
aorta,  has  been  already  traced  (p.  251).  We  now  propose,  before  surveying 
the  complicated  changes  which  ensue  in  the  heart  itself,  to  trace  the 
evolution  of  those  great  venous  channels  which  convey  the  blood  to  the 
heart — the  venae  cavae. 

1.  The  superior  vena  cava  arises  from  the  following  foetal  vessels  (Figs. 
313,  314)  : 

(a)  The  part  above  the  entrance  of  the  vena  azygos  is  the  terminal 
part  of  the  right  anterior  cardinal  (primitive  jugular)  vein  ; 

(6)  The  part  below  the  entrance  of  the  vena  azygos  major  represents 
the  right  duct  of  Cuvier.  The  condition  of  these  venous  trunks,  the 
anterior  and  posterior  cardinal  veins  and  ducts  of  Cuvier,  in  a  human 
embryo  of  the  4th  week,  is  shown  in  Figs.  314,  315.  The  condition  shown 
in  these  figures  is  retained  permanently  in  fishes. 


304 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


The  anterior  cardinal,  which  drains  the  anterior  half  of  the  body  on  each 
side,  with  the  posterior  cardinal  vein,  which  drains  the  posterior  half  of  the 
body,  receive  a  tributary  (segmental  vein)  from  each  body  segment. 
In  Fig.  315  the  anterior  and  posterior  cardinal  veins  on  each  side  are 
shown  uniting  to  form  the  duct  of  Cuvier,  which  conveys  the  blood  to  the 
sinus  venosus — a  contractile  chamber  opening  into  the  primitive  auricle. 
The  sinus  venosus  remains  as  a  separate  chamber  of  the  heart  in  lower 
vertebrates,  but  in  the  course  of  mammalian  development  it  becomes 
partly  merged  in  the  right  auricle  of  the  heart. 

It  is  important  to  notice  how  the  ducts  of  Cuvier  reach  the  sinus  venosus 
(see  Figs.  315,  330,  280).  They  pass  from  the  dorsal  to  the  ventral  surface 
of  the  body  in  the  somatopleure  or  body  wall,  and  enter  the  septum  trans- 


stomodaeum 
aorta 


jug.  vein 

card.  uein. 
somatopleure 

spfanchnopleure'/.^^^^^^ 


^prfrn.  auricle 
sinus  uenosus 
pericardium 
'uct  of  Cuuier 
pleura, 
peritoneum 


Fig.  315. — Diagram  to  show  the  manner  in  which  the  Ducts  of  Cuvier  encircle  the 
Coelom  at  the  junction  of  the  Pericardial  and  Peritoneal  Passages  at  the  4th 
week.    (After  His.) 

versum  to  reach  the  sinus  venosus,  thus  encircling  the  coelomic  passages 
passing  from  the  pericardial  to  the  pleuro-periteal  cavities.  Thus  the 
exit  from  the  pericardial  cavity  to  the  pleural  passage  is  surrounded  by  the 
great  venous  channels — the  ducts  of  Cuvier  ;  hence  the  exit  is  sometimes 
named  the  iter  venosum  or  pericardio-pleural  passage.  Ultimately,  by 
the  end  of  the  6th  week,  the  part  of  the  coelom  lying  in  front  of  the  ducts 
of  Cuvier  and  septum  transversum  is  cut  off  from  the  rest ;  the  part  so 
cut  ofi  forms  the  pericardium.  In  the  4th  week  the  dorsal  end  of  the 
septum  transversum  is  situated  opposite  to  the  2nd  cervical  segment ; 
by  the  end  of  the  6th  week,  the  embryo  being  about  10  mm.  long,  it  has 
shifted  backwards  so  as  to  lie  on  a  level  with  the  3rd  thoracic  segment, 
in  this  way  bringing  the  duct  of  Cuvier  into  an  oblique  position  (Fig.  330). 
Thus  the  ducts  of  Cuvier  are  instrumental  in  separating  the  pericardial 
from  the  pleural  cavity.    If  the  primitive  pleuro-pericardial  communication 


CIECULATORY  SYSTEM 


305 


(iter  venosum  of  Lockwood)  persists  between  them,  it  occurs  as  a 
foramen  in  the  pericardium  behind  the  part  of  the  superior  vena  cava 
derived  from  the  duct  of  Cuvier^  On  the  left  side  the  duct  of  Cuvier 
atrophies,  and  the  iter  venosum^  if  it  persists,  is  then  represented  by  an 
aperture  in  the  pericardium  in  front  of  the  root  of  the  left  lung  (Fig;  316). 
The  ducts  of  Cuvier,  and  the  folds  of  the  somatopleure  in  which  they  liCj 
are  separated  from  the  body  wall  and  buried  deep  in  the  thorax  by  the 
development  of  the  lungs  and  pleurae. 

2.  The  Vestigial  Fold  and  Oblique  Vein  of  Marshall.— In  the  human 
embryo,  during  the  4th  week,  and  for  two  weeks  afterwards,  there  is  a 
right  and  left  duct  of  Cuvier  and  corresponding  cardinal  veins  (Fig.  319). 
A  left  superior  vena  cava  is  present  and  may  persist  (Fig.  317).  The 
vestigial  fold  and  oblique  vein  of  Marshall  (Fig.  318)  are  all  that  usually 
remain  of  the  left  superior  vena  cava.     The  right  superior  vena  cava. 


PHRENi  NERVE. 

LI 

SUP; 

VEN 

CAvf 

^\          /     R]_g!JP:VEN;CA.V: 

_DUCT:ARTER: 

~~"^~— -. 

^StM    jk 

j-ROOTOFUUNG 

^^iM^^^^^^r  SINUS   VENOSUS 

ITER   VENOBUM 

PULM:V 

w 

^AORTA 

■^  PERICARD: 

^          RT  VENTRICLE 

~  DIAPH: 

Fig.  316. — Heart  of  a  Child,  showing  an  Abnormal  Aperture  in  the  Pericardium 
in  front  of  the  root  of  the  Left  Lung,  representing  a  patent  Iter  Venosum  or 
Pericardio-pleural  passage  of  the  Embryo.  The  left  auricle  is  seen  within  the 
aperture. 

Fig.  317. — Abnormal  Heart  of  a  Child  seen  from  behind,  showing  Persistence  of 
the  Left  Duct  of  Cuvier,  absence  of  the  Inferior  Vena  Cava,  and  with  the  Pul- 
monary Veins  terminating  in  the  Sinus  Venosus.  A  similar  condition  is  seen 
in  certain  fishes. 


within  the  pericardium,  passes  in  front  of  the  right  pulmonary  vessels, 
and  is  bound  to  them  by  a  mesentery  or  fold  of  serous  pericardium  ;  the 
left  has  a  similar  relationship  (Fig.  318)  ;  when  it  disappears  the  peri- 
cardial reflection  remains  in  front  of  the  left  pulmonary  vessels  as  the 
vestigial  fold.  The  intra-pericardial  part  of  the  left  vena  cava  or  duct  of 
Cuvier  becomes  the  oblique  vein :  it  turns  round  the  left  auricle  to  term- 
inate in  the  left  horn  of  the  sinus  venosus  (coronary  sinus).  The  extra- 
pericardial  part  of  the  left  duct  of  Cuvier  joins  the  superior  intercostal 
vein  (Fig.  318).  Both  right  and  left  superior  venae  cavae  persist  in  some 
lower  mammals,  and  occasionally  this  is  also  the  case  in  man.  The  left 
superior  vena  cava  begins  to  atrophy  when  the  common  auricular  chamber 
is  divided  into  a  right  and  left  compartment  in  the  6th  and  7th  weeks. 

The  left  superior  intercostal  vein  represents  the  following  embryonic 
vessels  (see  Fig.  318)  :  (a)  Anterior  part  of  the  left  posterior  cardinal  vein  ; 
(6)  The  extra-pericardial  part  of  the  left  duct  of  Cuvier  ;  (c)  The  terminal 
part  of  the  left  primitive  jugular  vein. 


306 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


3.  Left  Innominate  Vein  opens  up  as  a  channel  of  communication 
between  the  two  primitive  jugular  veins,  the  left  superior  vena  cava 
undergoing  a  simultaneous  process  of  atrophy  (Fig.  318). 

4.  Subclavian  Veins  are  developed  in  the  5th  week  with  the  outgrowth 
of  the  fore-limb  buds  ;  they  are  developed  from  the  vein  of  the  7th  cervical 
segment  and  at  first  end  in  the  posterior  cardinal  vein.  As  the  neck 
and  thorax  become  demarcated  in  the  2nd  month,  the  termination  of  the 
subclavian  veins  is  shifted  until  it  ends  in  the  anterior  cardinal  (primitive 
jugular)  vein. 

5.  Jugular  and  Cerebral  Veins. — In  the  6th  week  each  anterior  cardinal 
vein  commences  in  a  corresponding  primitive  head  vein  (see  page  133). 
Each  primitive  head  vein  passes  along  the  base  of  the  skull,  receiving  the 


rt.  d. 

of  Cuuier- 


sub.  clau. 

'"  ^        sub.  clau: 

(Jeft  sup.  intercost 
I  (left Jug.) 

left  sup.  intercost. 

(left  card. ) 
(     ,       pericard. 
p 
^left  duct  of  Cuuier 

-fold  of  Marshall 
pericardium 


sub.  clau. 


-left  jug. 
pericard. 


ypjMeft  d.  Cuuier 
^^\^left  card 

[ 


-left  horn, sinus  uenosus 
Eustach.  value 

Fig.  318. — The  Remnants  of  the  Left  Superior  Vena  Cava,  derived  from  the 

Structures  shown  in  Fig.  69. 
Fig.  319. — Diagram  of  the  Sinus  Venosus  and  Ducts  of  Cuvier  of  the  Human 

Embryo  about  the  4th  weels. 

veins  from  the  fore-,  mid-  and  hind-brains,  and  makes  its  exit  by  the  jugular 
foramen,  where  it  becomes  the  jugular  or  anterior  cardinal  vein. 

Posterior  Cardinal  Veins  and  their  Derivatives.^ — In  Figs.  320,  321 
a  schematic  representation  of  the  origin  of  the  inferior  vena  cava  and 
azygos  veins  is  given.  In  the  5th  week  the  posterior  cardinals  commence  by 
the  union  of  the  veins  from  the  limb  buds  and  sacrum  and  passing  forwards, 
dorsal  to  the  developing  Wolffian  ridge,  receive  as  they  go  a  tributary 
from  each  somite — tributaries  which  will  become  the  lumbar,  intercostal 
and  lower  cervical  veins,  and  end  in  the  veins  of  Cuvier.  With  the  de- 
velopment of  the  nephric  or  Wolffian  system,  a  large  tributary — the 
subcardinal  vein — appears  on  the  mesial  side  of  the  system  or  body, 
collecting  blood  from  and  pouring  it  into  the  posterior  cardinal  vein  at  the 

^  Florence  Sabin,  Gontrib.  Embryology,  1915,  vol.  3,  p.  5  ;  Huntington  and  M'Clure, 
Anat.  Record.  1908,  vol.  1,  p.  36;  ibid.  1920,  vol.  20,  p.  1. 


CmCULATORY  SYSTEM 


307 


cephalic  end.  ol  the  nephric  body.  Ultimately  the  subcardinal  veins  extend 
their  origin  in  a  caudal  direction  and  effect  a  connnunication  with  the 
hinder  part  of  the  posterior  cardinal  veins  (Fig.  320).  There  is  thus 
established  a  reno-portal  system  comparable  to  that  seen  in  amphibia 
(Fig.  322).  A  wide  cross  channel  (intern ephric)  opens  between  the  sub- 
cardinals. 

From  the  right  posterior  cardinal  vein  are  formed  (1)  the  vena  azygos 
major  ;    (2)  the  ascending  lumbar  vein. 


COM.i^/AC-l/ 


Fig.  320. — Scheme  of  the  arrangement  of  body  veins  about  the  end  of  the  6th  week  of 

development.     The  sites  of  new  channels  are  stippled. 
Fig.  321. — Scheme  showing  the  derivation  of  the  body  veins  of  the  adult. 

From  the  left  cardinal  arise  (Figs.  320,  321) — (1)  Part  of  the  left  superior 
intercostal  vein  ;  (2)  left  superior  azygos  vein  ;  (3)  left  inferior  azygos  ; 
(4)  the  left  ascending  lumbar  vein. 

Inferior  Vena  Cava. — The  transformation  of  the  cardinal  system  and 
the  development  of  a  new  caval  channel  to  convey  the  blood  in  the  systemic 
veins  of  the  abdomen  direct  to  the  heart,  take  place  during  the  2nd  month 
as  the  pulmonary  system  begins  to  expand.  With  the  evolution  of  lungs, 
respiratory  movements  of  the  body  wall  were  introduced — a  new  force 
which  was  utilized  to  assist  the  return  of  the  venous  blood  to  the  heart. 
The  development  of  pleural  cavities  made  the  old  or  cardinal  route  cir- 
cuitous and  difficult  and  hence  a  new  or  direct  passage  became  necessary — 
the  inferior  vena  cava.  It  became  fashioned  thus  ;  a  retrohepatic  anasto- 
mosis between  the  right  subcardinal  and  terminal  part  of  the  right  vitelline 
vein  (Fig.  320)  opened  up  and  thus  the  blood  of  the  subcardinal  system 
could  pass  straight  to  the  heart.     The  pre-renal  or  retrohepatic  part  of  the 


308 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


inferior  vena  cava  is  formed  out  of  this  new  channel.  The  post-renal 
part  is  formed  from  the  right  subcardinal  vein.  A  cross  channel  (pre- 
sacral) opens  up  between  the  right  and  left  cardinal  systems — forming  the 
greater  part  of  the  left  common  iliac  vein — and  so  all  the  blood  from  the 
pelvis  and  pelvic  limbs  passes  to  the  right  subcardinal  as  it  becomes  con- 
verted into  a  permanent  channel.  The  left  subcardinal  ^  vein  persists 
not  unfrequently — giving  rise  to  a  double  or  divided  inferior  vena  cava. 
The  internephric  channel  becomes  the  left  renal  vein,  the  terminal  parts 
of  both  subcardinal  veins  persist  as  communications  between  the  renal  and 
azygos  veins  (Fig.  321).     From  Figs.  320,  321,  it  will  be  seen  that  part  of 


sup.  uen.  cau. 

rt.  aur. 
diaph 
right  azygos — 

inf.  uen.  cau. 
(in  dors,  mesentj 


mesonephros 


card,  uein- 
(inf.  uen.  cau.) 


left  duct  ofCuu. 
^left  azygos 
hepatic  circ. 
■port,  uein 

umb.  uein 
:« (in  uent  mesent) 


^t^esonepliros 


uesic.  plex. 
fern,  uein 


Fid.  322. — The  Arrangement  of  the  Cardinal,  Umbilical  and  Inferior  Caval 
Veins  in  Lower  Vertebrates.  The  venous  blood  from  the  posterior  part  of 
the  body  passes  through  either  the  renal  or  hepatic  circulations  before 
reaching  the  heart.     (After  Hochstetter.) 

the  common  iliac  veins  are  derived  from  the  hinder  part  of  the  cardinal 
system  of  veins. 

Portal  Vein. — The  Portal  Vein  is  formed  out  of  the  terminal  parts  of 
the  two  vitelline  veins.  They  end  in  the  posterior  chamber  of  the  tubular 
heart  of  the  embryo — the  sinus  venosus.  The  vitelline  veins,  right  and 
left,  arise  from  ramifications  on  the  yolk  sac  and  pass  in  the  ventral  mesen- 
tery of  the  fore-gut  to  the  sinus  venosus  (Fig.  323).  The  nutriment 
within  the  yolk  sac  is  thus  carried  to  the  heart  and  distributed  by  the  heart 
to  the  tissues  of  the  embryo  and  yolk  sac.  With  the  differentiation  of 
the  gut  from  the  yolk  sac,  the  parts  of  the  vitelline  veins,  at  first  situated 

1  For  literature  and  description  of  cases  of  abnormal  development  of  the  posterior 
cardinal  veins,  see  Dr.  Gladstone's  article  in  Journ.  Anat.  and  Physiol.  1912,  vol.  46, 
p.  220  ;  J.  Cameron,  ibid.  1911,  vol.  45,  p.  416  ;  T.  B.  Johnston,  ibid.  1913,  vol.  47, 
p.  235  ;  H.  Rischbieth,  ibid.  1914,  vol.  48,  p.  290  ;  W.  E.  Collinge,  ibid.  1916,  vol. 
50,  p.  235. 


CIRCULATORY  SYSTEM 


309 


on  the  yolk  sac,  fuse  together  in  the  dorsal  mesentery.  Thus  while  the 
terminal  parts  of  the  vitelline  veins  lie  in  the  ventral  mesentery  of  the 
fore-gut,  the  three  tributaries  of  the  portal  vein — the  splenic  vein  from 
the  fore-gut,  the  inferior  mesenteric  from  the  hind-gut,  and  the  superior 
mesenteric  from  the  mid-gut  (Fig.  323) — lie  in  the  dorsal  mesentery.  They 
are  developed  as  tributaries  of  the  vitelline  veins,  for  we  have  already 
seen  that  the  veins  of  the  yolk  sac  may  persist  as  a  cord  which  joins  the 
superior  mesenteric  vein  below  the  pancreas  (see  Fig.  301).  The  duo- 
denum forms  a  loo]:)  between  the  vitelline  veins  (Fig.  324),  and  hence  on 


right  uit  vein 

hepatic  bud. 
vent,  mesentery 
left,  vitelline  vein. 


sinus  uenosus 

left  duct  of  Cuvier 

left  umb.  vein 

stomach 

spleen 
dorsal  mesentery 
splenic  vein 
inf.  mes.  vein 


hind  gut 


yolk  sac. 


sup.  mes.  vein 

Fig.  323.— The  Left  Vitelline  Vein  of  an  Embryo  of  the  5th  week. 

either  side  of  the  1st  and  3rd  stages  of  the  duodenum  the  vitelline  veins 
remain  separate,  while  in  front,  between  and  behind  these  stages,  they  are 
united  by  anterior,  middle  and  posterior  junctions  (see  Fig.  321:). 

The  portal  sinus  in  the  transverse  fissure  of  the  liver  is  formed  out  of 
the  anterior  junction  of  the  right  and  left  vitelline  veins  in  the  ventral 
mesentery  (Figs.  324,  282)  ;  the  part  of  the  portal  vein  in  the  gastro- 
hepatic  omentum  (ventral  mesentery),  and  behind  the  1st  stage  of  the 
duodenum,  is  formed  from  the  right  vitelline  vein  ;  the  corresponding 
part  of  the  left  vein  disappears  ;  the  commencement  of  the  portal  vein 
— in  the  neck  of  the  pancreas — represents  the  middle  junction  of  the  two 
vitelline  veins  (Fig.  324)  ;  the  terminal  part  of  the  superior  mesenteric 
vein,  which  in  the  adult  lies  in  front  of  the  3rd  stage  of  the  duodenum, 
represents  a  part  of  the  left  vitelline  vein — ^the  corresponding  part  of  the 
right   disappears   (Fig.    283).     To   understand   the   transmutation   which 


310 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


leads  to  the  formation  of  the  portal  vein,  it  must  be  remembered  (1)  that 
the  duodenum  forms  at  first  a  free  loop,  the  right  surface  of  which  after- 
wards becomes  applied  to  the  posterior  wall  of  the  abdomen  ;  (2)  the 
pancreas  is  developed  in  its  dorsal  mesentery  ;  (3)  the  ventral  mesentery, 
in  which  the  liver  is  developed,  is  attached  to  the  anterior  part  of  the 
loop  (Fig.  323). 

Hepatic  Veins  are  formed  out  of  the  terminal  parts  of  the  vitelline  veins. 
These  veins  end  at  first  in  the  sinus  venosus  (Figs.  282,  283,  324).  The 
liver  is  developed  between  and  around  their  terminal  parts  (see  p.  273). 
Thus  it  comes  about  that  the  vitelline  veins  are  transformed  into  the 


rt  umb. 

rt.  vit. 

inf.  uen.  cau. 

ductus  uen. 

in  transv.  fisX 
of  liver    j 

portal  uein 


.■:L  vein  of  Marshall 
stomach 

remnants  of  left  umb. 


rt  vit.  vein 


left  vit.  vein 
part  disappears 
left  umb.  vein 

rt.  umb.  vein  (disappears) 
duodeno-jej.  f leu  re 

left  vit.  vein. 

left  vit  uein 

sup.  mesent  vein 


Fig.  324. — Diagram  showing  the  Formation  of  the  Ductus  Venosus,  and  the  fate 
of  the  Umbilical  and  VitelKne  Veins.  The  arrows  show  the  parts  of  the 
vitelline  veins  which  become  the  portal  vein. 

veins  of  the  portal  and  hepatic  circulation.  All  the  foetal  and  umbilical 
blood  is  at  first  poured  through  the  liver. 

Ductus  Venosus  is  a  new  channel  formed  in  the  5th  week  between  the 
portal  sinus  and  the  terminal  part  of  the  right  vitelline  vein,  whereby  the 
greater  part  of  the  umbilical  blood  is  short-circuited  to  the  sinus  venosus 
without  passing  through  the  liver.  After  birth,  when  a  short  circuit  is  no 
longer  required  between  the  placental  circulation  and  heart,  it  becomes 
reduced  to  a  fibrous  cord.^  It  occupies  the  posterior  part  of  the  longi- 
tudinal fissure  of  the  liver  and  lies  within  the  hepatic  attachment  of  the 
gastro-hepatic  omentum  (Fig.  325). 

Umbilical  Veins. — The  umbilical  vein  at  birth  consists  of  two  parts  : 
(1)  A  part  within  the  umbilical  cord  ;  (2)  another  within  the  body,  enclosed 
in  the  falciform  ligament  and  anterior  half  of  the  longitudinal  fissure  of  the 
1  See  Scammon  and  Norris,  Anat.  Bee.  1918,  vol.  15,  p.  165, 


CIRCULATORY  SYSTEM 


311 


liver.  It  joins  there  the  ductus  venosus  and  portal  sinus  (Fig.  32.5).  The 
arrangement  of  the  umbilical  veins  in  a  human  embryo  of  the  3rd  week 
is  shown  in  Fig.  25,  and  of  the  5th  week  in  Fig.  326.  The  vessel  from  which 
the  umbilical  veins  have  been  evolved — the  lateral  vein  of  lower  verte- 
brates— is  illustrated  in  Figs.  27  and  322.  In  the  body  stalk  the  umbilical 
veins  have  already  fused  (Fig.  326),  but  in  the  body  wall  and  ventral 
mesentery,  in  which  they  pass  to  reach  the  sinus  venosus,  they  remain 
separate.  With  the  differentiation  and  closure  of  the  umbilicus,  the 
parts  of  the  body  wall  in  which  the  umbilical  veins  are  situated  are  drawn 
out  to  form  the  umbilical  cord.  The  intra-embryonic  parts  then  lie  within 
the  ventral  mesentery  of  the  fore-gut,  lateral  and  ventral,  to  the  vitelline 
veins.     By  the  umbilical  veins  the  blood  is  returned  from  the  placenta  to 


gastro-fiep.]  , 
oment  j-jr 

duct,  venosus 


diaph. 
J — inf.  u.  cau. 


falc.  lig.. 


round  lig.^ 


umb.  vein 
round  lig. 


umb. 


Fig.  325. — Diagram  of  the  Remnants  of  the  Umbilical  Vein  in  the  Adult — as  seen 
on  the  dorso-ventral  sm'face  of  the  liver. 


the  heart.  In  nearly  all  vertebrate  embryos  the  vitelline  veins  are  the  first 
of  all  the  vessels  of  the  body  to  be  developed,  but  in  the  Higher  Primates, 
including  man,  this  appears  not  to  be  the  case.  Professor  Eternod  found 
that  in  a  human  embryo,  of  about  21  days,  the  umbilical  veins  and  the 
venous  sinuses  of  the  chorion  were  already  in  process  of  formation,  while 
the  vitelline  veins  had  not  yet  appeared  (Fig.  25).  We  have  already  seen 
(Chap.  II.)  that  the  Higher  Primates  are  remarkable  for  the  precocious 
development  of  the  chorion  ;  this  early  differentiation  of  the  chorion  is 
attended  by  an  equally  early  formation  of  the  umbilical  vessels,  which 
return  the  blood  from  the  chorion  to  the  heart. 

The  outgrowth  of  the  liver-bud  within  the  ventral  mesentery  breaks 
up  not  only  the  vitelline  veins,  but  also  the  umbilical  at  their  junction  with 
the  sinus  venosus  (Figs.  282,  283).  The  intra-embryonic  part  of  the 
right  umbilical  vein  atrophies,  while  the  left  enlarges.  With  the  terminal 
parts  of  the  vitelline  veins  the  opposite  is  the  case.  Thus  the  umbilical 
blood  as  well  as  the  vitelline  comes  to  be  poured  into  the  liver.     The 


312 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


termination  of  the  left  umbilical  vein  is  gradually  transferred  during  the 
6th  and  7th  weeks  from  the  sinus  venosus  to  the  portal  sinus  (p.  273). 
The  left  umbilical  vein  thus  comes  into  communication  with  the  ductus 
venosus  (see  Figs.  324,  325). 

The  Heart  as  a  Placental  Pump. — Having  thus  traced  the  origin  of 
the  great  veins  which  conduct  the  blood  to  the  heart  we  now  turn  to  the 
development  of  this  organ.  In  the  4th  and  5th  weeks  the  umbilical 
veins  are  fully  established  (Fig.  326)  and  the  heart  is  receiving  the  major 
part  of  its  blood  from  the  chorion,  and  its  chief  task  is  to  serve  as  the 


right  duct, 
of  Cuuierj 


right  umb.  vein 
in  somato-pleure 


pericardium 

sinus  uenosus 
sept  transuersum 
rt.  uit.  vein  on  yolli  sac 

left  umb.  vein 


united  umb.  veins 
cord 


chorion 


rzny^ 


Fig.  326. — Diagram  of  the  Right  Umbilical  Vein  of  a  5th  weels:  Embryo  before  the 
Outgrowth  of  the  Liver  Trabeculae.     (Modified  from  His.) 

pump  of  that  organ.  Hence  the  large  size  of  the  heart  and  pericardium 
when  compared  with  the  actual  dimensions  of  the  embryo  itself  (Fig.  326) 
— or  the  individual  organs  such  as  the  stomach.  Angioblastic  cells  are 
being  transformed  into  vascular  structures  at  the  end  of  the  3rd  and 
beginning  of  the  4th  weeks,  and  although  vascularization  proceeds  at  an 
extremely  rapid  pace,  it  is  late  in  the  4th  week  before  an  effective  circulation 
has  been  established. 

Cardiac  Tubes  and  Pericardium. — In  Fig.  327  is  shown  a  coronal 
section  of  the  forward  projection  of  the  head  region  of  a  human  embryo 
in  which  the  neural  canal  is  still  open  and  in  which  only  five  body  segments 
are  demarcated — about  the  beginning  of  the  4th  week.  The  cardiac  tubes 
are  seen  in  process  of  fusion.     Under  the  fore-gut  is  seen  the  angioblastic 


CIRCULATORY  SYSTEM 


313 


cells — representing  the  endothelial  lining  of  the  heart  ;  the  walls  of  the 
tubes  clearly  represent  foldings  of  the  visceral  layer  of  the  mesoderm 
— for  they  are  seen  to  be  still  continuous  with  the  mesodermal  covering 
of  the  fore-gut.  The  pericardial  part  of  the  coelomic  space  is  already 
formed.  It  came  into  existence  during  the  latter  part  of  the  3rd  week 
— by  a  process  of  cleavage  which  separated  the  mesoderm  lying  under  the 
fore-gut  into  visceral  and  somatic  layers.  While  the  heart  tubes  are  separ- 
ated from  the  somatic  or  parietal  layer  of  mesoderm,  they  remain  attached 
to  the  floor  of  the  fore-gut  by  the  dorsal  mesocardium.  No  ventral  meso- 
cardium  is  formed.  Sections  showing  the  evolutionary  origin  of  the 
pericardium  and  of  the  mesodermal  wall  of  the  heart  are  shown  in  Figs. 
149,  327  and  352. 

In  Fig.  328  a  corresponding  section  of  an  embryo  a  few  days  older  is 
represented.     The  process  of  fusion  is  complete  and  already  the  cardiac 


NEUR    PLATE 


DORSAL   MESOC 
TH UNCUS 


6  somites 


ENOOTHEL.  TUBE: 

SUBENDOTH    HETIC 

15  somites 


myoge:nic  tissue 


Fig.  327. — Coronal  section  of  Pericardial  Region  of  a  Human  Embryo  with  6  somites — 

beginning  of  4th  week.     (After  Tandler.) 

Fig.  328. — Coronal  section  of  Pericardial  Region  of  a  Human  Embryo  with  15  somites 

— end  of  4th  weelk.    (After  Tandler.) 

tube  has  become  elongated  and  bent  so  that  it  is  laid  open  in  the  section  at 
two  places — near  where  it  enters  the  floor  of  the  fore-gut,  to  which  it  is 
bound  by  the  dorsal  mesocardium — and  across  the  segment  which  will 
become  the  ventricles.  The  angioblastic  mesenchyme  now  forms  the 
endothelial  lining  of  a  narrow  cardiac  lumen  ;  the  outer  wall — derived 
from  the  visceral  mesoderm — represents  the  muscular  and  epicardial 
strata,  but  as  yet,  although  its  cells  are  contractile,  they  are  still  in  a  pre- 
muscle  state.  Between  endothelial  and  mesodermal  strata  is  interposed 
a  thick  subendothelial  reticulum.  Into  this  subendothelial  tissue  the 
myogenic  cells  wfll  proliferate  and  establish  a  myocardial  sponge-work. 
The  spaces  of  the  reticulum  are  laden  with  fluid  ;  there  is,  then,  at  this 
time,  under  the  myocardial  wall,  a  fluid  subendothelial  cushion. 

Arterial  and  Venous  Mesocardia. — The  manner  in  which  the  tubular 
cardiac  pump  is  fixed  to  the  wall  of  the  pericardium  in  a  human  embryo 
in  the  4th  week  of  development  is  shown  in  Fig.  329.  The  myocardial  wall 
has  been  stripped  off,  showing  the  endothelial  lining  of  the  tube.     The  heart 


3U 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


is  fixed  at  two  points  only — behind  at  the  place  where  its  first  chamber, 
the  sinus  venosus,  is  embedded  in  the  septum  transversum — and  in  front, 
where  its  terminal  segment,  the  truncus  arteriosus,  perforates  the  roof 
of  the  pericardium  to  enter  the  wall  of  the  pharynx.  At  these  two  points 
of  attachment  the  epicardial  covering  of  the  cardiac  tube  becomes  con- 
tinuous with  the  lining  membrane  of  the  pericardial  cavity  ;  the  posterior 
reflection,  on  the  sinus  venosus,  is  the  venous  mesocardium,  the  anterior, 
enclosing  the  truncus,  is  the  arterial  mesocardium.  The  rest  of  the  heart 
is  free  within  its  bursa — the  pericardial  cavity.  For  a  brief  interval  there 
is  a  dorsal  mesocardium,  but  by  the  middle  of  the  4th  week  only  a  trace 
remains  on  the  dorsal  wall  of  the  pericardium  between  the  two  points  of 
attachment  (Fig.  329).     At  no  time  is  there  a  ventral  mesocardium.     The 

opal.pl.  truncus 

art.mesocard       /      phar.  floor  \  art.  mesoc. 


VENTRICLE 
BULBUS 


D0R9.MES0CAR 
BVLBUS 


DUCT  OF  CUV. 


ANT  CARD. 

POST.  CARD 


VEN.  MESOC.  y 

sept,  trans 
Sin.  ven. 


zi  m.m. 


DUCT  OF  CUV 


PER-PL.  PASSAGE 
UMB.VEIN  YEN.MESOCAR. 

UMB.VEIN 


LUNG  RECESS 
VIT.  VEIN 


'i-Z  m.m. 


Fia.  329. — The  Attachments  of  the  Cardiac  Tube — merely  its  lining  membrane  is 
depicted — in  a  Human  Embyro  2"1  mm.  long  and  in  the  4th  week  of  develop- 
ment.   (After  His.) 

Fig.  330. — The  Attachments  of  the  Heart  in  a  Human  Embryo  4'2  mm.  long  and  in 
the  5th  week  of  development.  As  in  the  preceding  figure,  only  the  endothelial 
lining  is  represented.     (After  His.) 

iter  venosum  leading  from  the  pericardial  to  the  pleuro-peritoneal  cavity 
is  still  open  ;  the  cardiac  tube  has  grown  in  length  and  assumed  certain 
definite  bends  and  twists. 

A  week  later,  as  shown  in  Fig.  330,  the  arterial  mesocardium  has  shifted 
backwards  along  the  roof  of  the  pericardium  and  become  approximated 
to  the  venous  mesocardium.  There  have  also  been  changes  in  the  hinder 
attachment,  for  the  septum  transversum,  which  is  also  migrating  back- 
wards, has  taken  up  a  more  oblique  position,  being  now  partly  on  the 
dorsal  wall.  The  iter  venosum,  which  is  reduced  in  size,  is  now  crossed 
by  the  vein  or  duct  of  Cuvier,  in  a  slanting  direction.  By  the  3rd  month 
the  mesocardia  have  approximated  and  the  heart  has  become  fixed  in  its 
final  position  (Fig.  354). 


CIRCULATORY  SYSTEM  315 

Bends,  Twists  and  Primary  Chambers. — In  the  previous  paragraph 
we  have  seen  how  the  arterial  and  venous  niesocardia  become  approxi- 
mated, thus  bringing  together  the  ends  of  the  original  simple  cardiac  tube. 
We  are  now  to  see  that  a  similar  process  takes  place  in  the  cardiac  tube 
itself,  whereby  its  auricular  (atrial)  segment  is  brought  in  contact  with 
its  terminal  or  aortic  segment.  The  bends,  twists  and  evaginations  of  the 
cardiac  tube  are  easily  understood  if  the  reader  keeps  in  mind  the  manner 
in  which  the  curvatures  of  the  stomach  are  produced — namely,  by  unequal 
growth.  The  greater  curvature  of  that  organ  is  due  not  only  to  its  growth 
being  more  rapid  than  that  of  the  lesser  curvature  but  also  to  the  localized 
expansion  or  evagination  of  the  fundus.  In  some  animals  there  is  an 
actual  reduction — an  absorption — of  the  lesser  curvature  which  brings  the 


prim,  uentn'clex .  /     . \  prim,  auricle 


sinus  uenosus 


vit  uein^-^"'^  ^^uit.  uein 

Fia.  331. — The  Primitive  Divisions  of  the  Embryonic  Heart. 

pylorus  in  contact  with  the  oesophagus.  The  bends,  twists  and  evagina- 
tions of  the  cardiac  tube  are  produced  in  a  similar  manner  ;  they  are 
expressions  of  asymmetrical  growth  leading  up  to  the  stage  reached  in 
the  fully  developed  heart. 

In  Fig.  331  the  embryonic  heart,  early  in  the  4th  week  of  development, 
is  seen  on  its  ventral  aspect  and  already  the  primitive  ventricular  segment 
of  the  tube  shows  a  greater  curvature  towards  the  right  and  a  sharply 
bent  lesser  curvature  towards  the  left.  These  curvatures  are  better  shown 
in  Figs.  329  and  330  ;  the  ends  of  the  primitive  ventricular  segment  are 
being  approximated.  The  limb  of  the  ventricular  loop  nearest  the  begin- 
ning of  the  heart — the  proximal  limb — will  give  rise  to  the  3rd  or  ventri- 
cular chamber  of  the  heart ;  the  distal  limb  will  produce  the  4th  chamber 
of  the  heart — the  bulbus  cordis.  Besides  the  ventricular,  there  is  another 
important  curvature  at  the  junction  of  the  auricular  with  the  ventricular 
segment.  The  lesser  curvature — the  sharp  angle — of  this  auriculo-ventri- 
cular  bend  is  on  the  right  and  ventral  aspect  of  the  tube  (Fig,  331).     The 


316 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


2nd  chamber  of  the  heart — the  auricular  or  atrial — is  scarcely  marked  in 
the  early  part  of  the  4th  week  (Figs.  331,  329),  but  by  the  5th  week  evagina- 
tions  are  produced  on  its  dorsal  side — at  the  side  opposite  to  the  auriculo- 
ventricular  bend  (Figs.  330,  332).  The  sinus  venosus  or  1st  chamber  of  the 
heart  is  partly  embedded  in  the  septum  transversum  in  the  4th  week,  while 
the  truncus  arteriosus  or  5th  segment  of  the  cardiac  tube,  which  is  elongated 
in  the  4th  week,  is  greatly  shortened  by  the  5th  (Fig.  329).  Further,  it  will 
be  observed  that  as  early  as  the  4th  week  (Fig.  329)  there  are  two  con- 
stricted segments  in  the  endothelial  lining  of  the  cardiac  tube — one  between 
the  auricular  and  ventricular  segments — the  auricular  canal,  and  one 
between  the  bulbus  and  truncus — the  bulbar  canal.     All  of  these  five 


sup.  uen.  cau. 


sin.  uen. 


Inf.  uen.  cau 


conus  aner. 


bulbus  arter. 


aur.-aort  angle 


\  c  i-'^f^i-  r  / 


Fig.  332. — Diagram  of  the  five  Segments  of  the  Primitive  Cardiac  Tube.  I.  The 
venous  segment.  II.  The  auricular  segment ;  on  its  dorsal  aspect  the  auricle 
proper  is  developed ;  the  venous  valves  are  shown  between  the  venous  and 
auricular  segments.  III.  The  ventricular  segment,  the  ventricle  proper  being 
developed  from  its  ventral  aspect.  IV.  The  bulbus  segment.  V.  the  truncus, 
conus  or  aortic  segment.  It  is  separated  from  the  last  by  the  aortic  and 
pulmonary  valves. 

parts  of  the  cardiac  tube  are  to  be  seen  in  the  heart  of  a  fish  (Fig.  332) 
such  as  the  shark.  The  sinus  venosus  serves  as  a  blood  reservoir  ;  the 
auricle  acts  as  a  pump  to  feed  the  ventricles,  the  ventricle  is  the  pump  of 
the  gills  and  body  ;  the  bulbus,  which  becomes  incorporated  in  the  right 
ventricle  of  the  mammalian  heart,  feeds  the  gills  in  diastole,  the  truncus 
serves  purely  as  a  canal. 

The  Sinus  Venosus. — The  sinus  venosus,  the  first  chamber  of  the 
foetal  heart,  is  formed  by  the  union  of  the  vitelline  veins  ;  the  umbilical 
veins  and  ducts  of  Cuvier  come  subsequently  to  open  in  it  (Fig.  333). 
The  ducts  of  Cuvier  reach  it  from  the  somatopleure  by  passing  round  the 
coelomic  passages  (Figs.  329,  330)  and  entering  the  septum  transversum. 
In  fish  and  in  the  human  embryo  the  sinus  serves  as  a  reservoir  during 
systole  of  the  auricle  ;  the  systolic  wave  always  commences  in  the  sinus 
venosus.     The  right  and  left  venous  valves  (Fig.  335)  at  the  juncture  of  the 


CIRCULATORY  SYSTEM 


317 


sinus  and  auricle  prevent  the  regurgitation  of  blood  during  systole  of  the 
auricle.  These  valves  become  more  or  less  atrophied  when  the  right  and 
left  sides  of  the  heart  are  completely  separated  by  the  formation  of  septa. 

Fate  of  the  Sinus  Venosus  (Fig.  334). — Since  the  sinus  venosus 
plays  such  a  dominant  part  in  the  physiology  of  the  heart  of  lower  verte- 
brates, it  is  extremely  important  that  we  should  follow  its  fate  in  the 
human  heart.  It  becomes  submerged  chiefly  in  the  right  auricle,  the 
sulcus  terminalis  (see  Fig.  337),  marking  the  line  at  which  it  became 
included  by  the  upgrowth  of  auricular  tissue.  Already,  at  the  end  of  the 
5th  week,  its  orifice  has  come  to  occupy  a  position  in  the  posterior  or 


-^n^i^^jii^ — stomodaeum 


vent  bend. 


oonus  arteriosus 
aur.  vent  bend 

_'  \ — pericardium 
prim.  aur. 
sinus  uenosus 
sept  trans 
duct  of  Cuuier 


pleura. 


uit  vein 


umb.  vein 

Fig.  333. — Showing  the  two  chief  Bends  which  occur  in  the  Heart  during  the  4th  weelc. 

dorsal  wall  of  the  right  part  of  the  common  auricle  (Fig.  335).  The  part 
which  it  forms  of  the  right  auricle  is  indicated  by  the  entrance  of  the 
following  vessels  which  primarily  terminate  in  the  sinus  :  (1)  The  superior 
vena  cava  (the  right  duct  of  Cuvier)  ;  (2)  The  inferior  vena  cava,  which 
also  opens  into  the  sinus  ;  (3)  The  oblique  vein  of  Marshall  (left  duct  of 
Cuvier),  which  opens  into  the  left  horn  of  the  sinus  venosus.  The  left 
horn  of  the  sinus  becomes  the  coronary  sinus.  The  sulcus  terminalis  is 
marked  on  the  interior  of  the  right  auricle  by  a  strong  muscular  band 
(taenia  terminalis),  which  runs  down  on  the  anterior  wall  of  the  right 
auricle  from  the  superior  to  the  inferior  vena  cava,  and  indicates  the 
junction  of  the  prinutive  auricle  with  the  sinus  venosus  (Fig.  336).  The 
musculature  which  surrounds  the  terminal  part  of  the  superior  vena  cava, 
and  that  contained  in  the  wall  of  the  coronary  sinus,  represents  the  muscula- 
ture of  the  sinus.  Elsewhere  the  nmscle  of  the  sinus  appears  to  be  replaced 
by  that  of  the  auricle. 


318 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


The  Valves  of  the  Sinus  Venosus. — Riglit  and  left  lateral  valves  (venous 
valves)  guard  the  entrance  of  the  sinus  to  the  primitive  auricle  and  prevent 
the  regurgitation  of  blood  when  the  auricle  contracts  (Fig.  335).     The 

sup.  uen.  cau. 

uena.  azi/5?,-t^yf======55i5^;,7e/if  swp.  intercost 


sup.  uen.  cau 


uein  of  Marshall 


part  of  rt  aur. 
left  venous  valve 

Eustach.  valve 

{Tight  venous 

value)  7""/      \      cor.  sinus 

open,  of  cor.  sinus 
Thebesian  ualue 
inf.  uen.  caua 

Fig.  334. — Showing  the  part  of  the  Right  Auricle  formed  from  the  Sinus  Venosus. 

valves  meet  above  and  form  a  superior  fornix  in  front  of  the  superior  caval 
opening  ;  they  meet  below  in  an  inferior  fornix,  which,  owing  to  the  great 
shortening  of  the  ventral  part  of  the  auricular  segment,  reaches  the  base 


sup  uen  cau 


right  uen.  ualue 
right  aur— 


inf.  uen.  cau. 


right  uent 


secundum 
•sept  primum 
left  uenous  ualue 

left  aur. 

open,  sinus,  uenosus. 

post 

endocard.  cushion 

aur.  uent.  canal 
left  uent. 


inter-uent.  sept. 


Fig.  335. — Section  of  the  Heart  of  a  6th  weelc  Human  Embryo  showing  the  Right 
and  Left  Venous  Valves  which  guard  the  Entrance  of  the  Sinus  Venosus  into  the 
Primitive  Auricle.     (After  His.) 

of  the  ventricle,  and  actually  fuses  with  the  posterior  endocardial  cushion 
(Fig.  342).  This  has  an  important  bearing  on  the  origin  of  the  auriculo- 
ventricular  (A.V.)  bundle  within  the  auricular  canal.     Along  the  base  of 


CmCULATOEY  SYSTEM 


319 


eacli  valve  is  arranged  a  band  or  taenia  of  the  auricular  musculature. 
Thus  each  valve  consists  of  a  membranous  marginal  part  and  a  muscular 
basal  part.  The  right  valve  in  the  adult  heart  becomes  (Fig.  336)  (1) 
the  Thebesian  and  (2)  Eustachian  valves  ;  (3)  the  musculature  at  its  base 
forms  the  taenia  terminalis.  The  left  valve  becomes  (1)  a  fretted  membrane 
on  the  septal  margin  of  the  inferior  caval  orifice,  (2)  a  band  of  musculature 
accompanying  this  remnant  (Fig.  336). 

The  Limbic  Bands.^ — Two  inflections  of  the  wall  of  the  sinus  venosus 
are  formed  (a)  between  the  superior  and  inferior  caval  orifices,  (6)  between 
the  inferior  caval  orifice  and  that  of  the  coronary  sinus.     In  these  inflections 


sup.  uen.  cau 

muse,  pectin, 
rt.  value 
open,  of  sup.  cau. 
rt.  taen.  term 
sup.  limbic  band 
open,  of  inf.  cau. 
Bust,  ualue 
inf.  uen.  cau. 


sup.  fornix 
left  ualue 
fossa  ovalis 

inf.  fornix. 

inf.  limbic  band 

open,  of  cor.  sin. 

Thebes,  ualue 

rt.  aur.  uent.  ualue 


base  of  uentr. 


Fig.  336. — Diagram  of  the  Right  Auricle  thrown  open  to  show  the  position  and 
relations  of  the  Uight  and  Left  Venous  Valves  and  the  manner  in  which  they 
are  broken  up  by  the  Superior  and  Inferior  Limbic  Bands. 

bands  of  auricular  musculature  cross,  forming  the  upper  and  lower  limbic 
bands  (Fig.  336).  Thus  the  mechanical  valves  which  prevent  regurgitation 
in  auricular  systole  are  replaced  by  a  muscular  mechanism  which  serves 
the  same  purpose.  In  amphibians  and  reptiles,  where  the  division  of  the 
heart  is  incomplete,  over-pressure  in  the  right  side  is  relieved  by  the  escape 
of  blood  to  the  left  side  of  the  heart ;  but  in  birds  and  mammals  such  an 
adjustment  is  impossible,  hence  the  mechanical  venous  valves  are  replaced 
by  a  "  safety  mechanism,"  which  will  allow  regurgitation  from  the  auricles 
to  the  veins  if  the  right  side  becomes  over-distended. 

Sino-auricular  Node.^ — The  musculature  of  the  sinus  venosus  of  fishes 
is  made  up  of  small  peculiar  fibres  rich  in  nuclei  and  in  nerve  supply.     It 

1  Keith,  Proc.  Anat.  Soc.  Nov.  1902  ;   Lancet,  Feb.  27th,  March  5th  and  12th,  1904  ; 
Journ.  Anat.  and  Physiol.  1905,  vol.  42,  p.  1. 

2  Keith  and  Flack,  Journ.  Anat.  and  Physiol.   1907,  vol.  41,  p.   172;    W.  Koch, 
Verhand.  Deutsch.  Path.  Gesellsch.  1909,  vol.  13,  p.  85. 


320 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


has,  more  than  all  the  musculature  of  the  heart,  the  power  of  automatic 
rhythmical  contraction.  In  human  and  mammalian  hearts  the  sinus 
musculature  is  replaced  by  fibres  similar  to  those  of  the  auricle — all  but  at 
the  sulcus  terminalis,  which  marks  the  junction  of  the  sinus  and  auricle. 
In  the  sulcus,  just  in  front  of  the  termination  of  the  superior  vena  cava 
(Fig.  337),  an  area  of  primitive  fibres  persists — the  sino-auricular  node. 
In  lower  mammals  like  the  mole,  the  sino-auricular  tissue  is  more  extensive  ; 
it  extends  along  the  greater  part  of  the  sulcus  terminalis,  and  passes  towards 
the  pulmonary  veins.  In  amphibia  and  reptiles  it  extends  to  the  part 
of  the  left  auricle  (vestibule),  in  which  the  pulmonary  veins  terminate. 
In  the  lowest  mammals — monotremes — the  muscular  tissue  of  the  node 
assumes  a  peculiar  form.^  Thus  the  higher  in  the  animal  scale  one  ascends, 
the  greater  is  the  reduction  of  the  sino-auricular  nodal  tissue.  It  is  in 
reality  a  neuro-muscular  tissue,  and  is  well  defined  by  the  5th  month  of 


sup:  ven: cav 

sino-aur:n0de 
Sulcus  term 

IHFVENrCAV: 


AORTA 
PULM: ART: 


R"!'    AUR: 

SINUS 

SEPT; PRIM: 

.DORSAL     MESOCARO: 


LEFT    SINUS 
EXTENSION 


BULBUS 


VENTRICLE 


Fig.  337.— Human  Heart  at  the  beginning  of  tlie  Brd  month  of  development  to 
show  the  position  of  tlie  Sino-auricular  Node.  The  unsubmerged  strip  of  sinus 
venosus  is  seen  between  the  superior  and  inferior  venae  cavae. 

Fig.  338. — The  Posterior  Wall  of  the  Common  Auricle  of  an  Embryo  of  the 
5th  week,  showing  the  Left  Extension  of  the  Sinus  Venosus.    (His.) 

development.  Dr.  T.  Lewis  found  that  the  contraction  of  the  heart  spread 
from  the  sino-auricular  node,  and  gave  it  the  name  of  the  "  pace-maker  " 
of  the  heart. 

Formation  o£  the  Right  Auricle. — The  right  auricle  or  atrium  is 
formed  by  the  combination  of  three  parts  :  (1)  the  right  primitive  auricle 
which  appears  as  a  diverticulum  from  the  right  dorso-lateral  aspect  of  the 
auricular  segment  of  the  cardiac  tube  (Fig.  332)  ;  it  forms  the  appendix 
and  all  that  part  of  the  right  auricle  which  is  furnished  with  musculi 
pectinati.  (2)  The  auricular  canal  (Fig.  332)  which  forms  the  inner  layer 
of  the  right  auriculo-ventricular  valve,  and  the  smooth  part  of  the  auricle 
above  the  base  of  that  valve.  The  morphological  and  physiological 
junction  between  the  auricle  and  ventricle  is  at  the  lower  or  free  margin 
of  the  auriculo-ventricular  cusps.  (3)  The  sinus  venosus  which  forms  the 
part  of  the  right  auricle  between  the  remnants  of  the  right  and  left  venous 
valves  (Fig.  336). 

1  Dr.  Ivy  Mackenzie,  Verhand.  Deutsch.  Path.  Gesellsch.  1910,  vol.  14,  p.  90. 


CIRCULATORY  SYSTEM 


321 


Formation  of  the  Left  Auricle. — The  left  auricle  is  also  formed  by  the 

combination  of  three  parts  :  (1)  the  vestibule  which  arises  as  an  extension 
round  the  terminal  parts  of  the  pulmonary  veins  (Figs.  339,  340),  (2)  the 
left  primitive  auricle,  and  (3)  the  auricular  canal,  all  of  which  arise  in  a 
manner  similar  to  the  corresponding  part  on  the  right  side.  In  the  human 
heart  the  vestibule  forms  a  large  part  of  the  left  auricle,  the  primitive 
auricle  being  reduced  to  form  merely  the  appendix  (Fig.  340).  The 
vestibule  is  marked  off  from  the  rest  of  the  auricle  by  a  prominent  muscular 
fasciculus — the  taenia  terminalis  sinistra. 

Origin  of  the  Vestibule  of  the  Left  Auricle. — The  representative 
of  the  pulmonary  veins  in  fishes — ^viz.  the  vein  of  the  swim  bladder — ends 
directly  or  indirectly  in  the  sinus  venosus,  a  condition  which  may  reappear 
as  an  abnormality  in  the  human  subject.  In  the  Dipnoi,  in  which  the 
swim  bladder  serves  as  a  real  lung,  the  pulmonary  vein  passes  along  the 


left  aur. 


pulm  vein, 
cor.  sin. 


auricle 

vein  of  Marsh 
sup.  uen.  eau. 

sin  ven.      ■' 
inf.  ven.  cau. 

aur.  can. 


ventr. 


left  vent. 


up.  ven.  cau. 

pulm.  art. 
pericard. 
rt  pul.  vein. 

rt.  aur. 
left  aur. 
^  I    cor.  sin. 
'pericard. 

._<^^^'-diaph. 
^""^inf.  ven.  eau. 


Fig.  339.— Reptilian  Heart,  viewed  on  its  Dorsal  Aspect,  to  show  (1)  the  manner 
in  which  the  Auricles  arise  from  the  Cardiac  Tube,  (2)  the  Auricular  Canal, 
(3)  the  Sinus  Venosus  and  Great  Veins,  (4)  the  Common  Pulmonary  Vem,  which, 
at  its  termination,  is  embraced  by  the  sinus  venosus. 

Fig.  340. — Heart  of  Adult  viewed  from  behind  to  show  the  Vestibule  and  the 
other  parts  of  the  Left  Auricle.  The  auricle  was  in  a  systolic  condition. 
The  remains  of  the  left  superior  vena  cava  (vein  of  Marshall)  and  the  attach- 
ment of  the  pericardium  are  also  indicated. 

left  wall  of  the  sinus  venosus  to  open  in  the  left  auricle  near  the  base  of  the 
left  venous  valve  in  a  manner  almost  identical  to  that  shown  in  some  abnormal 
human  hearts  (see  Fig.  371).  In  the  human  embryo  the  pulmonary 
veins  meet  in  the  venous  mesocardium,  and  open  by  a  single  orifice  as  in  the 
Dipnoi.  As  the  lungs  develop  they  grow  round  and  overlap  the  heart ; 
the  right  and  left  pulmonary  veins  separate  ;  their  orifices  move  apart ; 
later  the  right  and  left  veins  subdivide.  With  these  changes  the  venous 
mesocardium  is  widened,  and  the  part  of  the  auricle  in  which  the  veins 
end  is  greatly  extended  to  form  the  vestibule  (compare  Figs.  339,  340). 
It  is  highly  probable  that  the  vestibule  of  the  left  auricle  also  represents  an 
extension  of  the  sinus  venosus.  The  late  Professor  His,  who  laid  our 
knowledge  of  the  development  of  the  human  embryo  on  a  sure  foundation 
of  fact— he  died  in  1904— believed  this  to  be  the  case.  It  is  certainly  so 
in  the  heart  of  amphibians.     In  Fig.  338  the  sinus  area  will  be  seen  to 


322 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


extend  into  tlie  posterior  wall  of  the  left  auricle.     It  is  on  this  left  extension 
that  the  venous  channel  from  the  lung  buds  opens. 

Auricular  Septa. — During  the  6th  week  the  auricular  part  of  the  heart 
becomes  separated  into  right  and  left  chambers  by  the  formation  and 
union  of  the  three  following  elements  :  (1)  the  endocardial  cushions,  (2) 
the  septum  primum,  (3)  septum  secundum.  Two  endocardial  cushions 
arise  as  thickenings  of  the  endocardium,  one  on  the  dorsal  or  posterior  wall, 
the  other  on  the  ventral  or  anterior  wall ;  they  meet  and  fuse,  and  thus 
divide  the  common  auricular  canal  into  the  right  and  left  auriculo-ventri- 
cular  orifices  (Fig.  342).  In  amphibians  the  endocardial  cushions  form  the 
dorsal   and   ventral   cusps   of   the   common   auriculo-ventricular   valve  ; 


aorta 


sup.  uetis  cau 

sept  secund, 

for.  ovale 

sept.  prim, 
rif.cor.  sinus 
inf.  uen.  cau. 

Eustach  value 


pul.  art. 
post  cush.  of  bulb. 


ant.  endoc.  cush, 
pars  mem.  septi 

intervent  sept. 

-post,  endoc.  cush. 


Fig.  341.- 


-Diagram  of  the  opened  Right  Auricle  and  Ventricle  to  show  the  parts 
which  enter  into  the  Formation  of  the  Septum. 


in  reptiles  these  two  cusps  become  united,  and  thus  divide  the  common 
auriculo-ventricular  orifice  into  right  and  left  channels  ;  in  birds  and 
mammals  their  fusion  is  complete.  The  lower  fornix  of  the  venous  valves 
(Figs.  335,  342)  becomes  implanted  on  the  posterior  cushion  ;  thus  the 
sinus  comes  almost  to  reach  the  ventricular  chamber.  The  septum  primum 
(Fig.  341)  appears  at  the  beginning  of  the  6th  week  as  a  crescentic  fold  on 
the  roof  of  the  primitive  auricle,  and  while  it  may  actually  grow  downwards 
yet  appears  to  be  produced  mainly  by  the  expansion  of  the  two  auricular 
chambers  (Fig.  332).  Its  lower  margin,  which  is  covered  by  a  thickening 
of  endocardial  tissue,  is  attached  to  both  endocardial  cushions ;  the 
adjacent  margins  of  the  septum  and  endocardial  cushions  fuse,  but  occasion- 
ally the  fusion  is  incomplete,  an  inter-auricular  foramen  (foramen  primum) 
being  left  between  the  bases  of  the  auriculo-ventricular  valves  below  and 
septum  ovale  above  (Figs.  341,  348).  In  mammals  and  birds  the  upper 
part  of  the  septum  primum  breaks  down,  the  foramen  ovale  being  thus 
formed.     The  part  which  remains  forms  the  septum  ovale.     The  septum 


CmCULATORY  SYSTEM  323 

secundum  (Fig.  341)  is  formed  by  an  inflection  of  musculature  from  the 
roof  of  the  auricle  to  the  right  of  the  septum  primum.  It  forms  the  annulus 
ovalis  (limbic  bands)  (Fig.  336)  and  the  musculature  of  the  septum  above 
the  foramen  ovale  (Fig.  336).  The  foramen  ovale  thus  becomes  bounded 
above  by  the  septum  secundum,  below  by  the  septum  primum.  In  25 
per  cent,  of  people,  according  to  Fawcett's  statistics,  the  foramen  ovale 
fails  to  close  within  the  first  year  after  birth,  but  even  when  an  opening 
remains  blood  could  pass  from  the  right  to  the  left  auricle  only  when  the 
pressure  was  greater  in  the  right  than  in  the  left.  The  foramen  ovale 
is  an  adaptation  to  the  foetal  type  of  respiration  ;  by  it  the  purer 
blood  returning  from  the  placenta  can  pass  from  the  right  to  the  left 
side  of  the  heart  without  passing  through  the  lungs,  which  are  then 
only  partially  pervious. 


U'^  SUP-.VELN:CAV: 

LEFT    AUR: 
SEPT: PRIM; 

POST:  ENDO;CUSH: 
LEFT  VENT: 

INTERVENT:  SEPT: 


AORTIC   CUSHIONS 


Fig.  342. — Coronal  Section  of  the  Heart  of  a  Rabbit,  illustrating  the  condition  of 

parts  in  the  6th  week  of  Human  Development.    (Born.) 

The  cushions  labelled  "  aortic  "  should  be  marked  "  bulbar." 

Division  of  the  Truncus  Arteriosus. — While  the  auricular  segment  of 
the  cardiac  tube  is  undergoing  division  during  the  6th.  week  a  similar 
process  is  taking  place  in  its  terminal  segment — ^the  truncus  or  conus 
arteriosus,  leading  to  the  separation  of  the  pulmonary  from  the  systemic 
aorta.  We  have  seen  that  the  truncus  becomes  shortened  during  the  5th 
week  (Figs.  329,  330)  and  at  the  same  time  the  ventral  aorta  (Fig.  331) 
is  being  cleft  into  right  and  left  vessels.  In  the  6th  week  the  process  of 
cleavage  has  reached  the  origin  of  the  6th  pair  of  aortic  arches  from  which 
the  pulmonary  arteries  arise  (Fig.  360)  so  that  there  now  remains  but  a 
short  segment  of  the  common  aortic  stem  to  undergo  division  and  give 
rise  to  the  intrapericardial  part  of  the  aorta  and  common  pulmonary 
artery.  The  first  step  in  the  division  is  the  appearance  of  four  endocardial 
cushions  at  the  commencement  of  the  common  aortic  trunk  (Fig.  343,  A) 
the  two  larger  cushions  being  placed  right  and  left.  As  is  shown  in  Fig. 
343,  these  cushions  become,  for  the  chief  part,  converted  into  the  aortic 
and  pulmonary  valves — but  two  of  them,  the  right  and  left,  become  fused 


324 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


and  assist  in  forming  the  spiral  septum  wticli  separates  the  aortic  from  the 
pulmonary  passage.     By  the  end  of  the  6th  week  a  process  of  cleavage  has 


right 


ant 


pulm.  artenf 

Pig.  343. — The  Origin  of  the  Semilunar  Valves. 

A.  The  four  Endocardial  Cushions  of  the  Truncus  Arteriosus. 

B.  The  division  of  the  Lateral  Cushions  to  form  two  Aortic  and  two  Pulmonary 

Semilunar  Valves. 

reached  the  lateral  cushions  and  henceforth  the  pulmonary  artery  and 
aorta  form  distinct  channels. 

Bulbus  Cordis.-^ — We  have  seen  how  the  first  chamber  of  the  heart — 
the  sinus  venosus — becomes  included  in  the  auricles.     In  a  somewhat 


Fig.  344. — Heart  of  an  Embryo  of  4  weeks  seen  from  the  front. 

Explanation  in  text. 
Fig.  345. — Heart  of  a  Shark  viewed  from  the  front. 


(After  His.) 


similar  manner  the  fourth  chamber  of  the  heart — the  bulbus  cordis — ■ 
becomes  submerged  in  the  ventricles — principally  in  the  right  ventricle. 
In  Figs.  34:4:  and  345  the  heart  of  a  human  embryo  and  that  of  a  shark  are 
placed  side  by  side.     In  both  the  truncus  arteriosus  (ventral  aorta)  are 

1  See  Greil,  Morph.  Jahrb.  1903,  vol.  31,  p.  123  ;  Keith,  Lancet,  1909,  Aug.  7,  14,  21 ; 
Thompson,  Journ.  Anat.  and  Physiol.  1907,  vol.  42,  p.  159  ;  Prof,  D.  Waterston, 
Tra-ns.  Roy.  Soc.  Edin.  1918,  vol.  52,  p.  257. 


CIECULATORY  SYSTEM 


325 


present  (1)  ;  the  bulbus  cordis  (2)  ;  it  is  lined  with  valves  in  the  shark  and 
surrounded  by  cardiac  musculature  ;  the  bulbus  is  distinctly  marked  oS 
from  the  ventricle  at  4,  and  from  the  truncus  at  3.  The  ventricle  (5) 
in  the  shark  has  the  shape  of  a  stomach  ;  in  the  embryonic  human  heart  a 
diverticulum  or  evagination  indicating  the  left  ventricle  has  already 
appeared  (4th  week)  ;  the  auricular  canal  (6),  the  left  and  right  auricles 
(7)  (8)  are  also  present.  Thus  in  the  human  embryo  all  the  parts  of  the 
primitive  vert-ebrate  heart  are  represented. 

Fate  of  the  Bulbus  Cordis. — The  fate  of  the  bulbus  cordis  is  most 
easily  understood  by  a  reference  to  such  a  diagram  as  is  represented  in 
Fig.  346,  A,  B.  The  bulbo-ventricular  part  of  the  heart  in  the  human 
embryo  resembles  the  stomach  ;  there  is  a  greater  and  a  lesser  curvature. 
In  the  second  month  the  lesser  curvature,  represented  in  the  diagram  by 
a  heavy  black  line,  undergoes  a  process  of  atrophy.     The  result  is  (Fig. 


(A) 


(B) 


Fig. 


346,  A. — Diagrammatic  Section  of  the  Embryonic  Heart  in  the  3rd  week. 
B. — Diagrammatic  Section  of  the  Foetal  Heart  at  the  3rd  month. 
1,  sinus  venosus  ;    2,  auricle  ;    3',  3",  left  and  right  ventricles  ;    4,  bulbus  cordis ; 
5,  common    aorta  ;       bulbo-ventricular    junction  ;     7,  bulbo-aortic    junction ; 
8,  auriculo-ventricular  junction. 

346,  B)  that  the  cavity  of  the  bulbus  becomes  thrown  into  that  of  the 
ventricle  and  the  auriculo-ventricular  and  aortic  orifices  are  brought  side 
by  side.  At  this  time,  when  the  lesser  curvature  is  disappearing,  the 
cavities  of  the  ventricles  are  appearing  by  an  evagination  or  enlargement  of 
the  ventricular  wall,  leaving  the  interventricular  septum  between  the 
evaginations  (Figs.  288,  298).  The  conus  or  truncus  arteriosus  is  dividing 
then  into  systemic  and  pulmonary  aortae.  Thus  it  comes  about  that  the 
cavity  of  the  bulbus  cordis  is  converted  into  the  infimdibulum  of  the  right 
ventricle,  merely  a  trace  extending  across  to  the  left  ventricle  above  the 
interventricular  septum.  The  importance  of  recognizing  the  bulbus  cordis 
as  a  separate  constituent  of  the  heart  will  be  realized  when  it  is  remembered 
that  95  per  cent,  of  the  cases  of  congenital  malformation  are  the  result  of 
its  imperfect  transformation  to  form  the  infundibulum  of  the  right  ventricle 
of  the  heart.  In  nearly  every  case  of  what  is  described  as  congenital 
stenosis  of  the  pulmonary  orifice,  a  cavity  of  variable  size  wiU  be  found 
under  the  malformed  valves  representing  the  bulbus  cordis.     In  fishes  the 


326 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


bulbus  is  connected  witli  the  blood  supply  to  the  gills  ;  its  derivative,  the 
infundibulum  of  the  right  ventricle,  has  to  do  with  the  regulation  of  the 
blood  supply  to  the  lungs,  but  in  neither  case  do  we  know  the  exact  function 
of  this  part  of  the  heart. 

Bulbar  Cushions.^ — During  the  transformation  of  the  bulbus  in  the  6th 
week,  there  appear  within  it  two  endocardial  cushions — evolved  from  the 
series  of  valves  which  line  the  bulbus  of  the  primitive  heart  (Fig.  345). 
The  part  taken  by  them  in  building  up  the  interventricular  septum  can 
best  be  realized  when  the  infundibular  part  of  the  right  ventricle  is  exposed 
as  in  Fig.  341.  The  line  of  fusion  between  the  posterior  and  anterior 
bulbar  cushions  is  seen  to  descend  in  the  septal  wall  of  the  infundibulum 
from  the  pulmonary  valves  to  the  site  of  the  interventricular  foramen. 
When  the  bulbar  cushions  fuse  at  the  end  of  the  6th  week  the  small  sub- 


left  aur-uent 
orifice 


right  aur-vent  orif. 
tricuspid 


bicuspid  ualue 

muscular     , 
sponge-work 


intervent  sejjt 

Fig.  347. — Section  of  the  Ventricles  of  the  Foetal  Heart,  showing  the  Muscular 
Sponge-work  within  their  Cavities.     (After  His.) 

aortic  part  of  the  bulbus  becomes  separated  from  the  main  part  included 

in  the  infundibulum  of  the  right  ventricle  (Figs.  346,  A,  B).     The  bulbar 

cushions  at  an  early  stage  of  development  are  shown  in  Fig.  342,  where  they 

are  wrongly  labelled  aortic  cushions. 

Formation   o£   the  Ventricles. — -Along  the  lateral  and  convex  aspects 

of  the  ventricular  tube  the  musculature  grows  rapidly,  forming  a  dense 

superficial  layer  and  a  deep  sponge-work  system  of  trabeculae,  which 

almost  fill  the  ventricular  chamber.     In  the  hearts  of  fishes  and  amphibians 

the   sponge-work   persists,   but   in   birds   and  mammals  the  ventricular 

chambers  are  formed  as  diverticula  by  the  absorption  of  the  sponge-work. 

Between  the  right  and  left  excavations,  however,  part  of  the  sponge-work 

is  left  to   form  the   interventricular   septum   (Fig.   347).     In  front  the 

musculature  of  the  septum  is  attached  to  the  anterior  ciishion  of  the 

bulbus  arteriosus  (Figs.  341,  342)  ;   behind,  it  is  attached  to  the  posterior 

of  the  two  endocardial  cushions  in  the  auricular  canal.     On  its  upper  free 

crescentic  margin  is  a  thickening  of  endocardial  tissue.     The  closure  of 

the  interventricular  foramen  completes  the  separation  of  the  left  from  the 

^  For  full  details  regarding  the  formation  of  the  interventricular  septa,  see  Prof. 
Frazer's  research,  Journ.  Anat.  1917,  vol.  51,  p.  19. 


CIRCULATORY  SYSTEM 


327 


right  ventricle  of  the  heart.  It  is  bounded  below  by  the  margin  of  the 
interventricular  septum  ;  above,  by  the  bulbar  cushions  and  behind  by 
the  auricular  endocardial  cushions  (Figs.  341,  350).  The  pars  membranacea 
septi,  which  is  found  beneath  the  base  of  the  septal  cusp  of  the  tricuspid, 
and  below  the  septal  cusps  of  the  aortic  valve,  is  formed  towards  the  end 
of  the  7th  week,  by  the  fusion  of  the  endocardial  margins  of  the  inter- 
ventricular foramen.^  The  foramen  is  thus  closed  by  that  process  to 
which  the  name  of  zygosis  has  been  given  (p.  287).  Only  in  mammals  and 
birds  is  the  interventricular  foramen  closed,  the  foramen  ovale  opened  and 
the  venous  valves  replaced  by  a  muscular  mechanism. 

Abnormalities  of  the  Heart.^ — Thus  five  elements  enter  into  the 
formation  of  the  septum  of  the  heart,  the  two  interauricular  septa,  the  two 
endocardial  cushions  of  the  auricular  canal,  the  interventricular  septum, 
the  endocardial  cushions  of  the  bulbus  and  the  cushions  of  the  truncus 


Fig.  348. — ^Abnormal  Heart  of  a  Child  with  the  Left  Auricle  and  Ventricle  laid 
open,  a,  left ;  b,  right  pulmonary  veins  ;  c,  septum  primum  ;  d,  d',  posterior 
and  anterior  endocardial  cushions ;  e,  interventricular  septum ;  /,  left 
ventricle  ;  g,  left  auricular  appendix  ;  h,  aorta  ;  i,  sup.  vena  cava. 

Fig.  349. — Same  Heart  from  above,  a,  the  orifice  of  pulmonary  artery  with 
fusion  of  septal  cusps ;  6,  valves  of  aorta,  with  the  coronary  arteries 
risLug  above  septal  cusps ;  c,  d,  e,  f,  continuity  of  the  tricuspid  and  mitral 
valves  across  the  upper  border  of  septum. 


arteriosus  (Fig.  34:1).  Abnormalities  may  result  from  their  non-union, 
but  by  far  the  commonest  defect  found  is  a  patency  of  the  interventricular 
foramen  (Fig.  350).  This  is  accompanied  in  nearly  every  case  by  an 
arrest  in  the  expansion  of  the  bulbus  cordis  and  a  stenosis  or  narrowing 
at  the  orifice  of  the  pulmonary  artery  (congenital  pulmonary  stenosis). 
The  blood  of  the  right  ventricle,  in  such  cases,  is  pumped  into  the  aorta, 
through  the  interventricular  foramen  ;  blood  is  supplied  to  the  lungs 
through  the  ductus  arteriosus  or  by  the  bronchial  arteries  from  the  aorta. 

Aurieulo-Ventricular  Valves. — At  first  the  auricular  canal  is  exposed 
on  the  surface  of  the  heart  (Fig.  294),  but  it  soon  becomes  enveloped 

1  For  a  fuller  account  of  development  of  ventricles  see  F.  P.  Mall,  Amer.  Journ.  Anat. 
1912,  vol.  13,  p.  249  ;    Frazer,  Journ.  Anat.  1917,  vol.  51,  p.  19. 

2  A.  Keith,  Journ.  Anat.  and  Physiol.  1912,  vol.  46,  p.  211  ;   F.  T.  Le-svis  and  Maude 
Abbott,  Bulletin  Med.  Museums,  1916,  vol.  6,  p.  1. 


328     HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

by  the  upgrowth  and  excavation  of  the  bases  of  the  ventricles  (Fig.  338). 
The  auricular  canal,  with  an  attenuated  envelopment  derived  from  the 
ventricle,  thus  comes  to  hang  within  the  ventricular  chambers  and  forms 
the  lateral  cusps  of  the  tricuspid  and  mitral  valve  (Fig.  347).  The  septal 
cusps  are  formed  from  processes  of  the  endocardial  cushions  (Fig.  349). 
The  chordae  tendineae,  musculari  papillares,  columnae  carneae,  trabeculae 
and  moderator  band  are  derived  from  the  muscular  sponge-work  of  the 
ventricles. 

Various  maldevelopments  of  the  heart  throw  light  on  the  nature  of  the 
auriculo-ventricular  valves.  In  Fig.  348  an  abnormality  of  this  kind  is 
represented.  The  anterior  and  posterior  endocardial  cushions  have  not 
united,  hence  the  tricuspid  and  mitral  valves  are  continuous  across  the 
upper  border  of  the  septum  (Fig.  349).  The  aperture  seen  above  the 
interventricular  septum  is  the  foramen  'primum— not  the  interventricular 
foramen. 


CHAPTER  XXI. 

CIECULATORY   SYSTEM  {Covtinued). 

Purkinje  System  of  the  Heart. — About  the  middle  of  the  nineteenth 
century,  Purkinje,  Professor  of  Anatomy  at  Breslau,  discovered  large 
peculiar  muscle  fibres  beneath  the  endocardium  of  the  heart  of  the  sheep 
and  of  other  ungulate  animals.  In  1906  Tawara  showed  that  such  fibres 
were  connected  with  a  muscular  bundle,  which  rose  in  the  waU  of  the 
auricle  near  the  orifice  of  the  coronary  sinus  and  entered  the  ventricle  along 
the  upper  border  of  the  interventricular  system.^  In  many  cases  of  mal- 
formed heart  the  primitive  relations  of  the  auriculo-ventricular  (A.V.) 


Fig.  350. — The  Auriculo-ventricular  Bundle  in  a  Heart  with  open  Interventricular 
Foramen.  1,  aorta  ;  2,  on  the  site  of  the  pars  membranacea  septi ;  3,  left 
division  issuing  from  bundle  situated  on  the  upper  margin  of  the  inter- 
ventricular septum  (4) ;  5,  anterior  wall  of  left  ventricle  ;  6,  mitral  valve  ; 
7,  cut  wall  of  left  ventricle  ;  8,  left  auricle  ;  9,  pulmonary  artery. 

bundle  may  be  seen  (Fig.  350).  It  passes  along  the  upper  border  of  the 
interventricular  septum  below  the  interventricular  foramen.  Its  left 
branch  descends  on  the  septum  to  the  musculari  papillares  of  the  left 
ventricle  ;  the  right  division  or  branch  passes  along  the  moderator  band, 
which  marks  the  junction  of  the  bulbus  cordis  with  the  body  of  the  right 
ventricle.  When  it  is  remembered  that  the  ventricles  arise  from  evagina- 
tions  of  the  ventricular  tube,  it  will  be  seen  that  the  bundle  on  the  upper 
border  of  the  septum  occupies  the  least  disturbed  part  of  the  lumen  of  the 
primitive  cardiac  tube. 

The  evolution  of  the  Purkinje  system  may  be  realized  from  a  study  of 
Fig.  351.     Gaskell  found  in  1883  that  the  auricles  and  ventricles  were 

^  For  ap  account  of  Tawara's  discovery  see  Keith,  Lancet,  1906,  Aug.  11. 

329 


330 


HUMAN  EMBRYOLOaY  AND  MORPHOLOGY 


connected  in  j&shes,  amphibians  and  reptiles  by  the  musculature  of  the 
auricular  canal  (Fig.  351,  4,  4),  and  that  this  connection  conveyed  the  wave 
of  contraction  from  auricle  to  ventricle.  The  auriculo-ventricular  muscular 
collar  begins  in  a  ring  of  peculiar  muscle  cells  situated  as  shown  in  Fig. 
351,  3,  3.  In  the  mammalian  heart  the  primitive  muscle  of  the  auriculo- 
ventricular  canal  disappears — except  at  the  upper  border  of  the  septum, 
where  it  forms  the  bundle.  The  node  in  which  it  arises  represents  a  rem- 
nant of  the  ring  of  peculiar  muscular  tissue  which  surrounds  the  auriculo- 
ventricular  junction.  It  is  true  that  the  sinus  venosus  reaches  the  posterior 
endocardial  cushions  (Fig.  342)  near  the  site  of  the  node,  but  it  is  most 


Fig.  351 — Section  of  a  Generalized  Type  of  Heart  to  show  the  Origin  of  the  Auriculo- 
ventricular  Bundle  and  Node,  a,  sinus  venosus  ;  b,  auricular  canal ;  c,  auricle  ; 
d,  ventricle  ;  e,  bulbus  cordis  ;  /,  aorta  ;  1,  1,  sino-auricular  junction  ;  2,  2, 
auricular  junction  with  canal ;  3,  auricular  ring  of  peculiar  fibres  ;  4,  auriculo- 
ventricular  musculature  ;  5,  bulbo-ventricular  junction. 

improbable,  in  the  light  of  comparative  anatomy,  that  the  node  at  the 
commencement  of  the  bundle  should  represent  sinus  musculature. 

Changes  in  the  Circulation  at  Birth. — (1)  The  outflow  of  the  blood 
to  the  placenta  by  the  hypogastric  arteries  and  its  return  by  the  umbilical 
vein  is  arrested  when  the  umbilical  cord  is  tied.^  The  umbilical  vein  and 
ductus  venosus  gradually  become  ligamentous.  (2)  The  first  breath 
expands  not  only  the  air  spaces  of  the  lungs,  but  also  the  pulmonary 
vessels,  so  that  the  pressure  within  them  becomes  less  than  in  the  aorta  ; 
hence  the  blood  in  the  pulmonary  aorta  passes  through  the  lungs  instead 
of  gaining  the  aorta  by  the  ductus  arteriosus.  A  section  across  the  ductus 
arteriosus  and  aorta  (Fig.  353)  shows  that,  before  birth,  the  septal  wall  of 
the  ductus  is  invaginated  within  the  lumen  of  the  aorta  ;   after  birth  the 

1  For  changes  in  vessels  see  A.  W.  Meyer,  Amer.  Journ.  Anat.  1914,  vol.  16,  p.  477. 


CIRCULATORY  SYSTEM 


331 


septal  wall  is  bent  within  the  lumen  of  the  ductus,  thus  partly  closing  it. 

(3)  The  foramen  ovale  is  closed  by  the  pressure  within  the  left  auricle 
being  raised  by  the  inflow  of  pulmonary  blood,  the  pressure  in  the  left 
auricle  then  reaching  a  higher  point  than  in  the  right  auricle.  The  closure 
of  the  foramen  is  assisted  by  an  alteration  in  the  action  of  the  limbic  bands 
(Fig.  336)  brought  about  by  their  indirect  attachment  to  the  diaphragm. 

(4)  The  hypogastric  arteries,  beyond  the  origin  of  the  vesical  arteries, 
become  reduced  to  cords.  (5)  The  pressure  within  the  aorta  becomes 
three  times  that  in  the  pulmonary  arteries  ;  the  left  ventricular  wall  becomes 
three  times  as  thick  as  that  of  the  right.  Before  birth  the  ventricular 
pressures  were  equal  and  so  were  the  thicknesses  of  the  ventricular  walls. 

Remnants  of  the  Foetal  Circulation  in  the  Adult.— The  nature  of 
these  remnants  has  been  already  described  ;  they  need  be  only  enumerated 
here.     They  are  : 

(1)  The  Obliterated  Hypogastric  Arteries  ;  (2)  The  Umbilicus  ;  (3) 
The  Round  Ligament  of  the  Liver  ;  (4)  The  Fibrous  Remnant  of  the 
Ductus  Venosus  ;  (5)  The  Eustachian  Valve  ;  (6)  The  Foramen  Ovale  ; 
(7)  The  Fibrous  Remnant  of  the  Ductus  Arteriosus. 

Changes  in  the  Position  of  the  Heart. — The  alteration  in  the  position 
of  the  heart  from  a  subpharyngeal  to  a  thoracic  position  during  the  5th, 
6th  and  7th  weeks  of  development  is  brought  about  by  two  factors.  First, 
the  heart  is  primarily  a  pump  for  forcing  the  blood  through  the  organ  of 


neural  tube. 


mesobl.  seg. 
notoch 

visceral  arch 
per/card. 


aorta 
duct  art: 
left  branch 


per/card. 


heart 


Fig.  352.— Diagrammatic  Section  across  the  Head  Fold  of  a  developing  Salamander 
to  show  the  relationship  of  the  Pericardial  part  of  the  Coelom  to  the  Heart  and 
Fore-gut.    (After  C.  Rabl.) 

Fig.  353. — Section  across  the  Junction  of  the  Aorta  and  Ductus  Arteriosus  (viewed 
from  behind)  of  a  full  time  Foetus  to  show  the  Inflection  of  the  Wall  of  the  Ductus 
within  the  Lumen  of  the  Aorta. 

respiration  ;  hence  in  the  fish  it  lies  beneath  the  gills,  in  air-breathing 
vertebrates  it  is  situated  close  to  the  roots  of  the  lungs.  Secondly,  in 
reptiles,  birds  and  mammals  a  neck  is  developed,  the  head  and  pharyngeal 
region  being  gradually  forced  forwards,  while  the  heart  and  pericardium 


332 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


come  to  lie  opposite  the  middle  part  of  tlie  dorsal  region  of  the  spine.  The 
neck  is  difierentiated  in  the  human  foetus  during  the  second  month.  All 
the  structures  in  the  neck  become  elongated — ^the  oesophagus,  trachea, 
vagus  nerves,  jugular  veins  and  carotid  arteries.  During  this  change  the 
arch  of  the  aorta  and  its  branches  are  evolved  from  the  ventral  stems  of  the 
aortae  and  aortic  arches.  In  most  mammals  the  left  carotid  arises  in 
common  with  the  aortic  stem,  and  a  reversion  to  this  type  is  the  commonest 
abnormality  to  which  the  aortic  arch  is  liable  in  man  (Parsons).  The 
separation  of  the  left  carotid  from  the  innominate  m  man  is  due  to  the  large 
size  of  the  upper  aperture  of  his  thorax.  The  left  vertebral  artery  or  the 
thyroidea  ima  may  gain  an  origin  from  the  aortic  arch. 

Final  Fixation  of  Heart. — As  may  be  seen  from  Figs.  329  and  355,  the 
heart  of  the  human  embryo  is  fixed  within  the  pericardium  exactly  as  in  a 


SUP:  VEN.CAV: 


ART.MESOCARD. 
PERICARD 
SINUS   TRANSV: 

VENOUS    MESOCARO 

INF:VEN:CAV. 


inf.  uen.^ 
cau.    s 


yduct  of  Cuui'er     _, 

'"iS^ 


sept  transM 


pericard 


Fig.  354. — The  Heart  pulled  forwards  to  show  its  two  Attachments  hy  the  Arterial 

(d,  d)  and  Venous  (e,  e)  Mesocardia. 
Fig.  355. — Diagram  of  the  Heart  of  a  Fish  to  show :   (1)  the  Primitive  Parts  of  the 

Heart ;    (2)  the  Relationship  of  the  Heart  to  the  Pharynx ;    (3)  the  Septum 

Transversum  ;  (4)  the  fixation  of  the  Heart. 

fish — being  attached  behind  to  the  septum  transversum  by  the  venous 
mesocardium  and  under  the  pharynx,  by  the  arterial  mesocardium.  By 
the  8th  week  the  interventricular  septum  is  complete  and  the  heart  has 
taken  up  its  position  in  the  thorax,  being  fixed  within  the  pericardium 
in  the  same  manner  as  in  the  adult  (Fig.  354).  The  original  mesocardia 
can  still  be  recognized,  separated  by  the  transverse  sinus  of  the  pericardium 
(Fig.  354,  h).  The  sinus  is  also  shown  in  Fig.  356.  The  derivatives  of  the 
truncus  arteriosus — -the  aortic  root  and  pulmonary  arteries — ^lie  within  the 
reflections  of  the  arterial  mesocardium  ;  the  caval  and  pulmonary  veins 
reach  the  auricles  through  the  reflections  of  the  venous  mesocardium. 
That  part  of  the  septum  transversum  which  contained  the  sinus  venosus 
and  great  veins  has  become  an  intrinsic  part  of  the  dorsal  wall  of  the 
pericardium.  The  heart  has  so  doubled  on  itself  that  the  venous  and 
arterial  mesocardia  are  in  contact,  being  only  separated  by  a  potential 
space — the  transverse  sinus  (Fig.  356). 

The  venous  mesocardium  becomes  much  more  extensive  by  the  ingrowth 
and  separation  of  the  pulmonary  veins.  These  grow  in  from  the  lungs, 
and  pierce  the  pericardium  to  reach  the  left  auricle  (Fig.  356).     They 


CIRCULATORY  SYSTEM 


333 


reach  the  auricle  through  the  mesentery  or  venous  mesocardiuni  of  the 
sinus  venosus.  The  migration  of  the  left  pulmonary  veins  causes  a  pro- 
longation of  the  venous  mesocardiuni  to  the  left  side  ;  when  the  heart 
is  removed  the  venous  mesocardium  is  seen  to  be  F-shaped  in  section. 
The  oblique  sinus  lies  in  the  concavity  of  the  pulmonary  venous  meso- 
cardium (Fig.  356). 

Primitive  Relationships  of  the  Pericardium. — Were  one  to  restore  the 
head  and  pericardium  to  the  relative  positions  they  occupy  in  the  5th 
week  of  development,  then  the  pericardium  must  be  lifted  from  the  thorax 
and  placed  beneath  the  chin  and  larynx  so  that  the  septum  transversum 
is  opposite  the  origin  of  the  phrenic  nerve  from  the  4th  cervical  segment ; 
the  anterior  border  of  the  umbilicus  is  also  then  opposite  the  origin  of  the 
phrenic  nerve.     In  the  somatopleure  over  the  pericardium  and  between 


aorta 


sup.  u.  cau. 


venous  meso-card. 

right  puf.  veins 

venous  meso-card. 


aorta 

arterial  meso-cardium 

puim.  art. 

transuerse  sinus 
left  pul.  veins 


diaph. 


oblique  sinus 


FiQ.  356. — View  of  the  Interior  of  the  Pericardium  showing  the  Attachments  of  the 
Heart  to  its  Dorsal  Aspect  by  the  Arterial  or  Venous  Mesocardia. 


the  mandible  and  umbilicus  are  developed  the  depressors  of  the  hyoid, 
the  sternum  and  sternal  ribs.  The  pericardium  is  therefore  the  coelom 
of  the  neck  ;  its  fibrous  wall  represents  the  deepest  layer  of  the  cervical 
somatopleure,  corres^^onding  to  the  fascia  transversalis  of  the  abdomen. 
With  the  elongation  of  the  neck  and  separation  of  the  pharynx  and  peri- 
cardium, the  tissue  of  the  branchial  segments  which  surrounds  the  aortic 
arches  is  drawn  out  to  form  the  carotid  sheaths. 

Ectopia  Cordis. — Occasionally  children  are  born  with  their  hearts 
exposed  on  the  surface  of  the  chest.  In  extreme  cases  only  the  dorsal 
wall  of  the  pericardium  is  present,  and  it  is  flush  and  continuous  with 
the  skin  of  the  chest.  In  these  cases  the  sternum  is  partially  absent,  or 
if  present  it  is  cleft,  the  right  and  left  halves  being  widely  parted.  No 
satisfactory  embryological  explanation  of  this  condition  has  yet  been 
given. 

Dorsal  Aortae. — The  dorsal  or  descending  aorta,  like  the  heart,  is  bilateral 
in  origin.     At  the  beginning  of  the  ith  week,  as  somites  are  being  demar- 


334      HUMAN  EMBKYOLOGY  AND  MORPHOLOGY 

cated  in  the  cervical  region  of  the  embryonic  plate,  the  right  and  left  dorsal 
aortae,  commencing  at  the  upper  ends  of  the  pharyngeal  arches,  pass 
backwards  side  by  side,  supplying  branches  to  the  archenteron  as  they  go 
(Fig.  25).  From  their  terminal  branches  on  the  yolk  sac  commence  the 
umbilical  arteries  (Fig.  25).  By  the  end  of  the  4th  week  the  dorsal  aortae 
have  fused  to  form  one  vessel  from  the  1st  thoracic  to  the  1st  lumbar  seg- 
ment. At  this  date  the  radicles  of  the  umbilical  arteries  arise  from  the 
aorta  opposite  the  1st  lumbar  segment- ;  by  the  end  of  the  5th  week  their 
origin  has  migrated  backwards  to  the  level  of  the  last  lumbar  segment. 
Although  the  umbilical  arteries  appear  to  be  direct  continuations  of  the 
dorsal  aortae  in  later  embryonic  and  foetal  Hfe,  yet  there  can  be  no  doubt 
that  this  honour  falls  to  the  middle  sacral  artery,  for,  as  we  have  seen 
(p.  27),  the  umbilical  arteries  must  be  regarded  as  greatly  modified  vesical 
or  allantoic  branches  of  the  aorta.  The  middle  sacral  artery  is  formed 
by  the  fusion  of  the  caudal  arteries — morphological  continuations  of  the 
dorsal  aortae.  The  coeliac  axis,^  superior  and  inferior  mesenteric  arteries 
are  the  sole  survivors  of  the  numerous  branches  supplied  by  the  paired 
aortae  to  the  archenteron.  At  the  end  of  the  5th  week  the  coeliac  axis 
arises  from  the  aorta  opposite  the  7th  cervical  segment ;  by  the  end 
of  the  7th  week  its  origin  is  opposite  the  10th  thoracic  segment — its  per- 
manent position.  The  superior  and  inferior  mesenteric  arteries  undergo 
a  corresponding  degree  of  migration  backwards  during  the  6th  and 
7th  weeks. 

Formation  o!  Blood  Vessels.^ — The  development  of  blood  vessels  and 
blood  corpuscles  can  best  be  studied  in  the  mesoderm  which  covers  the 
yolk  sac,  for  in  the  human  embryo,  with  the  exception  of  the  chorion, 
the  wall  of  the  yolk  sac  is  the  site  at  which  vessels  and  blood  are  first  formed. 
The  mesodermal  cells  covering  the  yolk  sac  show  a  differentiation  into  two 
strata — a  superficial  or  mesothelial,  representing  the  peritoneum  and  a 
deeper  or  mesenchymal,  lying  between  the  mesothelium  and  the  entodermal 
lining  of  the  sac  (Fig.  357).  The  cells  of  the  mesenchyme,  as  already 
mentioned  (p.  40),  are  angioblastic  or  vaso-formative  in  nature.  Origin- 
ally their  cell-bodies  are  continuous  and  form  a  syncytium,  but  in  Fig. 
357,  A  this  continuity  is  disappearing  and  a  mass  of  mesenchymal  cells  is 
being  separated  to  lie  within  a  blood  space.  In  the  wall  of  the  space  certain 
cells  are  being  differentiated  to  form  a  lining  membrane.  The  blood  space, 
the  cells  within  it  and  the  enclosing  endothelium  constitute  a  blood  island. 
In  the  island  are  to  be  seen  certain  mesenchymal  cells — ^haemoblasts — 
which  represent  the  parent  type  of  all  blood  cells^both  white  and  red. 
In  the  same  island  (Fig.  357,  A)  are  to  be  seen  certain  haemoblasts  in  which 
haemoglobin  is  being  formed,  thus  becoming  erythroblasts — the  parent 
type  of  red  cells.     They  represent  cells  which  are  being  set  aside  as  oxygen- 

1  Broman,  Anat.  Hefte,  1908,  vol.  36,  p.  405. 

^  For  recent  literature  on  origin  of  blood  and  vessels  :  see  H.  E.  Jordan,  Amer. 
Journ.  Anat.  1916,  vol.  19,  p.  227  ;  C.  R.  Stockard,  ibid.  1915,  vol.  18,  pp.  227,  525  ; 
R.  D.  Lillie,  ibid.  1919,  vol.  26,  p.  209  ;  Vera  Danchakoff,  ibid.  1918,  vol.  24,  p.  1, 
Anat.  Bee.  1916,  vol.  10,  p.  415  ;  Florence  Sabin,  Contributions  to  Embryology,  1917, 
vol.  6,  p.  61  ;   1920,  vol.  9,  p.  213. 


CmCULATORY  SYSTEM 


335 


carriers  and  are  therefore  to  be  counted  units  of  the  respiratory  system. 
Further,  in  such  an  island  (Fig.  357,  A)  are  to  be  recognized  large  lympho- 
cytes— or  leucoblasts — the  parent  type  of  white  corpuscles. 

The  blood  islands  scattered  over  the  yolk  sac  become  confluent  by  the 
union  and  canaliculization  of  intervening  endothelial  cells.  In  this  manner 
a  vascular  network  is  produced  on  the  yolk  sac  ;  the  manner  in  which  the 
blood  islands  are  united  is  typical  of  the  manner  in  which  new  blood  chan- 
nels are  formed.  Within  the  body  of  the  embryo  mesenchymal  cells 
assemble  in  vasoformative  groups,  become  canaliculized  and  unite  with 
neighbouring  groups  to  form  both  arteries  and  veins.  The  endothelial 
cells  of  capillaries  retain  throughout  life  the  vasoformative  power  which 
characterizes  them  during  the  period  of  development  and  growth.  The 
cellular  processes  at  the  growing  point  of  a  capillary  are  permeable  at  first 


MESOTHEL . 
MESENCHY. 

ENTODERM 


MESOTHELIUM. 
ENDOTHELIUM] 


re 


d  c 


(A) 


Fig.  357. — A,  Section  of  the  Wall  of  the  Yolk  Sac  to  show  the  constitution  of  a  Blood 
Island.     (H.  E.  Jordan.) 
a,  Haemoblast,  dividing  ;    b,  Erythroblast,  dividing  ;   c,  Blood-space  ;   d,  Haemo- 
blast ;  e,  Endothelial  cell ;  /,  Leucoblast. 
B,  Wall  of  a  Blood-space,  showing  Blood  Cells  arising  from  its  Endothelium.     (H.  E. 
Jordan.) 
a,   Endothelial  Cell ;    6,  Haemoblast  being    produced   from    Endothelial    Cell ; 
c,  Haemoblast  arising  outside  Blood-space  from  Endothelium. 

to  the  plasma  only,  subsequently  the  lumen  becomes  large  enough  to  allow 
the  blood  cells  to  pass. 

Formation  of  Blood. — In  the  development  of  each  system  of  the 
human  body  the  various  parts  appear  in  the  same  order  as  they  are  seen 
to  occur  in  ascending  the  scale  of  the  animal  kingdom.  In  many  inverte- 
brates the  blood  is  formed  by  only  a  fluid  living  intercellular  substance — 
the  plasma  ;  when  the  human  heart  beats  first,  its  lumen  contains  no 
blood  cells,  only  plasma.  In  amphioxus  nucleated  uncoloured  corpuscles 
appear  in  the  plasma  ;  the  cells  which  appear  first  (during  the  4th  week) 
in  the  circulation  of  the  human  embryo  are  the  red  nucleated  corpuscles 
(erythroblasts)  formed  in  blood  islands.  In  all  vertebrates,  with  the 
exception  of  amphioxus,  nucleated  white  as  well  as  nucleated  red  cells 
appear  in  the  blood  ;  in  the  human  embryo  the  white  cells  (leucocytes) 
appear  somewhat  later  than  the  red.  In  mammals  only  do  the  nuclei 
disappear  or  become  extruded  from  the  er}i;hroblasts,  red  blood  corpuscles 
(erythroplastids)  being  thus  formed.  The  erythroplastids  begin  to  appear 
in  the  blood  of  the  human  embryo  before  the  end  of  the  2nd  month,  and 


336 


HUMAN  EMBRYOLOGY  AND  MORPHOLOaY 


gradually  replace  the  erythroblasts,  which  cease  to  appear  in  the  circulating 
blood  some  days  after  birth  (Ham).  At  every  period  erythroblasts  are 
formed  as  derivatives  of  the  endothelium  of  vascular  walls.  The  mode 
in  which  blood  ceUs  arise  from  the  endothelial  lining  of  blood  spaces  is 
illustrated  in  Fig.  357,  B. 

The  Germinal  Centres  for  Red  Blood  Corpuscles. — At  every  period 
of  life  the  red  blood  corpuscles  (erythroplastids)  arise  from  erjrfchroblasts. 
These  are  formed  first  in  the  blood  islands  of  the  chorion,  of  the  yolk  sac 
and  within  vascular  extensions  of  the  vasoformative  cells  throughout 
the  body.  The  formation  of  blood  corpuscles  in  the  liver  commences  at 
the  beginning  of  the  second  month  of  development,  and  ceases  in  the  later 
months  of  foetal  life.^  The  parent  erythroblasts  lie  side  by  side  with  the 
liver  cells.     The  splenic  blood  spaces  are  also  sites  of  blood  formation  in 

mucus  cell 


epithelium. 


B. 


lymphocytes. 


A. 


Fig.  358. — Section  of  a  tubular  part  of  the  Thymus  of  a  Frog,  showing  (1)  the  Pro- 
duction of  Lymphocytes  from  the  Thymic  Epithelium ;  (2)  the  Production  of 
Hassall's  Corpuscles.  In  J.  a  leucocyte  within  the  wall  of  a  capillary  has  become 
enlarged  and  shows  concentric  striae  ;  in  B  the  nucleus  of  the  leucocytes  has 
undergone  division ;  it  completely  fills  the  lumen  of  the  capillary,  the  nuclei 
of  which  are  seen  in  the  periphery  of  the  body.     (After  Nusbrum  and  Machowski.) 

the  latter  half  of  foetal  life.  About  the  middle  of  foetal  life  the  capillaries 
of  bone  marrow  begin  to  be  invaded  by  angioblastic  outgrowths,  and  from 
birth  onwards  the  capillaries  of  the  red  bone  marrow  become  the  breeding 
ground  of  erythroblasts,  from  which  the  red  corpuscles  arise  by  disappear- 
ance of  their  nuclei. 

Origin  o£  White  Blood  Corpuscles.^ — The  reticular  tissue  which  under- 
lies the  epithelial  lining  of  the  alimentary  tract  corresponds  to  the 
mesenchymal  angioblastic  stratum  of  the  yolk  sac  and,  from  the  5th  month 
of  foetal  life  onwards,  is  the  seat  of  a  prolific  production  of  lymphocytes. 
The  apparent  production  of  lymphocytes  direct  from  entodermal  ceUs 
(Fig.  358),  such  as  are  represented  in  the  tonsillar  and  thymic  outgrowths, 
is  probably  due  to  the  fact  that  such  outgrowths  always  are  in  the  closest 
apposition  to  this  lymphocyte-producing  mesenchymal  stratum. 

^  See  MoUier,  ArcJiiv.  fiXr  Mikroscopic  Anat.  1909,  Bd.  74,  p.  474. 

^  See  Retterer  et  Lelievre,  Journ.  d'Anat.  et  Physiol.  1912,  vol.  48,  pp.  14,  194  ; 
r.  Weidenreich,  Ergebnisse  der  Anat.  1909,  vol.  19,  p.  527.  See  also  references  on 
p.  261. 


CIRCULATORY  SYSTEM  337 

Leucocytes  are  also  profusely  produced  from  (1)  tlie  endothelium  of 
serous  cavities — such  as  the  peritoneum  and  pleura  ;  (2)  from  the  endo- 
thelium of  lymphatic  vessels ;  (3)  from  leucoblasts  of  bone  marrow  ;  (4) 
from  the  endothelium  of  capillaries,  and  possibly  (5)  from  connective 
tissue  cells.  As  yet,  however,  these  statements  must  be  accepted  with 
reserve,  for  there  is  still  a  degree  of  uncertainty  regarding  the  genetic 
relationship  of  one  form  of  leucocyte  to  other  forms.  Mollier,  who  has 
recently  studied  the  development  of  the  blood  corpuscles,  describes  the 
liver  as  the  chief  source  of  white  blood  corpuscles  during  foetal  life  ;  later 
the  site  of  their  formation  is  shifted  to  the  blood  spaces  in  marrow.  He 
regards  both  basophile  and  eosinophile  leucocytes  as  arising  in  the  liver 
from  the  same  parent  cells  (haematoblasts)  as  give  origin  to  the  red  nucleated 
corpuscles. 

Lymphatic  System. — In  all  vertebrate  animals  the  plasma  or  lymph 
from  the  tissues  of  the  body  is  drained  into  the  veins  by  a  special  system 
— the  lymphatic  vessels.  In  amphibia  the  lymph  collects  in  large  spaces 
lined  by  endothelium,  from  which  it  is  forced  into  the  venous  system  by 
two  pairs  of  lymph  hearts — one  pair  situated  in  the  angle  between  the 
jugular  and  subclavian  veins,  the  other  pair  between  the  internal  and 
external  iliac  veins.  In  mammals  the  lymph  hearts  disappear  ;  they  are 
no  longer  required,  for  the  negative  pressure  in  the  veins  of  the  thorax, 
set  up  by  the  evolution  of  a  separate  respiratory  cavity,  is  suflS.cient  to 
draw  the  lymph  into  the  venous  system.  It  is  remarkable,  however,  that 
Miss  Sabin  who,  by  a  paper  ^  published  in  1902,  inaugurated  our  knowledge 
of  the  development  of  the  mammalian  lymphatic  system,  found  that  the 
lymph  vessels  appear  first  at  those  four  points  where  the  amphibian  lymph 
hearts  are  situated. 

Recent  enquiries  by  American  embryologists  have  thrown  quite  a  new 
light  on  the  origin  of  the  lymphatic  system.  They  have  established  that 
the  formation  of  lymph  vessels  begins  at  definite  centres  and  from  such  a 
centre  vessels  spread  outwards,  vascularize  and  drain  a  definite  area. 
If  the  starting  centre  is  excised,  then  there  is  no  outgrowth  and  vessels 
from  neighbouring  areas  invade  and  drain  the  one  thus  deprived.  While 
the  angioblasts  of  the  blood  system  are  everywhere  and  have  established 
a  complete  vascularization  of  the  embryonic  tissues  before  the  end  of  the 
4th  week,  the  angioblasts  of  the  lymphatic  system  do  not  become  manifest 
until  the  end  of  the  6th  week,  when  they  form  a  capillary  network  in  the 
centres  of  initiation.  The  greatest  and  earliest  centre  is  situated  in  the 
angle  between  the  jugular  and  subclavian  veins,  where  the  termination 
of  the  thoracic  duct  is  afterwards  formed.     By  the  end  of  the  8th  week 

1  Florence  R..  Sabin,  American  Journ.  of  Anat.  vol.  1,  1902,  p.  367.  In  nearly  every 
subsequent  volume  will  be  found  some  of  the  important  contributions  made  to  our 
knowledge  of  the  development  of  lymphatics  by  modern  American  embryologists. 
F.  T.  Lewis,  Amer.  Journ.  Anat.  1909,  vol.  9,  p.  33  ;  Florence  R.  Sabin,  Amer. 
Journ.  Anat.  1909,  vol.  9,  p.  43  ;  Geo.  S.  Huntington  and  C.  F.  W.  McClure,  Amer. 
Journ.  Anat.  1910,  vol.  10,  p.  177  ;  F.  T.  Lewis,  Amer.  Journ.  Anat.  1905,  vol.  5, 
p.  95  ;  G.  S.  Huntington,  Anat.  Anz.  1911,  vol.  39,  p.  385;  E.  R.  Clark,  Amer.  Journ. 
Anat.  1912,  vol.  13,  p.  347;  G.  S.  Huntington,  Amer.  Journ.  Anat.  1914,  vol.  16, 
p.  259 ;  Ch.  F.  W.  McClure,  ibid.  1915,  No.  4 ;  E.  and  E.  Clark,  Contrib.  to 
Embryology,  1920,  vol.  9,  p.  447. 


338 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


tlie  capillary  network  of  lymph  vessels  have  fused  and  formed  the  extensive 
lymph  sac  shown  in  Fig.  359.  In  the  3rd  month  outgrowths  from  the 
jugular  sac  on  each  side  of  the  neck  spread  and  invade  the  tissues  of  the 
neck,  head  and  arm — all  save  the  central  nervous  system  and  voluntary 
muscles.  These  are  not  drained  by  the  lymphatic  system.  The  great 
lymph  sacs  are  merely  temporary  structures  ;  their  cavities  are  filled  by 
reticular  lymphoid  tissue  produced  by  the  lymphatic  endothelium  which 
lines  the  sacs.  As  soon  as  formed,  the  jugular  lymph  sac  effects  a  union 
with  the  jugular  vein,  the  orifice  being  guarded  by  valvular  folds. 


INT:    JUGUUAR    VEIN 

ExT: Jugular  vein 


JUGULAR   LYMPH  SAC, 


THORACIC   DUCT 


MESENTEt?lC   SAC 
INF:  VENA  CAVA 
POST:  LYMPH  •.  SAC 
INT      ILIAC  VEIN 


Fig.  359. — The  Main  Lymphatic  Vessels  and  Sinuses  of  the  Human  Foetus  at  the 
beginning  of  the  3rd.  month.     (After  Prof.  Florence  Sabin.) 

Another  pair  of  lymph  sacs  appear  in  the  pelvis — related  to  the  corre- 
sponding iliac  veins,  into  which  they  at  first  open  (Fig.  359).  From  the 
pelvic  or  iliac  sacs  outgrowths  invade  the  hind  limbs  and  tissues  of  the 
pelvis  and  buttocks.  In  the  mesenchymal  tissue  in  which  the  dorsal 
aorta  is  embedded  there  appear  a  series  of  endothelial-lined  lymphatic 
spaces,  which  become  united  and  place  the  posterior  or  iliac  sacs  in  com- 
munication with  the  jugular  sacs.  In  this  way  two  thoracic  ducts  are 
formed  at  the  end  of  the  second  month.  Two  other  retroperitoneal  centres 
appear,  one  at  the  root  of  the  superior  mesenteric  artery,  from  which 
arises  the  system  of  vessels  which  drains  the  alimentary  tract ;  the  re- 
ceptaculum  chyli  is  also  formed  from  a  special  centre.     The  lymphatic 


CIRCULATORY  SYSTEM  339 

system  is  just  as  mucli  a  "  closed  "  system  as  is  the  haemal  system  ; 
everywhere  its  walls  are  lined  with  endothelium.  Nowhere  does  it  open 
on  "  tissue  spaces." 

Lymphatic  Glands  make  their  first  appearance  during  the  fourth 
month  at  the  site  of  the  lymph  sacs  and  along  the  leashes  of  vessels  leading 
to  these  sacs.  They  appear  first  as  follicles  which  are  developed  within 
the  lumina  of  the  vessels  so  that  the  lymph  passing  along  is  exposed  to  the 
lymphocytes  developed  in  the  reticular  tissue  of  the  node.  Lymphocytes 
arise  by  proliferation  of  the  cells  lining  lymphatic  spaces  and  vessels. 
The  lymphatic  glands  and  nodes  grow  in  size  and  number  during  each 
month  of  foetal  life.  They  serve  as  germinal  centres  for  the  production 
of  lymphocytes. 

Interscapular  Gland.^ — Under  this  name  has  been  included  the  mass 
of  peculiar  tissue  which  occupies  the  posterior  triangle  of  the  neck,  and 
extends  under  the  trapezius  towards  the  posterior  border  of  the  scapula. 
It  represents  the  hibernating  gland  of  insectivora  and  bats.  It  begins  to 
form  in  the  2nd  month  of  foetal  life  at  the  site  of  the  jugular  lymph  sac. 
It  is  composed  of  a  stratum  of  three  tissues — lymphoid,  haemolymph 
(blood-forming)  and  fat. 

Haemolymph  Glands. — In  the  subperitoneal  fat  of  many  mammals 
numerous  red  bodies  may  be  seen  which  difier  from  lymphatic  glands 
in  the  following  points  :  (1)  the  sinuses  contain  red  blood  corpuscles  ; 
(2)  instead  of  aiierent  and  efferent  lymphatic  vessels,  arteries  and  veins 
open  into  the  sinuses.  They  occur  in  the  human  foetus,  and  apparently 
serve  the  same  function  as  the  spleen  (W.  B.  Drummond). 

Bone  Marrow. — Until  the  5th  month  of  foetal  life  the  marrow  is  com- 
posed of  branched  cells  embedded  in  a  jelly-like  matrix  (primary  marrow)  ; 
it  then  assumes  the  appearance  of  lymphoid  tissue,  and  contains  leuco- 
blasts  ;  in  the  6th  month  erythroblasts  and  erythrocytes  appear  in  the 
dilated  capillaries  forming  red  marrow  in  the  centres  of  ossification  (Ham- 
mar).  At  birth  the  marrow  of  all  the  osseous  tissue  is  red  ;  during  the 
years  of  active  growth  the  marrow  of  the  shafts  of  bones  is  gradually 
replaced  by  fat  cells,  yellow  marrow  being  thus  formed  (Hutchison). 
From  birth  onwards  the  red  marrow  forms  the  only  tissue  in  which  red 
blood  corpuscles  are  produced. 

^  For  an  account  of  this  structure  see  Bonnot,  Journ.  Anat.  and  Physiol.  1909, 
vol.  43,  p.  43. 


CHAPTER  XXII. 


RESPIRATOEY  SYSTEM. 


Stages  in  the  Evolution  of  the  Human  Respiratory  System. — The 

development  of  the  lungs,  the  pleural  cavities  and  chest  waU  forms 
one  of  the  most  complicated  chapters  of  human  embryology.  The  steps 
in  the  development  of  this  system,  as  seen  within  the  human  embryo,  are 
unintelligible  until  they  are  interpreted  by  a  study  of  comparative  anatomy, 
especially  of  those  animal  forms  that  show  the  manner  in  which  a  purely 
pulmonary  system  arose  from  one  which  was  purely  branchial.  Hence 
it  is  necessary  to  briefly  recapitulate  the  various  modifications  of  the 


rathke's  pock 


MOUTH 
MANDIBLE 


PULM:  ART 
LUNG    BUD 


DORSAL    AORTA 


PULM; POCKET 


Fig.  360. — Showing  the  Pulmonary  Artery  arising  from  the  6th  Aortic  Arch  in 

Human  Embryo  of  5  weeks.    (After  His.) 

Fig.  361. — Showing  that  the  Pulmonary  Diverticulum  arises  between  and  behind 

the  bases  of  the  last  or  6th  pair  of  Visceral  Arches.     (Frazer.) 

respiratory  system  which  are  seen  to  occur  in  ascending  from  the  lowest 
to  the  highest  class  of  vertebrates.     Four  stages  may  be  recognized  : 

Stage  I. — This  stage  is  represented  in  fishes,  in  which  the  respiratory 
system  is  made  up  of  three  parts  :  (1)  Branchiae,  in  which  the  respiratory 
exchange  of  blood  gases  is  efiected  ;  (2)  the  swim  bladder,  an  evagination 
from  the  oesophagus,  containing  oxygen,  and  surrounded  by  lymphoid 
tissue ;  (3)  the  musculature  of  the  branchial  arches  and  pharynx,  which 
pumps  water  through  the  branchial  clefts,  and  helps  to  force  the  blood 
through  the  branchiae  ;  (4)  nerve  system  with  centre — both  motor  and 
sensory — in  the  hind-brain,  and  visceral  nerves  supplied  by  the  vagus,  and 

340 


EESPIRATORY  SYSTEM 


341 


from  vasomotor  centres  in  the  dorsal  region  of  the  cord.  Although 
branchiae  are  never  developed  in  the  human  embryo,  yet  the  condition 
in  the  4th  and  5th  weeks,  when  the  heart  is  subpharyngeal  in  position  and 
the  visceral  and  aortic  arches  are  in  process  of  development,  can  only  be 
explained  by  the  supposition  that  at  one  stage  of  evolution  these  parts 
had  served  a  respiratory  purpose. 

Stage  II. — In  most  amphibians  four  parts  are  to  be  recognized  in  the 
respiratory  system.  (1)  The  swim  bladder  is  bifid ;  each  half,  now 
properly  called  a  lung,  projects  within  the  abdominal  cavity  above  the 
pericardium  and  liver  (Fig.  362).  (2)  A  respiratory  passage  leading  from 
the  pharynx  to  the  lungs,  and  formed  from  the  2nd,  3rd,  and  4th  branchial 


ext  oblig. 


pesoph. 

sup.  uen.  cau. 

r phrenic  nerue 
Pharynx. 


reel  abdoml 

sup.  red  obi 


ept.  transuX  ^sternum 


='SternO'COsta/. 
diaph. 


sterno-hyoid 


Fig.  362. — Diagram  of  the  Lung  and  Respiratory  Muscles  of  an  Amphibian  (Surinam 
toad)  to  show  the  Muscles  out  of  which  the  Diaphragm  is  evolved.  The  lungs 
lie  within  the  abdomen  as  in  the  6th  week  embryo.  The  arrow,  beginning  over 
the  apical  region  of  the  lung,  shows  the  direction  in  which  the  mammalian  lung 
develops.  The  shoulder  girdle  and  greater  part  of  the  external  oblique  are  cut 
away.     The  heart  lies  above  the  sternum. 

(4th,  5th,  and  6th  visceral)  arches.  (3)  The  vascular  system  for  each 
lung  rises  from  the  artery  of  the  6th  visceral  arch  (Fig.  360).  (4)  The 
branchial  muscles,  which  formerly  forced  water  through  the  gill  sHts, 
are  now  transmuted  into  pharyngeal  muscles  and  help  to  pump  air  into  the 
lungs — thus  acting  as  muscles  of  inspiration.  The  muscles  of  the  body 
wall  (see  Fig.  362)  are  modified  to  form  the  muscles  of  expiration.  Two 
parts  of  these  are  specially  worthy  of  notice,  because  in  mammals  they 
become  the  diaphragm  :  viz.  (a)  part  of  the  transversalis  sheet,  which 
rises  from  the  spine  and  ends  in  the  pericardium,  oesophagus  and  roots  of 
the  lung  ;  (6)  a  deep  lamina  of  the  rectus  abdominis  which  ends  in  the 
pericardium.  The  nerve  to  these  muscular  segments  descends  on  the 
outer  aspect  of  the  superior  vena  cava  exactly  in  the  same  manner  as 
the  phrenic  nerve  descends  to  the  diaphragm  (Fig.  362). 


342 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


Stage  III. — (1)  In  reptiles  the  lungs  are  abdominal  in  position,  but 
an  elaborate  series  of  septa  have  grown  up  within  them,  thus  exposing  a 
larger  vascular  surface  to  the  inspired  air.  (2)  The  respiratory  passage 
is  elongated  and  demarcated  into  larynx,  trachea  and  bronchi.  (3)  Ribs 
and  sternum  are  developed,  so  that  the  musculature  of  the  body  wall 
becomes  differentiated  into  inspiratory  and  expiratory  muscles. 

Stage  rV. — In  mammals  an  extraordinary  developmental  change  occurs 
which  leads  to  the  formation  of  two  pleural  cavities  and  their  complete 
separation  from  the  abdomen  by  a  diaphragm.     The  origin  of  the  dia- 


rt  d.  of  Ouuier^ 
rt  pleura 
lung  bud- 
dorsal  mesogast. 


dorsal  meso-card 

to  left  pleura 
pericardium 

liver  bud 

■septum  transuersum 

eck  of  yolk  sac. 

umbilical  diuertic. 


becomes  tunica  vaginalis 


Fig.  363. — Form  of  the  Coelom  in  a  Human  Embryo  of  the  5th  week.  The  arrow 
under  the  right  duct  of  Cuvier  is  in  the  right  passage  leading  from  the  peri- 
cardial to  the  peritoneal  cavity.  It  is  by  the  expansion  of  this  coelomic  passage 
that  the  right  pleural  cavity  is  formed. 

phragm  must  be  sought-  for,  not  in  the  reptiles,  present  or  past,  but  in  a  very 
low  form  of  amphibian.  To  understand  the  origin  of  the  pleural  cavities 
and  diaphragm  of  mammals  the  following  points  must  be  kept  in  mind  : 
(1)  That  the  septum  transversum,  in  its  fully  developed  condition,  as  seen 
in  the  frog,  is  the  fibrous  layer  of  tissue  which  separates  the  heart  from  the 
liver,  a  corresponding  structure  is  seen  in  the  human  embryo.  (2)  Into 
the  septum  transversum  are  inserted  the  deepest  layer  of  the  rectus 
abdominis  and  vertebral  fibres  of  the  transversalis  (Fig.  362).  (3)  The  ribs 
are  developed  in  the  intermediate  layers  of  the  body  wall — between  seg- 
ments of  the  external  and  internal  oblique  muscles.  The  muscular  fasciculi 
which  end  in  the  septum  transversum  are  deep  to  the  ribs  and  intercostal 


RESPIRATORY  SYSTEM  343 

musculature.  (4)  The  lung  buds  lie  at  first  in  tlie  mesentery  of  the  fore- 
gut  from  which  they  grow  outwards  on  each  side  into  a  narrow  (pleural) 
passage  of  the  coelom,  which  leads  from  the  pericardium  to  the  peritoneal 
cavity  (Fig.  363).  The  passage  is  situated  at  the  upper  border  of  the 
septum  transversum ;  its  pericardial  opening,  the  iter  venosum,  is  closed 
by  the  superior  vena  cava.  Now,  when  the  lung  buds  grow  out  in  the 
mammalian  embryo,  they  fill  these  passa:ges  and  their  hinder  ends  project 
into  the  abdominal  cavity.  Then  in  the  6th  and  7th  weeks  the  coelomic 
passage  undergoes  an  extremely  rapid  expansion,  growing  into  the  body 
wall  so  as  to  separate  the  pericardium  and  the  deeper  or  diaphragmatic 
layer  of  musculature  from  the  outer  or  intercostal  stratum.  Lung  growth 
follows  closely  on  pleural  expansion.  The  pleural  cavities  are  in  reality 
new  chambers  or  spaces  produced  by  an  enormous  expansion  of  the  narrow 
coelomic  or  pleural  passages  of  the  embryo.  We  shall  see  that  the  septum 
transversum  is  also  cleft  during  the  expansion. 

The  development  of  the  diaphragm  gave  mammals  two  advantages  : 
(1)  an  enormous  increase  in  the  power  of  inspiration  ;  (2)  the  respiratory 
negative  pressure,  which  afiects  all  the  viscera  within  the  body  cavity  in 
reptiles,  became  restricted  to  the  thorax  in  mammals. 

Morphological  Parts  of  the  Respiratory  System  are : — (a)  The 
respiratory  passage  which  extends  from  the  pharynx  to  the  bronchioles 
of  the  lung.  The  tissues  which  surround  this  passage  are  derived  from  the 
coverings  and  substance  of  the  4th,  5th  and  especially  the  6th  arch.  The 
nasal  cavities  continue  the  breath  passages  to  the  nostrils.  We  have  seen 
how  these  cavities  are  shut  ofi  from  the  mouth  in  the  later  part  of  the  2nd 
month.  (6)  The  pulmonary  tissue  made  up  of  (1)  a  diverticulum  from  the 
fore-gut  which  represents  the  swim  bladder  ;  (2)  a  vascular  network 
derived  from  the  capillaries  of  the  fore-gut,  into  which  opens  a  blood  supply 
from  the  last  (6th)  pair  of  aortic  arches  (Fig.  360).  (c)  The  respiratory 
muscles,  sternum  and  ribs  are  formed  in  the  somatopleure  of  the  body  wall. 

Development  of  the  Pulmonary  System. — In  the  4th  week,  towards 
the  end  of  it,  a  deep  groove  appears  in  the  floor  of  the  primitive  pharynx 
and  oesophagus.  The  groove  or  trough-like  depression  of  the  fore-gut 
commences  between  the  ventral  ends  of  the  6th  (or  5th  and  6th,  see  Fig. 
361)  arches  and  stretches  almost  to  the  stomach  (Fig.  364).  The  furcula, 
formed  from  the  central  mass  and  ventral  parts  of  the  4th  segments,  bounds 
the  pulmonary  groove  in  front  (Fig.  319)  ;  in  its  anterior  part,  which  is  the 
most  prominent,  is  developed  the  epiglottis  ;  the  anterior  parts  of  the 
lateral  margins  of  the  pulmonary  groove,  form  the  true  vocal  cords,  for 
the  arjrteno-epiglottidean  folds  are  secondary  formations  of  a  later  date 
(Frazer).  The  posterior  parts  of  the  margins  of  the  groove  unite,  and  in 
this  manner  the  posterior  part  of  the  groove  is  separated  as  a  diverticulum 
on  the  ventral  aspect  of  the  oesophagus  (see  p.  270).  The  anterior  part 
of  the  groove  represents  the  basis  of  the  pulmonary  passage  ;  the  posterior 
part,  the  basis  of  the  pulmonary  tissue.  Two  points  should  be  noted  in 
connection  with  the  relationships  of  the  oesophagus  at  the  4th  week  : 
(1)  like  that  of  a  fish,  it  is  extremely  short ;  (2)  it  lies  between  the  right 
and  left  cavities  of  the  coelom  in  the  dorsal  attachment  of  the  mesocardium 


344 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


of  tlie  sinus  venosus  (Fig.  367).  (3)  The  part  of  the  coelom  whicli  lies  at 
each  side  of  the  oesophagus  is  the  narrow  passage  connecting  the  pericardial 
and  peritoneal  cavities  which  becomes  expanded  to  form  the  pleura. 


tuberculum  impar. 


1st  recess  (salivary  glands) 

2nd  recess  (tonsil) 

median  thyroid  bud. 

-Jurcula  (epiglottis) 

3rd  recess  (thymus) 
4th  recess  (lat  thyroid  bud) 
aryteno.  epi.  fold 
coelom  (pericardium) 
pulmonary  groove 


stomach 


Fig.  304. — rioor  of  the  Pharynx  and  Oesophagus  of  a  Human  Embryo  of  4  weeks, 
showing  the  Furcula,  Pulmonary  Groove  and  Diverticulum.     (After  His.) 

When  the  pulmonary  outgrowth  is  viewed  from  the  side,  its  posterior 
extremity  is  seen  to  end  in  a  deep  pocket,  the  pulmonary  pocket  or  diver- 
ticulum (Figs.  276,  369).     The  wall  of  the  pocket  is  lined  by  a  mass  of 


Fig.  365. — The  Trachea,  Bronchi  and  Lung  Buds  in  the  5th  week  of  development. 
(After  Broman.) 

Fig.  366. — The  Lohulation  of  the  Lungs  early  in  the  6th  week.    (After  Merkel.) 

entoderm,  which  ultimately  forms  the  epithelial  lining  of  the  whole  re- 
spiratory tract,  from  the  ciliated  epithelium  of  the  trachea  to  the  pavement 
epithelium  lining  the  alveoli  of  the  lungs.  Round  the  pulmonary  bud  is 
grouped  a  mass  of  mesodermal  tissue  out  of  which  the  connective-tissue 
system  of  the  trachea,  bronchi  and  lungs  is  developed. 


RESPIRATORY  SYSTEM 


345 


In  the  5tli  week  the  pulmonary  pocket  produces  a  larger  right  and  a 
smaller  left  process,  the  right  and  left  lung  buds  (Fig.  365).  The  median 
j)art  of  the  pulmonary  outgrowth  separates  from  the  pharyngeal  floor 
and  forms  the  trachea.  The  anterior  part  forms  the  larynx  (see  p.  351). 
The  right  bud  forms  the  right  lung  and  bronchus  ;  the  left,  the  left  lung 
and  bronchus.  As  the  pleural  cavities  and  their  contained  lung  buds 
develop  the  stomach  is  forced  backwards ;  the  oesophagus  becomes 
elongated.  The  tracheal  part  of  the  bud  becomes  separated  from  the 
oesophagus,  but  both  retain  the  same  nerve  supply— the  recurrent  branch 
of  the  vagus — which  is  the  nerve  of  the  6th  arch.  The  rapid  development 
of  the  lung  during  the  4th,  5th,  and  6th  weeks  is  illustrated  by  Figs.  276, 
277,  365,  366.     In  the  4th  week  the  lung  bud  is  a  mere  diverticulum  ; 


dght  dorsal  aorta 


fore-gut 
right  lung  bud 

dorsal  mesocard. 


passage  between 
pericard.  and  pleura 


Isft  dorsal  aorta 
dorsal  mesent 

card,  vein  of  left  side 


pleuro-perit.  commun 
pleura 
uen.  mesocard. 
left  d.  of  Cuuier 
pericard. 
heart 


pericardium 


Fig.  367. — A  Section  of  a  Human  Embryo  to  show  the  Relationships  of  the  Pul- 
monary Buds  at  the  5th  week,  looking  backwards.     (After  Kollmann.) 

in  the  5th  the  trachea  and  buds  of  the  main  bronchi  are  apparent ;  in  the 
6th  week  the  secondary  bronchi  and  separate  lobes  are  in  a  process  of 
differentiation. 

In  Fig.  367  the  relationship  of  the  lung  buds  is  shown  to  surrounding 
structures  during  the  5th  week.  The  following  points  should  be 
noted  : 

(1)  As  the  lung  buds  grow  out  they  push  their  way  into  the  pleural 
passages — the  narrow  communications  between  the  pericardium  and 
peritoneum.  These  parts  of  the  coelom  form  the  pleurae.  The  part  of 
the  coelomic  lining  which  is  invaginated  as  a  covering  on  the  lung  bud 
becomes  the  visceral  pleura.  The  invaginating  or  ensheathing  lining  of 
the  isthmus  becomes  the  parietal  pleura.  As  the  lung  buds  grow,  they 
distend  the  originally  small  pleural  parts  of  the  coelom  until  at  the  time 
of  birth  the  right  and  left  pleurae  almost  meet  in  front  of  the  heart,  and 
completely  separate  the  chest  wall  from  the  pericardium  and  diaphragm. 


346 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


They  meet  after  birth  under  the  sternum,  enclosing  between  them  the 
anterior  mediastinum. 

(2)  As  will  be  seen  from  Fig.  363,  the  lung  buds  sprout  out  from  the 
mesentery  just  behind  the  duct  of  Cuvier.  This  relationship  is  retained 
in  the  adult,  the  vena  azygos  major  and  superior  vena  cava  lying  above 
and  in  front  of  the  root  of  the  right  lung.  The  roots  of  the  lungs  represent 
the  situation  at  which  the  embryonic  pulmonary  outgrowth  took  place. 
If  the  left  duct  of  Cuvier  persisted  it  would  lie  above  and  in  front  of  the 
root  of  the  left  lung.  The  ductus  arteriosus — ^part  of  the  6th  arch — 
lies  over  the  root  of  the  left  lung.  At  this  stage  (5th  week)  the  pleural 
passage  or  cavity  is  still  in  communication  with  both  pericardial  and 

aorta 
oesoph. 
lung 
,pleura 
sept,  iransu. 


diaph.  lamina. 

lung 

sept,  transut 

pen'c.  lamina'. 


left  aur. 


pulm.  uein. 
sm.  ven, 
rt.  aur. 


ericard.  cau 


ventricle 


bulb.  art. 


Fig.  368. — Transverse  Section  of  a  Human  Embryo  showing  (1)  the  Outgrowth  of 
the  Lung  Buds  from  the  Mesentery  of  the  Fore-gut ;  (2)  the  Separation  of  the 
Pericardium  from  the  Body  Wall  and  Formation  of  the  Pleural  Cavities  ;  (3)  the 
,  Separation  of  the  Diaphragmatic  Lamina  from  the  Septum  Transversum.  The 
arrow  shows  the  direction  in  which  the  left  pleura  invades  the  body  wall.  (After 
Lockwood.) 

peritoneal    cavities.     Its    communication    with    the    pericardium    closes 
at  the  end  of  the  6th  week. 

Formation  of  the  Bronchi  and  Lungs.^ — The  bronchi  are  the  stalks 
of  the  right  and  left  lung  buds.  The  right  bud  is  the  bigger  ;  the  left  is 
probably  repressed  by  the  heart  turning  to  the  left  side.  The  right  shows 
three  secondary  buds — ^the  forerunners  of  the  upper,  middle  and  lower 
lobes  of  the  lung  ;  the  left,  two,  which  form  the  upper  and  lower  lobes 
(Fig.  365). 

The  condition  of  the  lung  buds  during  the  6th  week  is  shown  in  Figs. 
366,  370.  Not  only  are  the  right  and  left  bronchi  formed,  but  so  also  are 
the  chief  bronchial  ramifications.  Each  ramification  ends  in  a  bud,  which 
divides  again  and  again  and  keeps  on  dividing  until  the  fourth  month. 
The  terminal  buds  form  the  bronchioles  and  infundibula.     Each  bud  is 


1  R.  Heiss,  Anat.  Anz.  1912,  vol.  41,  p.  62  (Dev.  of  Lobes  of  Lung). 


RESPIRATOKY  SYSTEM 


347 


solid,  and  carries  its  slieath  of  mesoderm  ;  the  appearance  on  microscopic 
examination  is  very  similar  to  that  of  a  gland,  such  as  the  pancreas  or 
parotid.  In  the  3rd  month  the  mesoderm  between  the  pulmonary  buds 
is  extremely  abundant ;  by  the  sixth  month  it  forms  merely  a  thin  stroma 
amongst  the  alveolar  air  sacs.  At  the  sixth  month  saccular  evaginations 
occur  from  the  infundibula  ;  they  form  the  air  cells,  or  alveoli.  Nothing 
is  known  definitely  of  the  growth  of  the  lung  tissue  after  birth,  but 
it  is  probably  formed  by  outgrowths  from  the  infundibula  occupying 
the  sub-pleural  layer.  The  opinion  usually  held  by  embryologists  is 
that  the  production  of  new  alveoli   ceases  at  the   7th  month  of   foetal 


dorsal  aorta 


7ct  aortic  arch 


transverse  sinus 

mesa-  /^^  ^"^^  ^^  ^""'^'^ 
cardium   /  lung  bud  in  mesentery 

dorsal  mesogast. 
torn. 


art.  meso-card. 


con.  arter 
pericard. 


amnion 


itelline  uein 
liuer  bud 
yolk  sac. 

ventral  mesentery 


septum  transv 
Fig.  369. — Diagram  to  show  the  manner  in  which  the  Heart  is  fixed  within  the  Peri- 
cardium by  the  Arterial  and  Venous  Mesocardia  in  a  Human  Embryo  of  4  weeks. 
The  "  dorsal  mesocardium  "  in  the  above  figure  forms  part  of  the  venous  meso- 
cardium. 

life.     After  that  time  there  is  merely  an  enlargement  of  the  elements 
already  formed. 

Changes  in  the  Shape  o£  the  Lung.— Even  in  the  6th  week  the 
lungs  are  merely  glandular  masses  round  the  terminal  parts  of  the  bronchial 
outgrowths.  As  in  the  frog,  the  hilum  at  this  time  forms  the  apex  of  the 
lung.  During  the  2nd  and  3rd  months  the  lungs  assume  their  definite 
shape.  The  upper  lobe  grows  towards  the  neck,  and  an  apical  region  is 
thus  formed.  The  diaphragmatic  or  basal  surface  is  at  first  absent,  but 
as  the  pleural  cavities  expand  and  the  basis  of  the  diaphragm  is  stripped 
from  the  body  wall,  this  surface  appears.  In  the  human  and  anthropoid 
foetus  the  diaphragmatic  or  basal  surface  becomes  remarkably  large. 
The  most  important  change,  however,  relates  to  the  anterior  or  ventral 
border  of  the  lungs  ;  at  first  situated  on  the  dorsal  side  of  the  pericardium 
the  lungs  expand  forwards  until  they  reach  almost  to  the  lateral  borders 
of  the  sternum.  In  man  and  anthropoids  the  ventral  or  sterno-costal 
part  of  the  lung  reaches  a  high  degree  of  development. 


348 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


Evolution  of  Air  Sacs.^ — In  reptiles  we  see  the  original  bladder-like 
lung  becoming  demarcated  into  two  parts — an  anterior  or  cephalic  part 
with  thick  spongy  walls  which  contain  cellular  recesses  for  air  and  are 
richly  supplied  with  blood  ;  and  a  posterior,  thin-walled  and  simple  part. 
The  thin-walled  hinder  part  serves  as  a  pulmonary  bellows  during  the  re- 
spiratory expansion  and  contraction  of  the  body  wall ;  it  naturally  acts 
on  the  most  yielding  part  of  the  lung.  In  birds,  the  anterior  or  respiratory 
part  of  the  lung  has  been  sharply  demarcated  from  the  posterior  or  "  bel- 
lows "  part.  The  latter  is  broken  up  into  abdominal  air  sacs.  In  mammals 
the  "  bellows  part,"  represented  by  the  pulmonary  infundibula  and  air  sacs, 
is  disseminated  amongst  the  "  respiratory  "  tissue  and  the  bronchi  are 
arranged  in  such  a  way  as  to  permit  every  part  of  the  lung  to  undergo 
expansion.     Thus  the  pattern  of  the  bronchial  tree  is  determined  by  the 

pharynx 
larynx 


right  cardin.  jwin 
right  branch 

I. 
put.  art: 
II. 


trachea 

left  bronchus 

I. 

mesoblast 
II. 
■pul.  buds 

III. 

mesoblast 


oesoph. 


Fig.  370. — The  condition  of  the  Right  and  Left  Pulmonary  Buds  in  an  Embryo 
at  the  end  of  the  6th  week.     (After  His.) 

nature  of  the  respiratory  movements.  Whereas  only  the  respiratory 
part  of  a  bird's  lung  is  supradiaphragmatic  the  whole  of  the  mammalian 
lung  occupies  this  position. 

There  are  certain  peculiarities  in  the  lungs  of  animals  which  are  adapted 
to  an  upright  posture  (Man  and  Anthropoids)  : 

(1)  Ramification  of  the  Bronchi. — In  quadrupedal  mammals  the  main 
bronchus  passes  backwards  in  the  lung  as  a  main  stem,  which  grows 
gradually  smaller  by  giving  off  four  dorsal  and  four  ventral  bronchial 
branches  (Fig.  371).  So  altered  are  the  human  lungs,  that  the  arrange- 
ment of  bronchi  seen  in  most  mammals  is  not  easily  recognized  in  them. 
The  ventral  bronchi  are  larger,  longer  and  more  branched  than  in  other 
mammals.  In  the  human  as  in  the  mammalian  lung  the  secondary  and 
terminal  bronchi  are  developed  by  a  dichotomy  or  subdivision  of  the 
pulmonary  buds. 

(2)  The  Lobes  of  the  Lungs. — In  the  embryonic  condition  (Fig. 
370)  it  is  seen  that  the  right  and  left  lung  buds  are  nearly  symmetrical. 
Aeby  supposed  the  upper  lobe  of  the  right  lung  to  be  absent  in  the  left ; 


RESPIRATORY  SYSTEM 


349 


and  this  is  also  tlie  conclusion  wliicli  Flint  arrived  at  after  a  minute  in- 
vestigation of  the  development  of  the  lungs  of  the  pig.  It  must  be 
remembered  that  the  point  of  origin  of  any  bronchus  may  easily  be  moved 
to  meet  new  physiological  conditions.  At  least  in  the  human  embryo 
each  main  bronchus  gives  ofi  three  primary  buds.  All  three  remain 
separate  on  the  right  side  ;  on  the  left  the  upper  and  middle  primary  buds 
arise  together  (Fig.  370).  Hence  the  upper  lobe  of  the  left  lung  represents 
the  upper  and  middle  lobes  of  the  right.  In  the  sheep  and  pig  the  upper 
right  lobe  springs  from  the  trachea.  The  bronchus  of  the  upper  right  lobe 
(the  reason  for  it  is  not  clear)  commonly  lies  above  its  artery — ^that  is  to 
say,  it  is  eparterial.  The  other  bronchi  are  hyparterial.  A  clue  to  the 
asymmetry  of  the  right  and  left  lungs  will  be  found  in  a  fuller  knowledge 
of  the  mechanism  of  respiration.^ 

'  (3)  The  Diameters  o£  the  Thorax. — The  peculiar  branching  of  the 
bronchi  in  man  and  upright  primates  is  due  to  the  shape  of  the  lungs, 


•^ 


vertebra 


D^  or  apic.  br. 
right  branch 

right  pulm.  art 
D2 


trach. 


left  branch. 


broad  sternum 
Harrow  sternum 

Fig.  371. — Scheme  of  the  Bronchial  Ramifications  In  Quadrupedal  Mammals.    B,  the 

dorsal  ramifications  ;  V,  the  ventral  ramifications. 
Fig.  372. — Diagrammatic  Section  of  the  Thorax  of  a  Quadrupedal  Mammal  {A), 
contrasted  with  a  corresponding  section  in  Man  (£). 

which  in  turn  is  due  to  the  shape  of  the  thorax.  In  quadrupedal  animals, 
such  as  the  horse  or  dog,  in  which  the  chest  rests  and  is  supported  between 
the  fore  limbs,  the  thorax  has  its  greatest  diameter  in  the  dorso-ventral 
direction  (Fig.  372).  In  upright  animals  (man,  anthropoids,  and  also  in 
some  water  living  mammals,  such  as  seals,  etc.)  the  transverse  diameter 
becomes  the  greater.  At  birth  the  diameters  of  the  child's  thorax  are 
nearly  equal.  The  thorax  is  flattened  by  the  spine  becoming  invaginated 
within  it ;  the  thorax  thus  comes  to  lie  within  the  axis  of  gravity  of  the 
upright  body. 

(4)  The  AzygOS  Lobe. — On  the  inner  side  of  the  right  lung  of  man  the 
azygos  lobe  is  frequently  present,  sometimes  as  a  mere  pulmonary  pro- 
jection or  trace,  sometimes  as  a  lobule.  It  represents  an  over-development 
of  the  second  ventral  branch  from  the  right  bronchus  (Fig.   366).     It 


^  Any  one  interested  in  this  problem  should  consult  Prof.  Huntington's  paper,  Amer. 
Journ.  Anat.  1920,  vol.  27,  p.  99. 


350 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


projects  into  and  fills  a  slight  recess  between  the  pericardium  and  dia- 
phragm, behind  the  intra-thoracic  part  of  the  inferior  vena  cava.  This 
lobe  is  always  well  developed  in  quadrupedal  mammals.  In  them  the 
pericardium  is  separated  from  the  diaphragm  by  a  diverticulum  of  the 
right  pleura— the  sinus  subpericardiacus  (Fig.  374).  With  the  assumption 
of  the  upright  posture  (in  man  and  anthropoids)  the  mechanism  of  respira- 
tion has  become  altered  and  the  heart  sinks  until  it  rests  on  the  diaphragm, 
the  subpericardiac  sinus  and  azygos  lobe  being  thus  obliterated.  The 
reappearance  of  the  azygos  lobe  as  a  separate  structure — ^for  a  buried  rudi- 
ment is  always  present — in  man  is  an  atavism — that  is  to  say,  a  recurrence 
of  an  ancestral  feature.     In  quadrupeds  the  contraction  of  the  diaphragm 


LUNG 
BUD 

CES 

Fig.  373. — Showing  the  Origin  of  the  Blood  Supply  to  the  Limg,  in  Cat  Embryo. 
(Huntington.) 

is  followed  by  an  expansion  of  the  lobus  azygos  and  a  corresponding 
elongation  of  the  highly  elastic  intra-thoracic  part  of  the  inferior  vena 
cava  ;  in  man,  on  the  other  hand,  the  contraction  of  the  diaphragm  is 
followed  by  a  descent  of  the  heart,  thus  indirectly  enlarging  the  pulmonary 
space. 

Blood  Supply  of  the  Lung.^ — The  pulmonary  aorta  is  formed  with 
the  ascending  part  of  the  aortic  arch,  out  of  the  truncus  arteriosus  (see 
p.  249).  The  right  and  left  pulmonary  arteries  spring  as  branches  from 
the  right  and  left  6th  aortic  arches  (Fig.  373).  The  lung  buds  are  at  first 
supplied  by  arteries  arising  from  the  dorsal  aorta  (Huntington),  but  in  the 
5th  week  this  primary  pulmonary  plexus  is  joined  by  a  communication 

^  J.  L.  Bremer,  Amer.  Journ.  Anat.  1901,  vol.  1,  p.  137  (Dev.  of  Pulmonary  Arteries) ; 
V.  Federow,  Anat.  Hefte,  1910,  vol.  40,  p.  529  (Dev.  of  Pulmonary  Veins) ;  Geo.  S. 
Huntington,  Anat.  Bee.  1919,  vol.  17,  p.  165. 


EESPIRATORY  SYSTEM 


351 


from  the  Btli  aortic  arches,  this  anastomosis  being  the  basis  of  the  pul- 
monary arteries  (Fig.  273).  At  first  the  pulmonary  arteries  descend  by  the 
side  of  the  trachea,  but  as  the  heart  becomes  intra-thoracic  in  the  6th  and 
7th  weeks  they  are  gradually  shortened  until  they  pass  horizontally  to  the 
roots  of  the  lungs.  The  pulmonary  veins  grow  out  from  the  pulmonary 
buds  and  enter  the  left  auricle  through  the  venous  mesocardium  about 
the  5th  week  (Fig.  368).  The  mesenchymatous  or  interstitial  tissue  of 
the  lungs  is  supplied  by  the  bronchial  arteries  which  represent  the  primary 
vessels  of  the  lung  buds  (Fig.  373).  These  arteries  also  supply  the  pleura 
on  the  mediastinal  and  diaphragmatic  surfaces  of  the  lungs. 

Changes  at  Birth. — When  the  child  begins  to  breathe  at  birth,  the 
expansion  of  the  lungs  opens  up  the  pulmonary  circulation  ;  the  foramen 
ovale  is  closed  and  the  ductus  arteriosus  begins  then  to  be  closed,  and  within 
the  1st  month  becomes  reduced  to  a  fibrous  cord.     The  ductus  arteriosus 


sup-uen.  cau. 


pericardium 


sin.  sub-pericard. 
of  rt  pleura 


ensiform 


inf.  uen.  cau. 


riG.  374. — The  Relationship  of  the  Heart  to  the  Diaphragm  in  Quadrupedal  Mammals. 


represents  the  dorsal  segment  of  the  6th  left  arch  ;  the  corresponding 
part  of  the  6th  right  arch  disappears  soon  after  it  is  formed.  It  is  not 
until  about  the  4th  day  after  birth  that  the  whole  of  the  lung  is  inflated. 
The  first  part  to  expand  is  the  costo-sternal  or  ventral  part ;  the  second, 
the  diaphragmatic  or  basal  part,  the  apex  is  the  third,  and  the  dorsal 
border  and  deep  part  the  last.^ 

The  Larynx.^ — The  larynx  is  developed  round  the  anterior  part  of  the 
pulmonary  diverticulum.  The  origin  of  the  cartilages  of  the  larynx  is 
shown  in  Fig.  375.  The  thyroid  cartilage  is  formed  by  the  expansion  and 
amalgamation  of  the  skeletal  bases  of  the  4th  and  5th  visceral  arches  ; 
at  least  this  is  true  of  lower  mammals,  but  in  higher  mammals  only  the  4th 

^  Eor  papers  relating  to  the  morphology  and  mechanism  of  the  lungs  see  Further 
Advances  in  Physiology,  edited  by  Leonard  Hill,  1909  ;  also  Keith,  Journ.  Anat. 
and  Physiol.  1905,  vol.  39,  p.  243. 

^  J.  E.  Frazer,  Journ.  Anat.  and  Physiol.  1910,  vol.  44,  p.  156  ;  H.  Lisser,  Amer. 
Joxirn.  Anat.  1911,  vol.  12,  p.  27  ;  F.  H.  Edgeworth,  Quart.  Journ.  Mic.  Sc.  1916, 
vol.  61,  p.  383. 


352     HUMAN  EMBRYOLOaY  AND  MOEPHOLOGY 

is  involved  (Edgewortli).  Tlie  skeletal  basis  of  tlie  6tli  or  pulmonary 
arch  in  man,  whicli  forms  tlie  two  lateral  cartilages  in  tlie  short  pulmonary 
passage  of  the  frog,  becomes  divided  into  a  dorsal  segment  which  forms 
the  arytenoid  cartilage,  a  ventral  segment  to  form  the  cricoid.  From  the 
posterior  part  of  the  primitive  lateral  cartilage  arise  the  rings  in  the  wall 
of  the  trachea,  chief,  secondary  and  ultimate  bronchi  (Fig.  375). 

Prof.  Frazer  has  made  a  very  thorough  investigation  of  the  development 
of  the  larynx.  At  each  side  of  the  primary  pulmonary  orifice  lies  a  mass 
of  tissue  representing  the  last  or  6th  visceral  arch  (Fig.  361).  In  this 
tissue  develops  the  various  parts  of  the  larynx.  The  cricoid  and  arytenoid 
are  the  primary  cartilages  ;  they  are  the  only  ones  present  in  the  larynx 
of  amphibia  and  reptiles.  The  thyroid  only  appears  in  mammals.  The 
true  vocal  cords  represent  the  primary  opening  of  the  larynx.     In  the  2nd 

-2nd, 


4th. 
5th 

rytenoid 


thyroid 


cricoid- 
traa 


Fig.  375. — Diagram  of  the  Cartilages  of  the  Larynx  to  show  the  parts  derived  from 
the  Skeleton  of  each  Visceral  Segment. 

and  3rd  months  of  human  development  the  part  of  the  laryngeal  cavity 
above  the  vocal  cords  (suprarimal  part)  is  produced  by  the  upgrowth  of 
the  lateral  masses  on  each  side  of  the  primary  opening.  In  these  masses 
are  developed  the  arytenoid  cartilages  and  the  aryteno-epiglottidean  or 
permanent  folds  which  bound  the  lateral  margins  of  the  secondary  laryngeal 
orifice.  The  epiglottis,  in  Prof.  Frazer's  opinion,  is  developed  out  of  the 
mass  of  tissue  (central  mass)  which  lies  behind  the  2nd  and  3rd  arches 
(Fig.  361). 

The  muscles  within  the  larynx  are  derived  from  the  6th  visceral  segment 
and  are  supplied  by  the  inferior  laryngeal  nerve,  while  the  crico-thyroid 
arises  from  the  musculature  of  the  4th  segment.  In  fish  the  pharyngeal 
orifice  of  the  oesophagus  is  guarded  and  kept  shut  by  a  sphincter  made  up 
of  striated  muscle.  When  a  pulmonary  system  is  evolved  the  laryngeal 
or  guarding  musculature  is  derived  from  the  primary  sphincter  of  the 
oesophagus  (Edgeworth). 

The  epiglottis  is  developed  in  the  furcula  (Symington)  ;  in  lower  verte- 
brates its  lateral  margins  extend  into  the  aryteno-epiglottic  folds.  The 
cartilages  of  Santorini  and  Wrisberg,  in  the  aryteno-epiglottic  folds,  are 
continuous  with  the  epiglottis  in  many  mammals  (Sutton).     Until  the  5th 


RESPIRATORY  SYSTEM  353 

month  of  foetal  life  the  epiglottis  lies  behind  the  palate  and  within  the 
naso-pharynx — a  position  which  is  normal  for  the  adults  of  many  kinds  of 
mammals. 

The  purposes  which  the  larynx  serves  in  all  air-breathing  vertebrates 
are  (1)  to  regulate  the  inflow  and  outflow  of  respiratory  air,  and  thus  the 
positive  and  negative  pressure  within  the  lungs  ;  (2)  to  prevent  food 
passing  into  the  air  passage.  The  production  of  voice  which  has  led  to  a 
marked  alteration  of  the  human  arytenoid  cartilage  is  a  secondary  function. 
Only  in  man  and  the  higher  anthropoids  are  the  true  vocal  cords  covered 
by  stratified  epithelium  ;  but  all  the  muscles  of  the  human  larynx  are 
represented  in  the  larynx  of  the  ape,  although  in  a  less  specialized  con- 
dition.^ 

Soon  after  the  upgrowth  of  the  lateral  masses  to  form  the  suprarimal 
cavity  of  the  larynx,  an  evagination  takes  place  above  each  vocal  cord 
to  form  the  ventricles.  In  the  5th  month  mucous  glands  are  developed 
from  the  membrane  lining  the  ventricles,  and  a  little  later  an  outgrowth 
is  developed  from  their  apices  to  form  the  saccules  of  the  larynx.  They 
project  against  the  thyro-hyoid  membrane.  Occasionally  the  saccule 
of  the  larynx  may  protrude  through  the  thyro-hyoid  membrane,  thus 
giving  rise  to  an  air  cyst  in  the  neck.  Laryngeal  air-sacs  are 
normally  developed  in  anthropoids  after  birth,  and  attain  a  great  size 
in  the  adults,  extending  to  the  chest  and  axillae.  Their  function  is 
unknown. 

Diaphragm.^ — The  diaphragm  constitutes  one  of  the  most  pronounced 
structural  characteristics  of  mammals.  The  ancestral  mammalian  types 
in  which  the  diaphragm  first  appeared  are  long  since  extinct ;  we  cannot 
study  the  evolution  of  the  diaphragm  among  modern  vertebrates.  There 
are  certain  facts  which  throw  light  upon  its  origin,  and  make  us  certain 
that  the  diaphragm  did  not  rise  up  gradually  as  a  partition  within  the 
coelom  and  shut  ofi  that  part  which  contains  the  lungs  from  the  part 
containing  the  abdominal  viscera.  During  the  4th  and  5th  weeks  of 
development  the  pleural  cavities  are  represented  merely  by  the  two  short 
passages  leading  from  the  pericardial  to  the  peritoneal  cavity.  In  the  5th 
week  the  passages  lie  in  the  cervical  region  under  the  4th  and  5th  spinal 
segments,  from  which  the  phrenic  nerve  arises,  and  from  which  the  muscula- 
ture of  the  diaphragm  is  derived.  It  is  clear,  then,  that  the  diaphragm 
entered  into  the  service  of  the  lungs  when  these  were  situated,  as  in 
the  frog,  below  the  cervical  region  (Fig.  362).  In  some  manner,  as  the 
lungs  developed  and  afterwards  took  up  a  thoracic  position,  the 
muscle  which  became  associated  with  them  in  the  neck  accompanied 
them  when  they  retreated  to  their  new  position  in  the  thorax.  If  we 
are  to  find  a  representative  of  the  early  form  of  the  diaphragm,  it 
must  be  amongst  amphibians  that  we  should  look.     We  can  also   get 

1  W.  H.  Duckworth,  Journ.  Anat.  1913,  vol.  47,  p.  82. 

2  See  Keith,  Journ.  Anat.  and  Physiol.  1905,  vol.  39,  p.  243  ;  Mall,  Bull.  Johns 
Hopkins  Hosp.  1901,  vol.  12,  Nos.  121-123,  pp.  158,  171  ;  I.  Broman,  Ergebnisse  der 
Anat.  1911,  vol.  20,  p.  1  ;  A.  Brachet,  Mem.  de  I'Acad.  Boy.  de  Med.  de  Belgique,  1906, 
vol.  19 ;  R.  Mazilier,  VEmbryologie  du  Diaphragme,  Lille,  1907  ;  Gladstone  and 
Cockayne,  Journ.  Anxit.  1918,  vol.  52,  p.  64. 

z 


354 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


light   on  its   origin   by   studying  certain  malformations  to   whicli  it  is 
liable  in  man. 

In  Fig.  376  is  shown  the  thoracic  aspect  of  the  diaphragm  of  a  newly 
born  child,  in  which  the  left  pleuro-peritoneal  opening  has  remained  patent. 
Through  the  opening  the  upper  end  of  the  left  supra-renal  body  and  the 
spleen  projected  within  the  pleural  cavity,  giving  rise  to  a  congenital 
diaphragmatic  hernia.  The  pleuro-peritoneal  opening  is  situated  on  each 
side,  between  the  muscular  fibres  which  rise  from  the  ribs  and  sternum, 
and  which  form  the  ventro-lateral  part  of  the  diaphragm,  and  the  muscular 
fibres  which  arise  from  the  spine  and  arcuate  ligaments,  forming  the 


costal  hiatus 


FIG.  376.— The  Thoracic  Aspect  of  the  Diaphragm  of  a  newly  born  Child  in  which 
the  communication  between  the  Peritoneum  and  Pleura  has  not  been  closed  on 
the  left  side  ;  the  position  of  the  opening  is  marked  on  the  right  side  by  the 
Spino-costal  Hiatus.  The  dorsal  mesentery  of  the  fore-gut  (represented  by  the 
posterior  mediastinal  pleura)  is  also  shown. 

dorsal  part  of  the  diaphragm.  The  phrenic  nerves,  when  they  reach  the 
diaphragm,  divide  into  two  branches,  a  ventral  to  the  right  and  left  ventro- 
lateral parts  (from  3rd  and  4th  cervical  nerves),  and  a  dorsal  branch 
(from  4th  and  5th  cervical  nerves)  to  the  right  and  left  dorsal  parts.  The 
central  tendon,  situated  between  the  four  parts  just  mentioned,  makes 
up  the  fifth  morphological  element  of  the  diaphragm.  Each  of  these 
five  parts — the  central,  the  two  dorsal  and  two  ventro-lateral,  has  its  own 
developmental  history. 

The  central  tendon  of  the  diaphragm  is  formed  from  the  septum  trans- 
versum  (Fig.  377).  The  manner  in  which  that  structure  is  cleft  into  its 
pericardial  and  diaphragmatic  elements  by  the  outgrowth  of  the  two 
pleural  passages  and  lung  buds  has  been  already  described  (p.  342).  The 
dorsal  and  ventral  mesentery  of  the  fore-gut  (Fig.  379)  are  included  in  the 


EESPIRATORY  SYSTEM 


355 


formation  of  the  septum  transversum  (p.  272),  and  hence  the  structures 
developed  in  these  mesenteries — ^the  aorta,  oesophagus,  azygos  veins, 
thoracic  duct,  vagus  nerves  and  inferior  vena  cava — perforate  the  median 
or  central  part  of  the  diaphragm.  The  structures  of  the  posterior  media- 
stinum lie  in  the  mesentery  of  the  fore-gut  (see  Figs.  376,  378). 

The  ventro-lateral  parts  of  the  diaphragm  are  derived  from  the  ventral 
longitudinal  muscular  sheets  which  give  rise  to  the  rectus  abdominis  and 
depressors  of  the  hyoid  bone  (Fig.  362).  Were  the  parts  of  this  sheet 
restored  to  their  embryonic  relationships,  then  the  pericardium  should  be 
placed  beneath  the  mandible,  so  that  the  central  tendon  of  the  diaphragm 
lies  opposite  the  4th  cervical  segment.     The  sternal  and  costal  origins  of 


pericardium 
ribs- 

lunglbud- 

cost,  mart: 

spinallpart 


pericard.  cau. 
\^septum  transu. 
liuer 


Fig.  377. — A  Lateral  Section  along  the  Thoracic  and  Abdominal  Regions  of  a  Human 
Embryo  in  the  5tla  week  of  development,  showing  the  Lung  Bud  gro^ring  within 
the  Septum  Transversum  and  separating  it  into  a  Pericardial  and  a  Diaphrag- 
matic (costal)  Lamina.  Tlae  arrow  points  to  the  dorsal  mesentery  of  the  fore-gut 
within  which  tlie  crura  of  the  diaphragm  are  developed.     (After  Mall.) 


the  ventro-lateral  segment  of  the  diaphragm  should  be  detached  in  the 
thorax  and  the  muscle  placed  ventrally  in  the  neck  so  that  it  is  continuous, 
at  its  insertion  to  the  septum  transversum,  with  the  depressors  of  the 
hyoid  bone.  Behind  the  detached  thoracic  origins  of  the  sternal  and 
costal  fibres  should  become  continuous  with  the  anterior  end  of  the  rectus 
sheet.  In  the  human  body  the  anterior  part  of  the  rectus  sheet  becomes 
divided  into  four  strata — (1)  the  ventro-lateral  fibres  of  the  diaphragm, 
(2)  the  interchondral  parts  of  the  intercostals,  (3)  the  rectus  abdominis, 
which  in  all  mammals,  except  man  and  the  anthropoids,  reaches  forwards 
to  the  1st  rib,  (4)  the  pectoralis  major,  minor,  subclavius  and  that  frequent 
human  abnormality — the  sternalis  muscle.  The  development  of  the 
lung  separates  the  deepest  part  of  the  rectus  sheet  from  the  chest  wall  to 
form  the  ventro-lateral  part  of  the  diaphragm.     The  ribs  are  formed  in 


356      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

the  chest  wall  and  to  the  posterior  six,  this  part  of  the  diaphragm  ultimately 
obtains  an  origin. 

The  dorsal  parts  of  the  diaphragm  are  formed  from  that  part  of  the  trans- 
versalis  sheet  of  the  body  wall  which  forms  the  subvertebral  musculature 
(Figs.  362,  376).  The  manner  in  which  these  parts  of  the  diaphragm  are 
detached  from  the  body  wall  and  carried  into  the  thorax  by  the  developing 
pleural  cavities  and  lungs  is  shown  in  Figs.  377,  378.  The  right  and  left 
spinal  parts  of  the  diaphragm  sink  within  the  dorsal  mesentery  of  the  fore- 
gut,  obtaining  anteriorly  an  insertion  to  the  pericardium  and  septum 
transversum,  while  posteriorly  they  retain  an  origin  from  the  spine  and 
costal    processes.     The    quadratus    lumborum,    longus    colli,    the    rectus 

PERICARDIUM 

\  ANT.  BODY    WALL 

\  I  LATERAL  WALL 

PHRENIC  ./V  \  /  1 

\  _— ^-V "^ • :?— L  PLEURAL  MARGIN 


\PERICARDIUM 
-j—PHREN/C.N. 
j—MESeNT. 
■COSTAL  D/APH. 
r-  CESOPH. 
\CRURAL  DIAPH. 
\-M£SENT. 


DORSAL   WALL 


L-r  PL    PER  IT  OPEN~l  RT  PL  .  PER  IT.  OPENING 

AORTA 

Fig.  378. — A  Dorsal  View  of  the  hinder  parts  of  the  Expanding  Pleural  Cavities  in 
a  Human  Foetus  16  mm.  long  and  in  the  7th  week  of  development.  (After 
Gladstone  and  Cockayne.)  The  pleuro-peritoneal  openings  are  at  the  point  of 
closure.  Compare  with  Fig.  376.  Arrows  show  the  direction  in  which  the 
Pleural  Cavities  expand  into  the  Body  Wall  and  separate  the  Pericardium  from 
the  Thoracic  Parieties. 

capitis  anticus  major  and  minor  are  also  derived  from  the  subvertebral 
musculature. 

Pleuro-peritoneal  Openings. — The  pleural  passages,  into  which  the 
lung  buds  develop  at  the  end  of  the  first  month,  open  into  the  pericardium 
by  the  iterinera  venosa  ;  behind  they  communicate  with  the  peritoneum 
by  the  pleuro-peritoneal  openings  (Figs.  329,  330).  These  lie  above  the 
septum  transversum  (Fig.  379)  and  are  separated  by  the  mesentery  of 
the  fore-gut.  In  the  mesentery  between  the  openings  are  developed  the 
spinal  fibres  of  the  diaphragm  ;  on  the  lateral  side  of  each  opening  arise 
the  costal  fibres.  The  condition  of  the  pleuro-peritoneal  openings  in  the 
7th  week  when  they  are  on  point  of  closing,  is  shown  in  Fig.  378.  The 
actual  closure  is  effected  by  that  form  of  embryological  healing  to  which 
the  name  of  zygosis  has  been  given  (p.  287),  but  certain  accessory  factors 
are  also  involved  in  approximating  their  margins.  (1)  The  spinal  fibres 
migrate  outwards  and  obtain  attachment  to  the  arcuate  ligaments  ;  the 


RESPIRATORY  SYSTEM 


357 


costal  fibres  migrate  inwards,  obtaining  an  origin  from  the  Uth  and  12th 
ribs.  Only  in  man  and  anthropoids  does  this  migration  occur,  and  the 
extent  to  which  they  approach  each  other  and  thus  close  the  opening 
is  extremely  variable.  (2)  The  collapsed  condition  of  the  lungs  allows 
the  abdominal  viscera,  developed  in  the  domes  of  the  diaphragm,  to  press 
the  spinal  and  costal  fibres  against  the  dorsal  wall  of  the  thorax,  thus 
mechanically  closing  the  aperture.  The  liver,  especially,  by  its  upgrowth 
within  the  septum  transversum  helps  to  close  the  apertures,  particularly 
on  the  right  side,  which  is  seldom  the  site  of  a  diaphragmatic  hernia. 
The  supra-renal  bodies  are  also  developed  just  behind  the  pleuro-peritoneal 


_^-  AORTA 


PLEURAL  PASSAGE. 


DORSAL  MESOCARO: 


SINUS  VELNOSUS 


PERICARDIUM 


Fig.  379. — Section  across  Mesentery  of  the  Fore-gut  to  show  its  relationship  to  the 
Pleuro-peritoneal  Openings  and  Septum  Transversum. 

orifices,  and  help  to  close  them.  Indeed,  the  mesentery  of  the  Wolffian 
body,  in  the  anterior  extremity  of  which  the  supra-renal  bodies  develop, 
are  attached  along  the  dorsal  wall  of  the  coelom  as  far  as  the  septum 
transversum,  where  it  forms  a  fold  upon  the  lateral  or  outer  margin 
of  the  pleuro-peritoneal  orifice.  The  developmental  representative  of 
this  mesentery  is  sometimes  named  the  pleuro-peritoneal  membrane,  and 
is  regarded  as  an  embryonic  form  of  diaphragm. 

Musculature  of  the  Body  Wall. — The  development  of  the  musculature 

of  the  body  wall,  also  of  the  ribs  and  sternum,  ought  rightly  to  be  included 
here,  for  all  are  closely  related  to  the  mechanism  of  respiration.  The  ribs 
have  been  already  considered,  and  it  will  be  more  convenient  to  reserve 
the  development  of  the  wall  of  the  thorax  and  abdomen  with  other  cor- 
related structures  for  another  chapter  (Chap.  XXV.), 


CHAPTER  XXIII. 


UROGENITAL   SYSTEM. 


Evolutionary  Stages. — The  association  of  the  genital  with  the  urinary 
system  has  to  be  sought  for  in  the  ancestry  from  which  vertebrate  animals 
arose,  for  even  in  the  lowest  vertebrates  they  are  already  associated. 
The  evidence  of  embryology  makes  it  certain  that  man  has  been  evolved 
from  a  type  in  which  each  segment  of  the  body  was  provided  with  its 
own  excretory  tubule  or  kidney.  The  parts  of  an  excretory  or  nephric 
tubule  are  diagrammatically  represented  in  Fig.  380,  A.     Into  its  dilated 


VOLLECTING    DUCT 

TUBULE 


Fig.  380. — Composition  and  Origin  of  Nephric  Tubules. 

A,  Diagram  of  an  Isolated  Nephric  Tubule.     (After  Semon.) 

B,  Showing  the  manner  in  which  the  Intermediate  Cell  Mass  (a,  b,  c)  gives  origin 
to  the  Nephric  Tubule  (a),  Peritoneal  Funnel  (b)  and  the  Nephrocele  (c). 

C'^The  isolation  of  these  parts  from  the  Somite  and  their  union  to  form  a  system. 
D,  The  Origin  of  a  Glomerulus  in  the  Wall  of  the  Nephrocele  (c).     (After  Felix.) 

head  or  beginning  projects  a  vascular  body — a  glomerulus — similar  to  the 
glomeruli  of  the  kidney  ;  at  its  commencement  the  tubule  is  also  connected 
with  the  peritoneal  cavity  by  an  open  funnel-shaped  structure — ^the 
peritoneal  funnel.  By  this  communication  ova  or  spermatozoa,  which 
are  shed  from  the  genital  glands,  may  escape  from  the  peritoneal  cavity 
and  enter  the  excretory  tubules,  and  thus  pass  outside  the  body.  We 
shall  see  that  the  openings  by  which  ova  still  escape  in  women  and  the 
passages  by  which  semen  leaves  the  testicle  in  men,  are  derived  from  the 

358 


UROGENITAL  SYSTEM 


359 


funnel  elements  of  the  nepliric  tubules.  The  essential  part  of  the  ex- 
cretory organ  is  the  epithelial-lined  wall  of  the  tubule  itself.  The  secretion 
of  the  tubules  is  conveyed  to  a  common  collecting  duct — the  nephric  duct 
— which  ends  in  the  cloaca. 

An  inspection  of  Fig.  380  {B,  C,  D)  will  show  how  the  various  parts  of 
the  nephric  tubule  just  named  arise  from  the  wall  of  the  intermediate 
part  of  the  coelom.  We  have  already  seen  (p.  41)  how  the  mesoderm  on 
each  side  of  the  embryo  becomes  demarcated  transversely  into  body 
segments  or  somites,  and  also  longitudinally  into  the  paraxial  mass,  the 
intermediate  cell  mass  and  the  parietal  laminae,  and  how  extensions  of  the 
coelom  are  included  in  each  of  these  longitudinal  divisioi>s.     From  Fig. 


AORTA 
NEPHRIC   DUCT 

NE.PHRIC  TOBOLE 
GENITAL  GLAND 


CLOSED 
INEPHROSTOME 


MESENTERY 
GUT 


Fig.  381. — Schematic  Section  to  show  the  Specialization  of  the  Dorsal  Part  of  the 
Coelom  into  Nephric  Tubules,  Peritoneal  Funnels  and  Glomeruli.  On  one  side 
the  tubule  is  connected  with  the  peritoneal  cavity  by  an  open  funnel  while  the  Glo- 
merulus is  intraperitoneal  as  is  usual  in  pronephric  tubules  ;  on  the  other  side 
they  are  buried  in  the  Wolffian  or  intermediate  ridge. 

380  it  will  be  seen  that  a  nephric  tubule  arises  by  an  evagination  of  the 
outer  wall  of  the  intermediate  part  of  the  coelom,  while  the  glomerular 
chamber  or  nephrocele  (c)  and  the  peritoneal  funnel,  are  produced  from  the 
coelomic  passage  which  originally  connected  the  peritoneal  cavity  with  the 
cavity  of  a  somite  (Fig.  380).  Thus  the  nephridial  and  genital  systems  must 
be  regarded  as  modified  parts  of  the  wall  of  the  original  coelomic  cavity.^ 

Succession  of  Renal  Systems. — In  the  evolution  of  the  higher  verte- 
brates there  has  been  a  succession  of  three  renal  systems,  the  third  being, 
the  present  functional  system — the  kidneys  or  metanephros .  All  of  them — 
pronephros,  mesonephros  or  Wolffian  body  and  metanephros,  are  com- 
pounded of  the  same  system  of  nephridial  elements  just  as  the  milk  and 

^  See  W.  Felix,  Keibel  and  MalVs  Manual  of  Human  Embryology,  vol.  2,  1912  ; 
Eliz.  A.  Fraser,  Journ.  Anat.  1920,  vol.  54,  p.  287  ;  Gynneth  Buchanan  and  Eliz. 
Fraser,  ibid.  1919,  vol.  53,  pp.  35,  97  ;  F.  T.  Lewis,  Amer.  Journ,  Anat.  1919,  vol.  26, 
p.  423  ;  J.  L.  Bremer,  ibid.  1916,  vol.  19,  p.  179, 


360 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


permanent  teeth  are  successive  manifestations  of  the  same  dental  system. 
In  the  human  embryo  of  the  6th  week  all  three  systems  may  be  seen  ; 
while  the  pronephric  system,  which  is  developed  in  the  last  four  or  five 
cervical  segments  and  first  two  or  three  thoracic,  is  then  undergoing 
retrogression,  the  metanephric  in  the  hinder  lumbar  segments  is  only 
appearing  ;  in  this  week  the  mesonephric  system  is  approaching  the  height 
of  its  development,  extending  from  the  5th  cervical  to  the  3rd  lumbar 
segment. 

It  is  clear  that  pronephros,  mesonephros  and  metanephros  are  parts  of 
the  same  linear  series  of  organs.  All  are  made  up  of  nephric  tubules  opening 
into  a  common  excretory  (Wolffian)  duct.  While  in  the  cervical  region 
the  tubules  are  simple  and  retain  their  segmental  arrangement,  in  the 


PRONEPHROS 


WOUFFIAN     DUCT 
WOLFFIAN    TUBULE.S 


CLOACA 
RE.NALTISSUE 
URETERIC    BUD 
l^?   SACRAL 


ALLANTOIS 


Fig.  382. — Condition  of  the  Nepliric  or  Renal  System  in  a  Human  Embryo  of  4  weeks. 

(After  Ingalls.) 

dorsal  and  lumbar  region  they  multiply  in  number  and  complexity  ; 
in  the  sacral  region  they  become  exceedingly  numerous  and  massed  round 
a  diverticulum  from  the  Wolffian  duct — which  forms  the  primitive  ureter. 
In  the  second  month  of  human  development  the  Wolffian  body  is  at  the 
height  of  its  development ;  in  the  3rd  month  the  permanent  kidney 
assumes  its  predominant  position,  and  its  predecessor — the  Wolffian  body 
• — is  converted  into  a  mere  appendage  of  the  genital  system. 

The  Wolffian  Body  or  Mesonephros  (Fig.  382). — In  lower  vertebrates 
(Fishes  and  Amphibians)  the  Wolffian  body  is  the  functional  kidney  ; 
in  higher  vertebrates  (Reptiles,  Birds,  and  Mammals)  it  is  merely  a  tempor- 
ary or  embryonic  structure,  the  renal  function  being  taken  over  by  the 
permanent  kidney.  Apparently  the  permanent  kidney  (metanephros) 
arose  by  a  hypertrophy  and  separation  of  the  hindermost  segment  of  the 
Wolffian  body.  The  presence  of  the  mesonephros  in  the  human  embryo 
and  in  the  embryonic  stages  of  the  three  great  classes  of  higher  vertebrates, 


UROGENITAL  SYSTEM 


361 


witli  the  presence  of  many  curious  stages  in  the  development  of  their 
genito-urinary  system,  can  be  explained  only  by  the  fact  that  these  higher 
forms  are  descended  from  ancestors  of  the  lower. 

In  Fig.  383  is  given  a  diagrammatic  representation  of  the  tubular  com- 
position of  the  Wolffian  Body  of  the  frog,  which  in  many  points  corresponds 
to  the  same  structure  in  the  human  embryo.  Each  body  is  made  up  of 
a  main  duct  and  a  series  of  tubules.  In  the  frog,  as  in  the  human  embryo, 
the  hind-gut  ends  in  a  dilatation,  the  cloaca.  In  the  cloaca  open  the 
rectum,  allantois  or  bladder,  and  the  two  Wolffian  ducts — right  and  left. 
In  the  frog,  the  Wolffian  bodies  lie  on  each  side  of  the  spine,  their  anterior 
ends  reaching  forwards  to  the  region  of  the  heart.     Each  duct  is  joined 


tubules  of 
pro-nephros 


Wolffian  duct. 


Wolf,  genit.  tubules--^ 


Wolf,  renal  tubules. 


Wolffian  duct. 


opening  into  coelom 
glomerulus 


genital  gland 

„^^glomerulus 
meso-nephros 


rectum 


allantois-^  *V==='^i- — 
^^cloaca 

Fig.  383.— Scheme  of  the  Wolffian  Body  of  the  right  side. 

by  numerous  convoluted  tubules — the  Wolffian  or  Nephric  tubules.  Each 
tubule  is  furnished  with  a  glomerulus  at  its  blind  extremity,  and  in  most 
features  agrees  with  a  secretory  tubule — such  as  is  seen  in  the  permanent 
kidney.  These  tubules  secrete  the  urine  ;  the  Wolffian  duct  conveys  the 
urine  from  the  tubules  to  the  cloaca.  The  anterior  tubules,  however,  lose 
their  secretory  function  and  become  associated  with  the  genital  gland. 
In  the  male  frog  they  convey  the  spermatozoa  to  the  Wolffian  duct,  which 
thus  carries  both  urine  and  spermatozoa.  In  the  female,  the  genital 
Wolffian  tubules  are  connected  with  the  ovary  but  are  quite  functionless 
(Fig.  383). 

The  WolfiBan  Body  in  the  Human  Embryo. — By  the  middle  of 
the  second  month  of  foetal  life,  the  Wolffian  body  is  well  developed  ;  by 
the  end  of  that  month  it  is  undergoing  a  process  of  atrophy,  except  those 
parts  connected  with  the  genital  organs.     Originally  extending  from  the 


362 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


intermediate 
cell-mass. 

Wolffian  body 

genital  gland 
gut. 


5th  cervical  to  the  3rd  lumbar  segment,  by  the  8th  week  it  is  confined  to  the 
region  of  the  lower  three  thoracic  and  upper  three  lumbar  vertebrae  where 
it  projects  at  the  dorsal  attachment  of  the  mesentery  (Fig.  384).  Its  anterior 
end  lies  at  the  pleuro-peritoneal  orifice  of  the  diaphragm.  To  its  inner 
side,  in  the  lower  dorsal  region,  lies  the  genital  ridge.  The  genital  and  the 
Wolffian  bodies  have  each  its  own  mesentery,  but  these  two  mesenteries 
have  a  common  attachment — the  common  urogenital  mesentery  (Fig.  384). 
On  section  the  Wolffian  ridge  is  seen  to  be  made  up  of  convoluted  tubules 
terminating  at  their  blind  extremities  in  glomeruli.  The  tubules  open 
into  the  Wolffian  duct,  just  as  in  the  frog  ;  the  duct  is  situated  on  the 
lateral  margin  of  the  ridge,  dorsal  to  the  Miillerian  duct.  It  runs  back- 
wards in  this  ridge  and  turns  into 
the  pelvis  to  end  with  the  Miillerian 
duct  (also  situated  in  the  Wolffian 
ridge)  in  the  cloaca  of  the  hind-gut 
(Fig.  382).  The  whole  arrangement 
is  similar  to  that  seen  in  the  frog. 
Further,  as  in  the  frog,  certain 
of  the  more  anterior  or  genital 
tubules  are  connected  with  the 
genital  glands,  and  are  not,  as  the 
posterior  are,  secretory  in  nature. 
If  the  testes  were  functional  at 
this  time — which  they  are  not — 
the  spermatozoa  and  urine  of  the 
Wolffian  body  would  pass  to  the 
cloaca  by  the  Wolfl&an  duct. 

Origin  of  the  WoMan  Duct  and 
Tubules. — The  tubules  which  com- 
pose the  Wolffian  body  are  de- 
veloped in  the  intermediate  cell 
mass,  in  the  manner  already  de- 
scribed (p.  359).  The  intermediate  cell  mass  is  divided  from  before  back- 
wards into  segments  ;  two  or  three  tubules  arise  in  each  segment.  The 
tubules,  although  of  the  nature  depicted  in  Fig.  380,  appear  in  the  course 
of  human  development  as  minute  vesicles  in  the  intermediate  cell  mass  ; 
these  vesicles  become  tubular  ;  one  end  opens  into  the  Wolffian  duct ;  at 
the  other  a  glomerulus  is  developed  (see  Fig.  381).  The  duct  is  developed 
in  the  outer  part  of  the  intermediate  cell  mass.  Its  anterior  or  cervical 
part  appears  early  in  the  4th  week  as  a  solid  rod  of  cells  formed  by  the 
union  of  the  terminal  ends  of  the  pronephric  tubules.  By  the  end  of  the  4th 
week  the  caudal  end  of  the  pronephric  duct  has  reached  the  cloaca  and  thus 
the  pronephric  duct  forms  the  basis  of  the  Wolffian  duct — the  duct  into 
which  the  tubules  of  the  Wolffian  body  open.  At  first  the  hinder  or  pelvic 
ends  of  the  Wolffian  bodies  are  separate,  but  in  the  8th  week  they  become 
approximated  and  fuse  to  form  the  genital  cord.  The  genital  cord  contains 
the  terminal  parts  of  the  Wolffian  and  Miillerian  ducts.  The  Miillerian 
ducts  being  situated  nearest  to  the  middle  line,  fuse  to  form  the  uterus. 


Fig.  384. — Diagrammatic  Section  to  show  the 
Position  of  the  Wolffian  and  Genital  Ridges 
on  the  Dorsal  Wall  of  the  Abdomen. 


UROGENITAL  SYSTEM 


363 


The  Pronephros. — Pronephric  differ  from  mesonepliric  tubules  in  retain- 
ing open  peritoneal  funnels  attached  to  them  (Fig.  383)  and  in  having  their 
glomeruli  occasionally  situated  within  the  peritoneal  cavity  (Fig.  381). 
They  reach  their  highest  development  in  anterior  segments  of  the  human 
embryo  during  the  4th  week  and  then  disappear,  leaving  no  trace.  Their 
duct  becomes  the  Wolffian  duct,  and  if  a  remnant  did  persist  we  should  seek 
for  it  at  the  commencement  of  this  duct. 

The  Fate  o£  the  Wolffian  Body  (mesonephros)  and  Pronephros. — 
1.  In  the  Female. 

In  Fig.  385  are  shown  the  various  remnants  of  the  embryonic  renal 
formations  which  may  persist  in  the  adult  female.  The  Miillerian  duct, 
the  upper  part  of  which  becomes  the  Fallopian  tube,  is  situated  in  the 

Miillerian  duct 

^^^ -hydatid 
epoophoron,     ^^         ^"^ 

right  ouary 
Wolffian  duct 

paroophoron 

passing  into 
wall  of  uterus 

duct  of  Gartner 
rectum 

uagina 


opening  at  margin  of 
vaginal  orifice 

Fig.  385. — Remnants  of  the  Wolffian  Body  in  the  Female  (see  also  Fig.  387). 

Wolffian  ridge  (Fig.  384).  Hence  when  the  ovary  and  tube  migrate  to  the 
pelvis,  the  Wolffian  mesentery,  which  comes  to  form  the  mesosalpinx, 
is  also  drawn  within  the  pelvis,  and  with  it  all  the  Wolffian  remnants. 
A  hydatid  attached  to  the  mesosalpinx  (part  of  the  broad  ligament)  at  the 
fimbriated  extremity  of  the  Fallopian  tube  (Fig.  385)  represents  the  most 
anterior  (cephalic)  part  of  the  Wolffian  formation.  The  Wolffian  duct 
(Fig.  385)  runs  towards  the  body  of  the  uterus  in  the  mesosalpinx  ;  it 
reaches  the  side  of  the  uterus,  but  from  that  point  onwards  it  has  dis- 
appeared by  the  commencement  of  the  3rd  month.  Occasionally,  however, 
remnants  of  the  lower  or  distal  part  of  the  duct  persist.  They  lie  in  the 
roof  of  the  vagina.  The  point  of  termination  of  the  duct  is  sometimes 
represented  on  the  trigone  of  the  vulval  cleft  a  little  distance  from  the  side 
of  the  opening  of  the  urethra.  Only  the  upper  part  of  the  duct  (meso- 
salpingeal  part)  persists  in  women.  The  uterine  and  vaginal  segments, 
when  they  persist,  get  the  name  of  duct  of  Gartner.  The  genital  tubules, 
those  attached  to  or  connected  with  the  ovary,  persist  and  form  the 


364 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


epoophoron.  Organ  of  Rosenmiiller,  or  parovarium.  The  renal  Wolffian 
tubules — those  which  acted  as  renal  structures  in  the  embryo — also  persist, 
sometimes  unconnected  with  the  duct.  They  lie  between  the  ovary  and 
uterus  and  form  the  paroophoron.  Fluid  may  collect  in  their  cavities, 
and  thus  those  vestiges  may  form  cysts,  but,  as  we  shall  see  (p.  366),  the 
Wolffian  remnants  which  are  the  usual  source  of  cystic  formations  in  the 
female  lie  along  the  ovarian  fimbria  and  are  derived  from  the  elements 
which  give  rise  to  the  rete  testis  in  the  male. 

2.  In  the  Male. 

In  the  male  (Fig.  386)  the  Wolffian  duct  forms  : 

(1)  The  tube  of  the  epididymis,  which  is  coiled  up  in  the  globus  major, 
body  and  globus  minor  of  the  epididymis  ;  ^ 


bpidiaymis 


Mullerian 
duct 


uasa  aber. 


•paradidymis 


urethra. 


open,  com 
ejac.  duct' 


sessile  hydatid 
stallied  hydatid 
right  testicle 


uas.  deferens 
(Wolf,  duct) 


-uesic.  semm. 
^miL  duct. 


uterus  masculinus 

Fig.  386. — Remnant  of  the  Wolffian  Body  in  the  Male  (see  also  Fig.  387). 

(2)  The  vas  deferens  and  common  ejaculatory  duct.  The  duct  opens 
at  each  side  of  the  uterus  masculinus  in  the  prostatic  urethra,  a  site  cor- 
responding to  the  vestibule  of  the  vagina  in  the  female  ; 

(3)  The  vesiculae  seminales  arise  from  the  Wolffian  ducts  as  tubular 
diverticula  at  the  end  of  the  3rd  month  ;  the  terminal  part  of  the  duct 
also  becomes  dilated  to  form  an  ampulla. 

The  stalked  hydatid  frequently  seen  on  the  upper  extremity  of  the 
testicle  corresponds  to  the  hydatid  at  the  fimbriated  extremity  of 
the  Fallopian  tube  in  the  female,  and  is  of  similar  origin  (Figs.  385 
and  386). 

The  genital  tubules  of  the  Wolffian  body  become  the  vasa  efierentia 
and  coni  vasculosi. 

1  J.  L.  Bremer,  Amer.  Journ.  Anat.  1911,  vol.  11,  p.  393  (Dev.  of  Vasa  Efferentia) ; 
Otto  Petersen,  Anat.  Hefte,  1907,  vol.  34,  p.  239  (Dev.  of  Vesiculae  Seminales)  ;  E.  M, 
Watson,  Amer,  Journ,  Anat,  1918,  vol,  24,  p,  395, 


UROGENITAL  SYSTEM 


365 


The  renal  tubules  of  the  Wolffian  body  form  : 

(1)  The  vasa  aberrantia  found  in  the  globus  minor  ; 

(2)  The  paradidymis  or  organ  of  Giraldes  situated  in  the  cord  above 
the  globus  major  but  not  always  present.  The  vas  aberrans  represents 
an  elongated  Wolffian  tubule,  which  has  effected  a  communication  with 
the  Wolffian  duct,  but  not  with  the  genital  gland.  The  tubules  of  the 
paradidymis  represent  blind  tubules,  which  retain  the  embryonic  cystic 
form.  All  these  tubules,  both  genital  and  renal  of  the  Wolffian  body,  are 
situated  originally  in  the  mesentery  of  the  Wolffian  body  (Fig.  384). 

Thus  it  will  be  seen  that  while  in  the  male  the  Wolffian  tubules  and  duct 
become  part  of  the  genital  system,  in  the  female  they  become  functionless 
and  only  of  pathological  importance.     Their  presence  in  the  female  is  due 


JUNCT    TUB 


VEST:  GEN   GU-. 


JUNCT ; TUBES 


—     UTERUS 


GLOB:  MAJOR 


W:D:(VAS) 


TESTICLE 


A. 


B 


Fig.  387. — Diagrams  showing  the  fate  of  the  Junctional  Cords  in  the  Ovary  and 
in  the  Testis.  A,  ovary  and  Fallopian  tube,  showing  the  rudiments  of  the 
junctional  tubules  in  the  ovario-fimbriate  margin  of  the  broad  ligament ;  B,  the 
origin  of  the  junctional  system  ;  C,  the  junctional  system  of  the  testis.  M.D. 
Miillerian  duct ;   W.D.  Wolffian  duct ;  W.T.  Wolffian  tubules. 

to  their  being  inherited  from  the  male,  just  as  the  breasts  in  the  male 
persist  because  of  their  utility  in  the  female. 

Bete  Testis. — The  junction  between  the  genital  tubules  of  the  Wolffian 
body  and  the  seminal  tubules  of  the  testes  is  effected  by  the  development 
of  a  separate  element  to  which  the  names  of  rete-cords  or  junctional  cords 
have  been  given.  In  Fig.  387,  B,  is  represented  the  origin  of  the  junctional 
tubules,  according  to  the  account  given  by  Dr.  Allen. ^  The  ovary  and 
testis  represent  only  the  middle  part  of  the  original  genital  ridge  ;  the 
anterior  and  posterior  parts  atrophy  and  disappear  by  the  end  of  the  2nd 
month.  In  the  anterior  vestigial  part  of  the  ridge  solid  cords  representing 
peritoneal  funnels  (Fig.  380),  grow  into  the  mesentery  of  the  Wolffian 
body,  and  from  these  cords,  as  shown  in  Fig.  387,  B  (where  only  two  cords 
are  represented),  is  formed  the  rete  testis.  The  rete  testis  effects  com- 
munications with  the  seminal  tubules  by  means  of  outgrowths,  which  form 
the  vasa  recti,  and  also  with  the  glomerular  or  blind  extremities  of  the 
genital  tubules  of  the  Wolffian  body  (Fig.  387,  C).     In  the  female  the 

^  See  Bennet  M.  Allen,  Amer.  Journ.  Anal.  1906,  vol.  5,  p.  79. 


366 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


junctional  cords  are  formed ;  vestiges  usually  remain.  Frequently 
fimbriated  hydatids  (peritoneal  funnels)  are  attached  to  them  (Fig.  387, 
A).  The  majority  of  parovarian  cysts  ^  are  formed  from  the  junctional 
rudiments.  Isolated  vestiges  may  also  be  seen  in  the  testicle  between  its 
upper  pole  and  the  globus  major  (Fig.  387,  C).  They  also  may  give  rise  to 
cysts. 


THE   KIDNEY. 

Origin  of  the  Permanent  Kidney.^— In  Fishes  and  Amphibians  the 
Wolfiian  body  alone  acts  as  a  kidney.  In  Reptiles,  Birds  and  Mammals 
the  permanent  or  hind  kidney  appears,  and  supplants  the  Wolffian  kidney. 
Like  the  Wolffian  body  the  kidney  arises  by  the  combination  of  two  ele- 
ments which  are  developed  separately — a  duct  or  collecting  system,  and  a 


NEPHROGENIC  CAP 
PELVIS 


WOLFFIAN  DUCT 


PELVIC  BUOS 
URETER 
VFfOGENITAL    SINUS 
RECTUM 
CLOACA 

CLOACAL  MEMB. 


Fig.  388a. — The  Ureteric  Bud  and  Nephrogenic  Cap  at  the  beginning  of  the  6th  week. 
Fig.  388b. — The  same  parts  later  in  the  6th  week.     C  indicates  the  stage  of  Renal 
development  reached  in  the  7th  week. 

nephric  or  secretory  system.  The  collecting  system  arises  as  an  outgrowth 
from  the  hinder  end  of  the  Wolffian  duct,  and  forms  the  ureter,  the  pelvis 
of  the  ureter  and  the  collecting  tubules,  which  compose  the  main  part  of 
the  medullary  pyramids  of  the  kidney.  The  secretory  part  arises  from  the 
hinder  end  of  the  nephridial  system — just  behind  the  part  which  gives 
origin  to  the  mesonephros  ;  it  forms  the  cortex  of  the  kidneys — the  glom- 
eruli, convoluted  tubules  and  loops  of  Henle  ;  in  short,  the  secretory 
substance  of  the  kidney  (Fig.  389).  Already,  at  the  beginning  of  the  5th 
week,  the  ureteric  part  of  the  kidney  is  apparent  as  a  dilatation  or  slight 
evagination  at  the  hinder  end  of  the  Wolffian  duct,  near  the  cloaca.     The 

^  For  the  pathological  significance  of  this  structure  see  Alban  Doran,  Journ.  of 
Obstetrics  and  Oynaec.  of  Brit.  Empire,  Oct.  1910,  vol.  18,  p.  me. 

2  G.  C.  Huber,  Amer.  Journ.  Anat.  1905,  vol.  4,  Supplement  (Dev.  of  Renal  Tubules)  ; 
F.  T.  Lewis,  Amer.  Journ.  Anat.  1919,  vol.  25,  p.  423  ;  A.  F.  Dixon,  Journ.  Anat.  and 
Physiol.  1911,  vol.  45,  p.  117  (Supernumerary  Kidney)  ;  E.  Muthmann,  Anat.  Hefte, 
1907,  vol.  32,  p.  577  (Horse-shoe  Kidney). 


UROGENITAL  SYSTEM 


367 


stage  reached  by  the  beginning  of  the  6th  week  is  shown  in  Fig.  388,  A  ; 
the  ureteric  bud  is  stalked,  the  stalk  representing  the  ureter  and  its  dilated 
pelvic  end  the  renal  pelvis  and  collecting  tubules.  The  nephrogenic  tissue 
forms  a  cap  on  the  pelvic  dilatation.  At  this  time  the  kidney  lies  under 
the  4:th.  and  5tli  lumbar  segments.  At  a  later  stage  in  the  6th  week  (Fig. 
388,  B)  the  ureteric  stalk  has  become  elongated,  the  pelvic  dilatation  has 
given  rise  to  primary  evaginations  representing  the  calyces  of  the  kidneys  ; 
round  the  evaginations  is  massed  the  nephrogenic  cap.  The  kidney  now 
lies  dorsal  to  the  Wolffian  body  and  under  the  2nd  and  3rd  lumbar  segments. 


I?r  CONVOL 


collect:  tube 


URETERIC  BUD 


NEPHRIC    BUD 


CROWING  END 
(ureteric) 


NEPHRIC   TUBULE 


Glomerulus 


,CpLLECT:TUBE. 
(Ureteric) 


COLLCCTINQ  TUBES 


Fig.  389.— Illustrating  the  Development  of  the  Renal  Tissue.  A,  growing  end  of 
collecting  tubule  with  bud  of  nephric  tube  attached  to  it ;  B,  first  stage  in  the 
development  of  a  nephric  bud  into  a  nephric  tubule  ;  C,  fully  developed  renal 
tubule  ;  the  part  formed  from  the  ureteric  bud  is  represented  in  outline  and  the 
part  from  the  nephric  tubule  is  shaded.     (After  Huber.) 

The  separation  of  the  ureter  from  the  Wolffian  duct  has  commenced. 
In  Fig.  388,  C  a  still  later  stage  is  shown.  Tubules  now  begin  to  form  in 
the  nephrogenic  cap  and  collecting  tubules  to  bud  out  from  the  pelvic  bud. 
Collecting  tubules  arise  by  the  division  and  redivision  of  the  growing  end 
of  the  pelvic  outgrowths.  In  the  third  month  the  process  of  outgrowth 
from  the  ureteric  bud  continues  ;  the  growing  end  of  each  bud  divides 
and  redivides,  and  in  this  manner  the  collecting  tubules  of  the  pyramids 
are  formed  (Fig.  389,  C).  In  Fig.  389,  A,  the  growing  extremity  of  such  a 
collecting  duct  is  represented.  Near  one  of  its  terminal  buds  is  represented 
one  of  the  numerous  tubule-rudiments,  formed  from  the  nephrogenic  tissue 
surrounding  the  ureteric  outgroA^ths.  Like  a  Wolffian  tubule,  it  appears 
in  a  vesicular  form.     At  one  extremity  it  establishes  a  communication 


368     HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

with  the  collecting  tubule  ;  at  the  other  a  glomerulus  develops  (Fig.  389, 
B).  The  tubule  elongates,  becomes  convoluted,  and  quickly  assumes  the 
adult  form  represented  in  Fig.  389,  C.  Glomeruli  appear  at  the  com- 
mencement of  the  3rd  month  ;  a  capsule  becomes  difierentiated  for  the 
kidney  from  the  surrounding  mesodermal  tissue  in  the  3rd  month.  The 
kidneys  have  by  then  reached  their  final  position — extending  from  the  11th 
thoracic  to  the  4th  lumbar  segment.  Up  to  the  time  of  birth,  tubular  and 
glomerular  formation  are  seen  in  full  activity  within  the  subcapsular  zone 
of  the  kidney.  The  deep  tubules  are  the  first  to  difierentiate.  Soon  after 
birth  the  formation  of  new  elements  ceases  ;  increase  in  size  is  then  due  to 
pure  growth  of  the  parts  already  formed.  The  collecting  tubules,  arising 
from  each  primary  evagination  of  the  ureteric  bud  become  massed  in 
pyramids  ;  the  bases  of  the  pyramids,  clothed  by  nephrogenic  caps,  appear 
on  the  surface  of  the  kidney  and  give  it  a  lobulated  structure.  In  the 
fissures  between  the  lobules  cortex  is  formed  ;  soon  after  birth,  as  new 
cortical  tissue  is  laid  down,  the  depressions  between  the  lobules  are  filled 
up.  In  many  mammals  (ox,  bear,  seal)  the  renal  substance  remains 
broken  up  into  numerous  lobules. 

The  upper  pole  of  the  kidney  reaches  the  11th  rib  in  the  5th  month, 
and  is  then  in  juxtaposition  with  the  adrenal,  which  is  developed  at  the 
anterior  end  of  the  Wolffian  body.  At  their  first  appearance  the  renal 
buds  receive  temporary  branches  from  the  common  iliac  artery  and 
from  the  aorta,  but  when  they  come  to  lie  on  the  dorsal  aspect  of 
the  Wolffian  body  in  the  7th  week,  the  arterial  network,  supplying 
the  tubules  of  that  body,  invade  the  nephrogenic  tissue  of  the  renal 
buds  and  thus  the  kidneys  annex  the  series  of  Wolffian  arteries— stretch- 
ing from  the  11th  thoracic  to  the  4th  lumbar.  The  definite  arteries  are 
derived  from  those  of  the  2nd  lumbar  segment  but  frequently  more  than 
one  pair  persists. 

With  the  development  of  the  lumbar  and  sacral  regions  of  the  body 
the  ureter  becomes  elongated.  The  termination  of  the  ureter  becomes 
separated  from  the  Wolffian  duct  early  in  the  7th  week,  by  a  process  to  be 
mentioned  later. 

As  the  kidney  grows  forwards  its  hilum  at  first  looks  towards  the  pubic 
region,  and  even  when  it  has  reached  its  final  position  and  the  poles  become 
upper  and  lower,  the  hilum  of  the  kidney  still  looks  towards  the  ventral 
wall  of  the  abdomen.  In  the  4th  and  5th  month  an  anterior  lip  is  formed 
to  the  hilum  by  the  development  of  cortical  tissue,  and  the  hilum  then 
assumes  its  normal  form  and  position.  The  anterior  lip  is  usually  absent 
from  horseshoe  kidneys,  an  abnormality  which  arises  from  a  fusion  of  the 
right  and  left  nephrogenic  buds.  Such  kidneys  are  usually  supplied  with 
multiple  renal  arteries.  In  other  cases  the  renal  buds  grow,  not  towards 
the  loins  but  towards  the  sacral  region,  becoming  developed  in  the  pelvis 
and  drawing  their  arteries  from  the  sacral  and  iliac  vessels.  The  ureteric 
bud  may  divide,  and  give  rise  to  a  forked  ureter,  or  to  double  or  even 
triple  ureters.  The  nephrogenic  element  may  remain  single,  or  it  also  may 
become  divided,  giving  rise  to  two  kidneys  on  one  side.  Another  common 
developmental  error  is  the  failure  of  the  nephric  tubules  to  effect  a  union 


UROGENITAL  SYSTEM 


369 


with  the  collecting  tubules.     The  nephric  tubules  then  become  dilated  and 
cystic,  giving  rise  to  congenital  cysts  of  the  kidney. 


THE  MULLERIAN  DUCTS. 

The  Miillerian  Ducts  ^  or  Oviducts  are  present  in  almost  all  vertebrates, 
and  convey  the  ova  from  the  peritoneal  cavity  to  the  surface  of  the  body. 
In  fishes,  amphibians,  reptiles,  birds  and  lower  mammals  (Marsupials) 
the  ducts  terminate  in  the  cloaca.  This  is  also  the  case  in  the  embryonic 
stages  of  man  and  all  higher  mammals.  The  development  of  the  duct 
in  man  is  very  simple  (Fig.  390).  The  first  part  to  be  formed  is  the  ostium 
abdominale  which  appears  on  the  ventro-lateral  aspect  of  the  Wolffian 
ridge  (Fig.  390)  as  a  funnel-like  invagination  of  the  coelomic  mesothelium. 
This  invagination,  which  appears  in  the  6th  week  at  the  anterior  end  of 
the  Wolffian  ridge  (under  the  3rd  thoracic  segment)  represents  a  modified 


Wolffian  duct 
Mailer,  duct, 
todij  wall. 


inteniKd.  cell,  mass 


aorta 


glomerulus 


Wolffian  duct 
M'u'ller  duct 

body  wall. 


nepnrostome 


ridae. 


mesentery 


genital  ridge 


Fig.  390. — Diagrammatic  Section  across  the  "Wolffian  and  Genital  Ridges  to  show 
the  Origin  and  Relations  of  the  Miillerian  Duct  to  the  Duct  and  Tubules  of  the 
Wolffian  Body.     (After  Pasteau.) 

peritoneal  funnel  (Fig.  380)  or  nephrostome  (Fig.  390).  From  the  apex  of 
the  funnel-like  invagination  of  coelomic  epithelium,  a  solid  rod-like  process 
of  cells  grows  backwards  on  the  Wolffian  ridge,  ventral  to  the  Wolffian 
duct  (Fig.  384)  reaching  the  region  of  the  cloaca  in  the  7th  week.  Although 
the  ostium  is  developed  thus,  the  fimbriae  which  surround  it  are  not  formed 
until  the  3rd  month,  when  they  appear  as  outgrowths  of  the  lining  mem- 
brane of  the  tube.  More  than  one  ostium  may  be  developed,  representing 
neighbouring  nephric  funnels.  As  it  passes  backwards  in  the  Wolffian 
ridge  the  Mullerian  duct  lies  below  and  internal  to  the  Wolffian  duct  and 
comes  in  contact  with  its  neighbour  of  the  opposite  side  in  the  pelvis 
(Fig.  392).  The  Miillerian  duct  is  formed  in  the  embryo  later  than  the 
Wolffian  duct ;  its  posterior  growing  end  does  not  acqmre  a  lumen  until  late 
in  the  3rd  month  when  it  opens  on  the  dorsal  wall  of  the  urogenital  sinus 
—a  derivative  of  the  cloaca  (Fig.  391).  The  openings  of  the  two  Mullerian 
ducts  are  situated  between  the  orifices  of  the  Wolffian  ducts.  Mullerian 
ducts,  although  they  only  reach  their  full  development  in  woman,  are 
yet  as  completely  and  strongly  formed  in  the  male  embryo  as  in  the  female. 


1  S.  E.  Wichmann,  Anat.  Hefte,  1912,  vol.  45,  p.  629. 
2  a 


370     HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

By  the  beginning  of  the  3rd  month  the  fimbriated  tube  has  retreated  to  the 
level  of  the  2nd  lumbar  segment. 

The  Genital  Cord.— During  the  3rd  month  the  Miillerian  ducts  show 
two  distinct  stages  in  their  course  : 

(1)  Lumbar,  which  lies  in  the  Wolffian  ridge  and  is  suspended  from  the 
posterior  abdominal  wall  by  the  Wolffian  mesentery.  This  stage  after- 
wards forms  the  Fallopian  tube  (Fig.  392). 

(2)  Pelvic,  which  is  embedded  in  the  genital  cord.  The  posterior  ends 
of  the  Wolffian  ridges,  with  their  contents,  the  Wolffian  and  Miillerian  ducts, 
fuse  in  the  pelvis  during  the  8th  week,  and  thus  form  the  genital  cord. 

RT  MULLERiAN    DUCT 

-WOLFFIAN    DUCT 

-GE.NITAU   GLAND 
BLADDER 


GENITAL  CORD 


LEFT  mul:  Duct 


GENIT:  TUBER;  /  Y  '//-j^-^^  POUC  H   OF  DOUGLAS 

URO-GEN:  SINUS 

CAUDA 

Fig.  391. — Diagram  of  the  Genital  Ducts  at  the  commencement  of  the  3rd  month  of 
Foetal  Life.     Lateral  view. 

With  their  fusion  the  peritoneal  space  of  the  pelvis  is  separated  into  a  deep 
posterior  part — the  pouch  of  Douglas  and  a  shallow  anterior  depression 
— the  utero-vesical  (Fig.  391).  The  parts  of  the  Miillerian  ducts  within 
the  cord  form  the  uterus  and  vagina.  The  ureter  is  also  enclosed  within 
the  mesodermal  tissue  of  the  genital  cord,  but  afterwards  becomes  separated 
from  it. 

The  genital  cord  of  the  foetus  at  the  beginning  of  the  3rd  month  shows 
the  two  Miillerian  and  two  Wolffian  ducts — in  the  male  as  well  as  in  the 
female  (Fig.  391).  One  of  the  first  signs  of  sexual  differentiation  is  to 
be  observed  in  the  genital  cord.  Whereas  the  genital  cord  in  the  male 
embryo  is  closely  appHed  to  the  bladder,  so  that  there  is  no  utero-vesical 
pouch,  in  the  female  the  cord  remains  separated  from  the  bladder  by  a 
deep  peritoneal  pocket. 

The  Round  Ligament  of  the  Uterus,  which  is  apparent  early  in  the  3rd 
month,  is  attached  to  the  Miillerian  duct  on  each  side  (Fig.  392).     The 


UROGENITAL  SYSTEM 


371 


point  of  attachment  marks  the  junction  of  the  uterine  and  Fallopian 
segments  of  the  Miillerian  ducts.  The  round  ligament  corresponds  to  the 
gubernaculum  testis  in  the  male  and  its  development  is  similar.  Both 
are  developed  in  the  following  manner  : 

Part  of  the  Wolffian  ridge  is  continued  backwards  as  a  peritoneal  fold 
to  the  groin,  this  part  forming  the  inguinal  fold  (Fig.  392).  Within  the 
inguinal  fold,  in  the  mesenteries  of  the  Wolffian  body  and  genital  gland 
and  in  the  subperitoneal  tissue  of  the  genital  cord  a  stratum  of  non-striated 
muscular  tissue  is  developed.  The  mesodermal  tissue,  in  the  lower  end 
of  the  inguinal  fold,  begins  to  pierce  the  abdominal  wall  external  to  the 
deep  epigastric  artery  in  the  3rd  month,  the  piercing  force  being  obtained 
purely  from  the  inertia  of  growth.     The  growing  end,   at  first  merely 


rectum. 
ing.  fold 

genii,  cord. 


inguin.  fold 


bladder 


TPia.  392. — Diagram  of  the  Miillerian  Ducts  at  the  commencement  of  the  3rd  month. 

Ventral  view. 

represented  by  fine  strands  of  tissue,  in  later  months  increases  in  mass, 
and  carries  over  it  and  in  front  of  it,  into  the  scrotum  or  labium  ma  jus,  a 
process  of  the  peritoneum  and  attenuated  representatives  of  each  stratum 
of  the  belly  wall  (Fig.  421).  The  inguinal  canal,  the  round  ligament  of 
the  uterus  and  the  gubernaculum  testis  are  thus  formed  by  the  extension 
of  the  substance  of  the  inguinal  fold.  From  the  stratum  of  muscular 
tissue  which  is  found  everywhere  under  the  pelvic  peritoneum,  particularly 
around  the  genital  cord,  are  formed  the  round  ligament  of  the  ovary,  the 
muscular  tissue  in  the  utero -rectal  (utero -sacral)  ligaments  and  in  the  broad 
ligaments,  and  also  the  outer  muscular  coat  of  the  uterus. 

Formation  of  Uterus  and  Vagina. — The  parts  of  the  Miillerian  ducts 
lying  side  by  side  in  the  genital  cord  (Fig.  392)  begin  to  unite  in  the  3rd 
month,  and  by  their  fusion  the  uterus  and  vagina  are  formed.  In  all  the 
members  of  the  vertebrate  series  below  and  including  the  Monotremes, 
the  Miillerian  ducts  remain  separate  and  open  in  the  cloaca  (Fig.  393,  A). 
The  process  of  fusion  begins  with  the  formation  of  the  genital  cord  in  the 
8th  week  and  is  continued  throughout  the  3rd  month.  The  septum  formed 
by  the  fused  mesial  walls  (Fig.  394)  disappears  first  below  the  region  of 


372 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


the  uterine  cervix  ;  the  process  may  be  arrested  at  this  stage — a  stage 
shown  by  some  adult  marsupials.  Next,  the  lower  or  cervical  part  of  the 
uterine  septum  disappears  ;  the  human  uterus  then  (2|  months)  resembles 
that  of  higher  mammals  (carnivora,  etc.,  Fig.  393,  C).  It  may  be  arrested 
at  this  stage  (uterus  bicornis).  Lastly  the  upper  part  of  the  septum 
disappears  (3|  months,  Fig.  394).  The  fundus,  which  is  the  last  part  to 
be  developed  and  is  only  found  in  the  highest  primates,  is  quite  well  marked 
in  the  child  at  birth. 

The  musculature  ^  appears  in  the  wall  of  the  uterus,  vagina  and  tubes 
during  the  4th  month,  the  inner  or  circular  layer  appearing  before  the 


fallop. 
tube 


rd.  lig. 

uterus 
Muller.  duct 

cloaca. 


tpen.  of.  rect      Q 


'M-°"^)Uallop. 
-)_  tube 

'm^rd.  lig 


uterus 


fallop. 
'  tube 

round  lig. 
uterus 


G  0 

Fig.  393. — Evolution  of  the  Human  Form  of  Uterus. 

A,  form  seen  in  lowest  mammals,  reptUes,  amphibians,  fishes,  and  in  the  2nd  month 
human  foetus  ;  B,  form  of  Miillerian  ducts  in  rodents  ;  C,  form  in  carnivora,  etc., 
and  in  the  3rd  month  human  foetus  ;  D,  form  found  in  man  and  higher  primates. 

outer  or  longitudinal.  Sometime  after  birth  additions  are  made  to  the 
musculature  of  the  uterus,  and  the  distinction  between  the  two  primary 
layers  becomes  obliterated.  Glands  begin  to  form  in  the  uterine  mucosa 
during  the  4th  month  and  at  the  same  date  the  cervix  becomes  differentiated 
from  the  vagina.  At  this  time,  too,  the  ovarian  extremity  of  the  Fallopian 
tube  becomes  wide  and  trumpet-shaped  ;  the  mucous  membrane  within 
it  becomes  plicated.     The  fimbriae  are  then  formed  by  the  extremities 


H.  R.  Clarke,  Journ.  Obstet.  Gynae.  1911,  vol.  20,  p.  85. 


UROGENITAL  SYSTEM 


373 


of  the  plicae  growing  out  at  the  ostium  abdominale.      Secondary  or  acces- 
sory ostia  may  also  be  produced. 

By  the  7th  month  (Fig.  402)  the  foetal  uterus  is  divided  into  two  parts, 
the  cervix  or  lower  segment  and  body  or  upper  segment.  The  lower 
segment  or  cervix  forms  then  two-thirds  of  the  uterus  ;  its  walls  are  thick 
and  its  upper  part  is  lined  by  columnar  non-ciliated  epithelium,  containing 
racemose  mucous  glands.  Its  mucous  membrane  is  arranged  in  palmate 
folds.  The  upper  or  uterine  segment  composes  only  a  third  of  the  uterus. 
It  is  lined  by  columnar  epithelium,  which  becomes  ciliated  at  the  end  of 
foetal  life.  At  puberty  the  body  of  the  uterus,  instead  of  being  half  the 
size  of  the  foetal  cervix,  becomes  larger  than  it.     The  cervix  takes  no 


Fallop,  tube 


from  right  i 

Mil  liar  H II nil 


last  part  to  disappear 
from  left  Muller.  duct 


Muller.  duct} 


1st  part  to  disappear 
3rd  month 


^\^[2nd  part  of  septum  to 
I    disappear 


Fig.  394. — Showing  the  manner  in  which  the  Miillerian  Ducts  fuse  to  form  the  Uterus 

and  Vagina. 

part  in  menstruation  nor  in  containing  the  foetus  ;  its  true  function  is 
unknown.  The  external  os  is  formed  at  the  junction  of  the  vaginal  cords 
with  the  uterine  segment  of  the  Miillerian  ducts  ;  it  becomes  demarcated 
at  the  end  of  the  4th  month.  For  some  time  after  birth  the  body  of  the 
uterus  actually  undergoes  a  reduction  in  size  (Bayer)  ;  growth  does  not 
become  marked  until  the  7th  year. 

Metamorphosis  of  the  Vagina. — About  the  middle  of  the  3rd  month 
the  lower  ends  of  the  Miillerian  ducts  of  the  human  embryo  undergo  a 
remarkable  metamorphosis,  first  fully  described  by  Berry  Hart  and  lately 
reinvestigated  by  F.  Wood  Jones.^  The  epithelium  lining  the  vaginal 
tracts  of  the  Miillerian  ducts  proliferates,  forming  two  cords  of  cells,  while, 
at  the  same  time,  the  mesodermal  tissue  in  the  lower  end  of  the  genital 
cord,  which  surrounds  the  terminal  segments  of  the  Miillerian  and  Wolffian 
ducts,  undergoes  a  rapid  growth,  pushing  downwards  that  part  of  the 
cloaca  in  which  they  end — the  urogenital  sinus  (Fig.  395, 1.,  II.).     The  vaginal 

1  Brit.  Med,  Journ.  1904,  Dec,  17th  ;   Journ.  Anat.  1914,  vol.  48,  p.  268. 


374 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


cords  formed  by  the  Miillerian  linings  (Fig,  396)  proliferate  into  the  tissue  of 
the  genital  cord  and  fuse  together,  the  vagina  being  formed  by  the  breaking 
down  of  the  epithelial  core  in  the  4th  month.     Thus  the  orifice  of  the 


I. 


Fig.  395.— Diagrams  showing  the  Termination  of 'the]Vagina  about  the  seventh  weelc 
(I.)  and  about  tlie  thirteenth  week  (II.)-  (After  Wood  Jones.)  A,  Miillerian 
ducts  (vagina  and  uterus) ;  B,  urogenital  sinus  ;  C,  bladder  ;  D,  rectum  ;  E, 
vagina  represented  by  a  cord  of  epithelium. 

vagina,  originally  situated  high  in  the  urogenital  sinus,  is  carried  down- 
wards until  it  opens  in  the  vulval  cleft.  Atresia  of  the  vagina  results  from 
a  failure  of  the  process  of  canaliculization.  Septa  in  the  vagina  result 
from  incomplete  fusion  of  the  two  cords.     Only  the  tip  of  the  vaginal  cords 


Fallopian  tube 


bulbous  ends 
urogenital  sinua 


Fig.  396. — Diagram  Illustrating  the  manner  in  which  the  Vagina  is  formed  by  the 
Fusion  of  two  solid  Processes  or  Cords.     (Wood  Jones.) 

reach  the  urogenital  sinus  ;  hence  a  partial  septum — the  hymen  ^ — 
marks  the  opening  of  the  vagina  into  the  urogenital  sinus.  The  extent 
to  which  the  terminal  septum  breaks  down  varies  widely  ;  hence  the 
numerous  forms  assumed  by  the  hymen. 

^  For  development  of  hymen:    D.  Berry  Hart,  Edin.  Med.  Journ.   1911,  vol.  6, 
p.  577  ;   F.  J.  Taussig,  Amer.  Journ,  Anat.  1908,  vol.  8,  p.  89. 


UROGENITAL  SYSTEM 


375 


An  explanation  of  this  remarkable  change  may  be  found  in  the  formation 
of  a  new  vagina  in  lower  marsupials  which  was  discovered  by  J.  P.  Hill.^ 
In  lower  marsupials  the  vaginal  segment  of  the  Miillerian  ducts  are  separ- 
able into  two  parts — upper,  which  lie  side  by  side,  and  reach  towards 
the  cloaca  (Fig.  397)  ;  lower,  which  form  lateral  loops  before  terminating 
in  the  cloaca.  Hill  found  that  the  young  were  born  by  passing  from  the 
upper  or  median  segments  into  the  cloaca  by  the  formation  of  a  new 
passage  (Fig.  397).  In  higher  marsupials  he  found  that  the  upper  parts 
of  the  vaginal  segments  became  fused  to  form  a  median  vagina,  and  that 
the  new  passage  to  the  cloaca  was  not  temporary  as  in  lower  marsupials, 
but  permanent.  In  monotremes,  the  Miillerian  ducts  have  to  serve  only 
for  the  passage  of  unhatched  ova,  but  with  the  evolution  of  gestation  the 


j„^  Fallopian  tube 


fat  uag.- 


commenc.  of  uag^ 


Fallopian  tube 


lat.  uag. 

cervix 
commenc.  of  uag. 
— nem  median  vagina 


opening  of  new  median 
vagina 


FIG.  397. — Diagram  showing  the  arrangement  of  the  Miillerian  Duct  in  a  Marsupial 
Mammal  and  the  manner  in  which  a  New  Vagina  is  formed  for  the  Passage 
of  the  Young  at  Birth.    (F.  Wood  Jones  after  J.  P.  Hill.) 

ducts,  which  could  convey  ova,  were  unfitted  for  the  transmission  of 
young,  and  a  new  passage  or  median  vagina  was  formed.  The  evidence  is 
conclusive  that  there  was  a  phase  in  human  evolution  when  the  Miillerian 
ducts  terminated  in  a  cloaca,  and  the  metamorphosis  which  takes  place 
in  the  lower  ends  of  these  ducts  of  the  human  embryo  is  evidently  an 
abbreviated  recapitulation  of  the  formation  of  the  median  vagina  of 
marsupials. 

The  Miillerian  Ducts  in  the  Male.— In  the  male  foetus  of  the  3rd 
month  the  Miillerian  ducts  are  undergoing  atrophy,  the  distinction  between 
the  testis  and  ovary  being  quite  marked  by  that  time,  and  the  process  of 
sexual  diiierentiation  already  to  be  seen  on  close  examination.  All  that 
remain  of  the  Miillerian  ducts  in  the  adult  male  are  their  fused  terminal 


1  Proc.  Linnean  Soc.  New  South  Wales,  1899,  March  29th,  p.  42  ;   1900,  Aug.  29th, 
p.  519. 


376 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


or  vaginal  segments  forming  the  sinus  pocularis  or  uterus  masculinus  in 
the  prostate  (Figs.  386,  398).  Its  depth  is  commonly  about  3  or  4  mm., 
but  occasionally  such  a  form  as  is  represented  in  Fig.  399  occurs  and  shows 
the  real  nature  of  the  sinus  pocularis.  The  vagina  and  uterus  can  be 
recognized  in  such  cases  (Primrose).  The  fimbriated  ends  of  the  MiiUerian 
ducts  persist  as  the  sessile  hydatids  on  the  testicle  (Fig.  386).  The  inter- 
mediate part  of  the  tube  disappears  in  the  3rd  month  and  its  site  becomes 

bladder 


open, 
ejao.  duct 


triang.  lig. 


.3rd  lobe  prosi 

sejac.  duct 
[  from  W.D. 

(Sinus  pocularis 
[    from  M.D. 

from  uro-genital  sinus. 


FIG.  398.- 


-A  Section  of  the  Prostate  showing  the  Remnants  of  the  Lower  Ends  of 
the  MiiUerian  Ducts  in  the  Male. 


greatly  stretched  during  the  descent  of  the  testicle.  A  remnant  of  its 
upper  end  can  be  found  in  the  sharp  anterior  border  of  the  epididymis 
until  quite  a  late  period  in  foetal  life.^  The  mesosalpinx  shrinks  and  com- 
pletely disappears  in  the  anterior  border  of  the  epididymis. 

The  Urogenital  Sinus  or  Canal. — The  Miillerian  ducts  open  into  the 
cloaca  of  the  embryo  side  by  side,  between  and  below  the  openings  of  the 
Wolffian  ducts  (Fig.  391).     The  passage  which  serves  as  a  common  channel 


bladder 


uterus 

Fallop. 
tube 


4  ejac. 
uagina     {duct 


triang.  lig. 


Fig.  399. — A  Section  of  a  Prostate  showing  an  unusually  developed  Uterus  Mascu- 
linus.    (After  Primrose.) 

for  bladder,  Miillerian  and  Wolffian  ducts  is  the  urogenital  sinus  (Fig.  400, 
A,  B).  In  the  female  foetus  at  the  3rd  month  it  is  still  well  marked.  In 
all  mammals  except  man  this  passage-like  sinus  is  retained.  By  the 
beginning  of  the  4th  month  in  the  female  foetus  (Fig.  400,  B)  it  will  be 
seen  that  the  urogenital  sinus  has  become  shortened  and  opened  out  to 
form  the  floor  of  the  pudendal  or  vulval  cleft  from  the  glans  clitoris  in 
front  to  the  fossa  navicularis  behind,  and  thus  the  end  of  the  Miillerian 

1  J.  H,  ^a.tson,' Journal  of  Anat.  1902,  vol,  36,  p.  147. 


UROGENITAL  SYSTEM 


377 


ducts  (vagina)  and  urethra  come  to  have  separate  openings.  The  meta- 
morphosis in  the  genital  cord  which  leads  to  the  formation  of  the  vagina 
plays  a  large  part  in  the  transformation  (Fig.  395).  In  the  male  (Fig. 
401)  the  early  foetal  form  is  retained,  and  the  urogenital  sinus  becomes  that 
part  of  the  urethra  between  the  sinus  pocularis  and  the  fossa  navicularis 


uterus 


cord- 
bladder, 
symph. 


uro-gen.  canal 


cordr 


bladdec 
rectum 
vagina 


uterus 


uagina 


rectum 


symph 


clitoris 


A. 


uro-gen.  canal 

B. 


Fig.  400. — Section  showing  the  Urogenital  Sinus. 
A,  in  the  3rd  month  female  human  foetus  ;  B,  in  the  5th  month  female  human  foetus  ; 
a,  the  vesico-vaginal  septum. 


in  the  glans  penis.  The  female  urethra  corresponds  to  the  prostatic  part 
above  the  opening  of  the  sinus  pocularis  of  the  male  urethra  (Figs.  400, 
401). 

The  Hymen  ^  is  formed  at  the  junction  of  the  vagina  with  the  uro- 
genital sinus,  being  covered  on  its  outer  surface  by  epithelium  derived  from 


Cord 


-w.^'-^Hj//   //    /  .      .    uterus  and 
uagina 

rectum 

uro-gen.  canal 
t'riang.  lig. 

penile  urethra 

Fig.  401. — Section  showing  the  Urogenital  Sinus  in  the  Male  Foetus. 
a  indicates  the  part  corresponding  to  the  vesico-vaginal  septum  of  the  female, 
occupied  by  the  3rd  lobe  of  the  prostate. 


It  is 


the  urogenital  sinus,  and  on  its  deep  surface  by  epithelium  of  the  vaginal 
cord.     Usually  at  one  point  on  the  hymen,  but  occasionally  at  several, 

^  See  references,  p.  374, 


378 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


the  epithelial  coverings  fuse  and  break  down,  leading  to  one  or  more 
perforations.  On  the  other  hand,  the  vaginal  cords  may  not  reach  the 
urogenital  sinus,  the  hymen  being  then  imperforate.     In  reptiles,  as  in 


OS.  externum 
rectum 


vagina 


Fia.  402. — A  Section  to  show  the  condition  of  the  Vagina  and  Uterus  at  the  7th  month 

of  Foetal  Life. 


the  human  embryo,  the  part  of  the  urogenital  sinus  into  which  the  Miillerian 
and  Wolffian  ducts  open,  forms  the  trigone  of  the  bladder  (see  p.  381). 
In  such  animals  the  hymen  prevents  the  reflux  of  urine  into  the  Miillerian 
ducts. 


CHAPTER  XXIV. 

UROGENITAL   SYSTEM   {Continued). 

Evolution   of  the  Penis.^ — The   transformation    of   the    mesonephros 
to  form  an  adjunct  of  the  genital  system  of  the  male  is  of  ancient  origin, 
but  those  remarkable  changes  which  are  seen  to  occur  in  the  perineum 
of  the  human  embryo  represent  a  much  later  evolutionary  movement. 
Even  in  the  lowest  mammals — monotremes  and  marsupials — the  rectum 
and  urogenital  ducts   end  in   a   common  terminal  passage — the  cloaca 
(Fig.  403,  B).     In  the  human  embryo,  until  the  7th  week  of  development, 
this  is  also  the  case  ;  but  about  the  beginning  of  this  week,  when  the  embryo 
is  12  mm.  long,  changes  occur  which  separate  the  rectal  and  urinary 
passages.     These  changes  have  been  occasioned  by  the  evolution  of  an 
external  or  extra-cloacal  penis.     In  Fig.  403  stages  in  the  evolution  of  the 
penis  are  represented.     In  the  tortoise  the  penis  lies  on  the  pubic  or  ventral 
wall  of  the  cloaca  ;    during  copulation  the  cloaca  is  partially  everted  and 
the  open  groove  of  the  penis  is  converted  into  a  canal  by  the  application 
of  the  dorsal  or  opposite  waU  of  the  cloaca.     In  Echidna — a  primitive 
mammal — ^the  penis  is  still  intra-cloacal ;    its  groove  is  converted  into  a 
canal,  except  posteriorly,  where  there  is  still  a  communication  between  the 
urogenital   and   cloacal   passages — which   represents   the   primitive   uro- 
genital orifice,  for  the  penile  canal  is  a  new  passage  (Fig.  403,  B,  4).     In 
marsupials  (Fig.  403,  C)  the  penis  is  still  partially  intra-cloacal,  but  the 
primitive  urogenital  orifice  is  closed,  and  the  urogenital  passage  is  now 
separated  from  that  which  serves  for  the  faeces.     In  man  the  penis,  as  in 
all  primates,   has  been  permanently  extruded  and  is  now  completely 
extra-cloacal,  and  a  perineal  body  separates  the  rectal  orifice  from  the 
urogenital  passage.     The  metamorphosis  of  the  cloaca  is  thus  a  result 
of  the  evolution  of  the  penis.     The  external  penis  with  a  complete  penile 
urethra  appears  with  the  evolution  of  a  vagina,  uterus  and  the  intra- 
uterine nourishment  of  the  young.     The  cloacal  passage  is  seen  in  oviparous 
mammals  ;  in  viviparous  mammals  the  penis  is  evolved  as  an  intromittent 
organ,  and  the  urogenital  passage  is  separated  from  that  of  the  rectum. 

Twofold  Origin  of  the  Cloaca. — The  primitive  cloaca,  as  represented 
in  Fig.  403,  A,  is  of  double  origin,  the  deeper  part  in  which  the  rectum  and 
urogenital  sinus  end  is  derived  from  the  hind-gut  and  is  thus  of  entodermal 

1  See  articles  by  Prof.  Wood  Jones,  Journ.  Anat.  1914,  vol.  48,  p,  73  ;  1915,  vol.  49, 
p.  393  ;  1916,  vol.  50,  pp.  1,  189. 

379 


380 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


origin.  The  more  superficial  part,  enclosed  by  the  cloacal  lips,  arises  as  a 
perineal  depression  and  is  thus  of  ectodermal  origin.  Students  of  embry- 
ology, however,  when  they  speak  of  cloaca,  have  in  mind  only  the  part 
derived  from  the  hind-gut — the  entodermal  cloaca.  The  development 
of  the  perineal  region  cannot  be  understood  unless  it  is  remembered  that, 
both  ectodermal  and  endodermal  elements  play  a  part  in  fashioning  the 
anal  and  vulval  orifices'of  the  human  embryo. 


Fig.  403. — stages  in  the  Evolution  of  the  Penis.  A,  stage  seen  in  tortoise  ;  B,  stage 
seen  in  Echidna  ;  C,  stage  seen  in  marsupial  (kangaroo) ;  D,  stage  seen  in  man. 
1,  bladder  ;  2,  Wolffian  duct  (vas) ;  3,  rectum  ;  4,  urogenital  sinus  ;  5,  anus  ; 
6,  phallic  groove  and  canal ;  7,  glans  ;  8,  cloaca  ;  9,  cloacal  orifice  ;  10,  floor 
of  phallic  canal ;  11,  Cowper's  glands  ;  *  position  of  primitive  orifice  of  urogenital 
sinus. 


The  Cloaca  of  the  Embryo.^ — Having  in  the  previous  chapter  traced 
the  origin  and  fate  of  the  genital  ducts,  it  is  now  necessary  to  follow  the 
changes  which  are  undergone  by  the  cloaca — the  common  vent  for  the 
rectum  and  genital  passages.  We  have  already  seen  that  the  cloaca  appears 
early  in  the  4th  week  (Fig.  274)  ;  its  precocious  origin  being  undoubtedly 
due  to  the  fact  that  it  gives  origin  to  the  allantois,  by  means  of  which  the 
chorionic  circulation  is  established.  Thus  in  the  4th  week  (Fig.  407,  A) 
the  cloaca  forms  a  relatively  large  cavity,  into  which  open  the  rectum  and 
allantois,  while  the  Wolffian  duct  is  also  establishing  a  communication 

^  A.  G.  PoUman,  Amer.  Joum,  Anat,  1911,  vol,  12,  p.  1  (Dev.  of  Cloaca). 


UEOGENITAL  SYSTEM 


381 


with  its  more  ventral  part.  At  this  time  the  outline  of  the  cloaca,  as  seen 
on  making  a  median  section  of  the  embryo,  is  triangular  in  outline  ;  its 
dorsal  wall  follows  the  curve  of  the  notochord  to  the  point  of  the  tail ; 
a  large  part  of  its  ventral  wall  is  formed  by  the  cloacal  membrane — which 
is  composed  of  only  the  two  primitive  layers — the  entoderm  which  lines 
the  cloaca,  and  the  ectoderm  which  covers  the  embryo.  It  will  be  re- 
membered (see  p.  35)  that  the  hinder  end  of  the  embryonic  body  is  produced 
on  each  side  of  the  primitive  streak.  The  cloacal  membrane  occupies  the 
site  of  a  part  of  the  primitive  streak,  thrust  into  a  ventral  position  by  the 
outgrowth  of  the  tail  (Fig.  404).  The  hinder  apex  of  the  cloaca  extends 
beneath  the  tail  and  behind  the  cloacal  membrane  and  forms  that  transitory 
structure  known  as  the  tail  gut.     In  the  4th  week  the  cloaca  has  no  perineal 


ALLANTOIS 


V .CLOACA 


(A) 


(B) 


Fig.  404. — The  Formation  of  the  Cloaca  from  the  Hind-gut  during  the  4th  week. 
(Wood  Jones.)  A,  section  of  the  posterior  end  of  a  liuman  embryo  early  in  the 
4th  week  ;  B,  later  in  the  4th  week  when  the  hind  fold  is  more  produced  and  the 
cloaca  assuming  its  triangular  form. 


opening  ;   that  opening  is  first  established  near  the  end  of  the  2nd  month 
by  an  absorption  of  the  cloacal  membrane. 

Evolution  o£  Cloacal  Structures. — To  understand  the  significance 
of  the  changes  undergone  by  the  cloaca  in  the  human  embryo,  one  must 
have  first  a  clear  conception  of  the  various  evolutionary  stages  known  to 
the  comparative  anatomist.  We  have  already  seen  that  some  of  these 
changes  are  related  to  the  differentiation  of  an  external  penis  ;  it  is  now 
necessary  to  see  how  the  cloaca  becomes  modified  to  assume  its  mammalian 
and  human  form.  The  essential  stages  are  represented  in  Fig.  405  ;  in 
the  frog  {A)  the  cloaca  receives  the  bladder,  rectum  and  Wolffian  duct, 
the  duct  opening  distal  to  the  rectum,  being  thus  nearer  the  cloacal  vent. 
In  the  tortoise  {B)  the  rectum  has  passed  distal  to  the  Wolffian  duct,  which 
now  opens  with  the  bladder  into  a  common  part  of  the  cloaca — the  uro- 
genital sinus  (Fig.  405,  B,  TJG).  In  the  lowest  mammals — monotremes 
(C) — the  urogenital  sinus  has  become  elongated  and  assumed  the  form 
of  a  urethra  ;  the  ureter  is  now  severed  from  the  Wolffian  duct,  but  still 
opens  on  the  floor  of  the  urogenital  sinus  ;  the  urine  thus  has  to  pass  across 
the  urogenital  sinus  to  reach  the  bladder.  In  marsupials  [D)  a  further 
stage  is  reached  ;  the  cloacal  anus  of  the  rectum  has  migrated  backwards 
on  the  posterior  wall  of  the  cloaca  until  it  almost  reaches  the  perineum. 
This  posterior  migration  of  the  rectal  opening  (anus)  is  already  seen  in 


382 


HUMAN  EMBEYOLOGY  AND  MORPHOLOGY 


Echidna  (C),  where  the  urogenital  sinus — which  represents  the  proximal 
part  of  the  cloaca — has  assumed  a  considerable  length.  Thus  in  the 
evolution  of  mammals  we  see  that  the  rectum  migrates  backwards  until 
its  vent  or  anus  almost  reaches  the  surface  of  the  perineum,  leaving  the 
greater  part  of  the  cloaca  as  the  urogenital  sinus. 


RECTUM 


Cloacal  Orifice 
(e) 
(A) 


(B) 


CLITORIS 


(D) 

Fig.  405. — Diagrams  to  show  the  manner  in  which  the  Cloaca  is  modified  and  the 
Termination  of  the  Rectum  transferred  from  the  Cloaca  to  the  Perineum  in 
Higher  Vertebrates.  A,  the  amphibian  form;  1,  bladder;  2,  Wolffian  duct 
(ureter  and  vas) ;  3,  cloaca  ;  4,  rectum;  5,  intra-cloacal  anus  ;  6,  cloacal  orifice  ; 
M.D.,  MiilJerian  duct ;  B,  form  found  in  the  tortoise  ;  C,  form  in  monotremes; 
D,  form  found  in  female  marsupial ;  X,  floor  of  urethra. 

Ectodermal  Cloaca. — The  forms  of  cloaca  depicted  in  Fig.  405  are 
not  entirely  derived  from  the  gut  or  entodermal  cloaca,  which  is  seen  in 
the  human  embryo.  The  terminal  or  superficial  part  is  derived  from  a 
cloacal  depression  or  pit  formed  on  the  perineum,  and  lined  by  epithelium 
derived  from  the  ectoderm.  The  glans  of  the  penis  and  also  of  the  clitoris 
are  formed  in  the  ectodermal  part :    the  rest  of  the  penis  and  clitoris  is 


UROGENITAL  SYSTEM 


developed  in  tlie  entodermal  part  (Fig.  403).  We  have  already  seen  how 
the  urethral  groove  of  the  cloacal  penis  becomes  closed  ofi  as  a  separate 
channel  by  the  union  of  two  vestibular  folds — seminal  guides  Prof.  Wood 
Jones  has  named  them — ^the  penile  urethra  being  thus  enclosed.  In 
Echidna  (Fig.  403,  B)  one  sees  that  the  urethra  is  made  up  of  two  parts 
— an  upper  derived  from  the  urogenital  sinus,  and  a  lower  or  penile  from 
the  channel  enclosed  by  the  lateral  vestibular  folds.  At  the  junction  of 
these  two  parts  of  the  urethra  there  is  still  an  orifice  forming  a  communica- 
tion between  the  urogenital  sinus  and  the  cloaca  and  representing  the 
primitive  opening  of  the  urogenital  sinus  (Fig.  403,  B).  In  marsupials 
the  primary  urethra  (urogenital  sinus)  and  secondary  or  penile  urethra 
have  united  by  the  closure  of  the  primitive  opening  of  the  urogenital  sinus, 
and  thus  the  passage  for  the  urine  and  semen  become  completely  separated 


cloaca/  sept,  imperfect 


rectum 


anal  plate 
'anal  dep. 


uagina 


perineal  sept,  not  united 
to  cloacal  septa. 


Fig.  406. — Case  of  a  Female  Child  in  which  the  Rectum  opened  on  the  Vestibule 
while  the  Normal  Anus  remains  closed  by  the  Anal  Plate.  The  opening  on  the 
vestibule  represents  the  ancient  cloacal  orifice  of  the  rectum. 

from  the  passage  for  the  faeces.     The  rectum  is  detached  from  the  uro- 
genital sinus  and  opens  directly  into  the  ectodermal  cloaca. 

Differentiation  o£  the  Human  Cloaca. — We  are  now  in  a  position 
to  interpret  the  changes  which  occur  in  the  human  cloaca  during  the 
5th,  6th  and  7th  weeks  of  development  (Fig.  407).  In  the  5th  week  the 
rectum  ends  proximal  to  the  Wolffian  duct  as  in  the  frog  ;  in  the  6th  week 
the  cloacal  orifice  of  the  rectum  has  moved  backwards,  leaving  the  proximal 
part  of  the  cloaca  as  the  urogenital  sinus,  a  condition  similar  to  that  seen 
in  Echidna  (Fig.  405,  C).  As  in  that  animal,  the  Wolffian  ducts  and 
ureters  open  close  together  in  the  sinus.  The  appearance  presented  by  the 
backward  migration  of  the  rectal  orifice  is  exactly  the  same  as  if  the  cloaca 
had  been  divided  into  rectal  and  urogenital  cavities  by  the  septum  marked 
"  A  "  in  Fig.  407,  B,  C.  It  is  convenient  to  apply  the  term  given  by 
Retterer  to  this  septal  formation — the  urorectal  septum.  In  the  7th 
week  (Fig.  407,  C)  the  orifice  (cloacal  anus)  of  the  rectum  reaches  the 
cloacal  depression  (ectodermal  cloaca)  and  thus  become  separate  from 
the  urogenital  sinus — which  now  represents  practically  the  whole  of  the 
entodermal  cloaca.     During  the  6th  week  the  ventral  or  pubic  part  of  the 


384 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


entodermal  cloaca  is  extended  forwards  to  provide  the  lining  of  the  penile 
urethra.  It  represents  a  direct  extension  of  the  urogenital  sinus  carried 
within  the  outgrowing  genital  eminence  (Fig.  407,  C).  The  cloacal  mem- 
brane on  the  floor  of  this  penile  extension  of  the  cloaca  breaks  down  towards 
the  end  of  the  7th  week  and  the  urogenital  sinus  thus  presents  a  fissure- 
like opening  on  the  perineum.  This  fissure  corresponds  to  the  groove 
on  the  open  urethra  of  the  tortoise  (Fig.  403,  A).     On  each  side  of  the 


Fig.  407. — Showing  the  manner  in  which  the  Rectum  becomes  separated  from  the 
Urogenital  Sinus  (entodermal  cloaca)  during  development  of  the  Human  Embryo. 
A,  Human  embryo  of  5th  week,  4  mm.  long  ;  after  Keibel.  B,  From  human 
embryo  of  6th  week,  11  mm.  long  ;  after  Keibel.  C  and  D,  Later  stages  of 
development ;  1,  bladder  ;  2,  Wolffian  duct  (ureter  and  vas) ;  3,  entodermal 
cloaca  ;  4,  rectum  ;  5,  anus  ;  CM.,  cloacal  membrane  ;  U.G.,  urogenital  sinus  ; 
A,  Urorectal  septum  ;  B,  penis  ;  **,  the  limits  of  the  perineal  depression  (ecto- 
dermal cloaca). 

perineal  fissure,  towards  the  end  of  the  7th  week  there  appears  a  fold — 
the  vestibular  fold  or  seminal  guide.  The  hinder  ends  of  the  vestibular 
folds  are  continuous  with  the  urorectal  septum  ;  they  unite  together  in 
the  middle  line,  union  commencing  at  the  urorectal  septum  and  spreading 
forwards.  Their  union  closes  the  ancient  cloacal  orifice  of  the  rectum, 
but  cases  frequently  occur  in  which  the  closure  is  imperfect  and  the  ancient 


UROGENITAL  SYSTEM 


385 


cloacal  anus  persists  (Figs.  406,  408,  409).  It  will  thus  be  seen  that  in 
the  human  embryo  the  rectal  orifice  migrates  backwards  until  it  opens 
in  the  posterior  part  of  the  perineal  depression  (ectodermal  cloaca),  leaving 
the  w^hole  of  the  entodermal  cloaca  of  the  embryo  as  a  urogenital  passage 
or  urethra.  All  these  changes  take  place  during  the  latter  part  of  the  2nd 
month. 

Malformations  of  the  Rectum  and  Anus. — When  the  rectum  reaches 
the  perineal  depression,  it  is  in  contact  with  and  closed  by  the  cloacal 
membrane  (Fig.  407,  C).  The  union  of  the  urorectal  septum  with  the 
hinder  ends  of  the  vestibular  folds  gives  rise  to  the  perineal  body  which 
separates  the  anus  from  the  vestibular  cleft.  The  posterior  part  of  the 
cloacal  membrane  proliferates,  and  forms  the  anal  plug.  The  plug  breaks 
down  at  the  commencement  of  the  8th  week,  and  the  permanent  anus  is 


Fig.  408. — Section  of  Pelvis  of  a  Male  Child,  showing  the  Rectum  ending  in  the 
Prostatic  Part  of  the  Urethra.  J.,  bladder;  5,  rectum;  C,  recto-vesical  pouch  ; 
D,  uterus  masculuius  ;  E,  intra-cloacal  anus  ;  F,  prostate  ;  G,  anal  depression 
(ectodermal) ;  H,  external  and  internal  sphincters  ;  /,  Cowper's  gland. 

thus  formed.  This  process  may  fail,  giving  rise  to  the  condition  known  as 
atresia  ani  or  imperforate  anus.  A  common  degree  of  malformation  is 
shown  in  Fig.  408.  The  migration  of  the  rectum  has  failed  ;  it  opens  into 
the  urethra  by  the  ancient  cloacal  anus  and  a  thick  stratum  of  mesodermal 
tissue  separates  the  rectum  from  the  anal  depression  formed  by  the  ecto- 
dermal plug  derived  from  the  hinder  end  of  the  ectodermal  cloaca.  In 
Fig.  409  an  exactly  similar  condition  is  represented  in  a  female  infant. 
The  rectum  opens  in  the  male  below  the  orifice  of  the  uterus  mascubnus, 
in  the  female  at  a  corresponding  point  below  the  orifice  of  the  vagina. 
The  urorectal  septum  and  vestibular  folds  in  the  female  form  merely  the 
perineal  body,  which  separates  the  terminal  part  of  the  rectum  from  the 
vulva  ;  in  the  male  they  form  the  floor  of  the  urethra  and  perineum  from 
the  sinus  pocularis  to  the  lacuna  magna  in  the  glans  penis.  The  terminal 
part  of  the  male  urethra,  as  we  shall  see  presently,  has  a  separate  origin. 
The  downward  migration  of  the  vaginal  orifice  in  women  brings  the  cloacal 
opening  of  the  rectum  into  the  vulva — ^the  floor  of  the  vulval  cleft  being  a 
derivative  of  the  urogenital  sinus.     In  many  cases  of  imperforate  anus 

2b 


386 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


(Fig.  410)  tlie  cloaca!  anus  is  closed,  and  tte  rectum  terminates  an  inch  or 
more  from  the  anal  depression.  In  other  cases  merely  a  thin  septum 
separates  the  anal  depression  from  the  termination  of  the  rectum.  In 
extreme  cases,  which  are  by  no  means  rare,  no  anal  depression  is  formed 
and  the  sacral  part  of  the  rectum  is  absent. 


Fig.  409. — Section  of  Pelvis  of  Female  Infant,  shoM'ing  the  Rectum  opening  into  the 
Navicular  Fossa  of  the  Vulva.  A,  bladder  ;  B,  rectum  ;  C,  recto-uterine  fold  ; 
D,  symphysis  ;  E,  vulval  anus  ;  F,  cervix  ;  G,  anal  depression  (rarely  present  if 
rectum  opens  in  vulva) ;  E,  urethra  ;  /,  clitoris  ;  K,  hymen  ;  L,  Labium  minus. 

There  is  some  confusion  as  to  how  much  of  the  anal  canal  is  formed 
from  rectum  and  how  much  from  anal  depression.  As  may  be  seen  from 
Figs.  408  and  409  certain  folds  are  formed  in  the  wall  of  the  anal  depression. 
At  the  upper  end  of  these  anal  folds,  which  are  scarcely  recognizable  in 


cloaca/,  sept. 


vagina 


anal  depress. 


perineal  septum 


Fig.  410. — A  Case  of  Imperforate  Anus  in  which  the  Rectum  has  been  arrested  in  its 
migration  from  the  Cloaca  to  the  Perineum. 


the  fully  developed  anal  canal,  are  develoj)ed  certain  valve-like  folds  of 
mucous  membrane,  the  anal  valves.  Above  the  valves  are  the  well-marked 
columns  of  Morgagni  formed  in  the  rectum.  The  valves  mark  the  junction 
of  the  rectum  with  the  anal  depression. ^ 

'  See  research  by  F.  P.  Johnson,  Amer.  Journ.  Anat.  1914,  vol.  16,  p.  1. 


UROGENITAL  SYSTEM 


387 


External  Genital  Organs  and  Perineum. — That  tlie  interpretation 
just  given  of  the  embryological  parts  entering  into  the  formation  of  the 
rectum  and  urethra  is  right  is  seen  when  the  development  of  the  external 
genital  organs  is  traced.  The  stages  in  the  development  of  the  human 
urethra,  penis  and  scrotum  during  the  latter  part  of  the  2nd  month  and 
earlier  part  of  the  3rd  are  shown  in  Fig.  411.  Stage  I.  represents  the 
condition  seen  in  the  perineum  about  the  end  of  the  8th  week.  The 
circular  fold  A — cloacal  fold  ^  it  may  be  named,  for  it  represents  the 
opening  or  margin  of  the  primitive  (ectodermal)  cloaca.     Within  its  anterior 


Fig.  411. — Stages  in  the  development  of  the  Human  Penis  and  Perineum.  (Drawings 
by  Dr.  Stanley  Beale  after  figures  given  by  Kollmann,  Keibel  and  Hertzog.) 
I."  human  embryo  25  mm.  long  (8  weeks)  ;  II.  29  mm.  long  (9th  week)  ;  III. 
31  mm.  long  (9th  week)  ;  IV.  45  mm.  long  (about  10th  week).  A,  lips  of  cloaca 
(labit  majora) ;  £,  urogenital  orifice  being  carried  to  the  surface  between  labia 
minora  (a,  a) ;  C,  penis  becoming  extra-cloacal ;  D,  Tail ;  E,  urogenital  orifice  ; 
F,  anus  ;  G,  meatus. 

or  pubic  fornix  is  rising  up  the  genital  eminence  to  form  the  penis  or 
clitoris — according  to  sex,  for  at  this  time  the  external  parts  of  both  sexes 
are  alike,  although  the  ovary  is  being  differentiated  from  the  testicle. 
There  is  a  groove  or  furrow  on  the  under  or  cloacal  aspect  of  the  genital 
eminence,  as  on  the  cloacal  penis  of  the  tortoise  ;  it  represents  the  roof  of 
the  penile  urethra,  and  leads  backwards  to  the  urogenital  sinus.  The 
folds  at  each  side  of  the  furrow  {a,  a)  are  the  vestibular  or  perineal  folds 
which  form  the  penile  urethra  in  Echidna.     In  Stage  II.  (Fig.  411),  reached 

^  Usually  named  the  outer  genital  fold. 


388     HUMAN  EMBEYOLOGY  AND  MOEPHOLOGY 

during  the  9th  week,  two  further  changes  are  seen  in  progress.  The 
lateral  perineal  folds  {a,  a)  have  now  united  behind  the  genital  or  urethral 
furrow,  and  by  so  doing  have  separated  the  anal  part  of  the  ectodermal 
cloaca  (perineal  depression),  in  which  the  rectum  now  terminates,  from  the 
anterior  urogenital  part.  By  the  union  of  the  posterior  ends  of  those 
lateral  perineal  folds  the  perineal  body  is  formed.  The  cloacal  folds 
{A,  A)  are  still  well  marked,  but  it  is  apparent  that  the  genital  eminence 
and  its  attached  folds  are  being  extruded  from  the  cloaca.  In  Stage  III., 
reached  at  the  commencement  of  the  3rd  month,  a  condition  is  represented 
which  is  common  to  both  male  and  female  foetuses.  The  anus  is  now 
extruded  from  its  depression,  and  lies  within  the  flattened  posterior  fornix 
of  the  cloacal  folds.  The  lateral  perineal  or  vestibular  folds  meet  behind 
in  the  perineal  body,  where  their  free  margin  forms  a  semilunar  fold — the 
primitive  fourchette.  Anteriorly  the  folds  unite  on  the  perineal  aspect  of 
the  glans.  Between  the  folds  opens  the  penile  urethra  ;  the  opening 
between  the  folds  is  the  orifice  of  the  urogenital  sinus  ;  it  represents  the 
primitive  meatus  of  the  penile  urethra.  On  the  under  or  perineal  aspect 
of  the  glans  a  depression  (the  phallic  groove)  packed  with  an  ectodermal 
plug  is  also  present ;  it  forms  the  part  of  the  urethra  within  the  glans. 
Stage  IV.  represents  a  condition  peculiar  to  the  male.  A  median  raphe  or 
suture  is  now  seen  extending  from  the  anus  behind  to  terminate  in  front 
in  the  two  perineal  or  vestibular  folds — perhaps  it  would  be  well  to  name 
their  anterior  parts,  which  are  confined  purely  to  the  urethra  of  the  male 
and  vestibule  of  the  female — urethral  or  inner  genital  folds.  The  primitive 
urethra  is  now  small  in  size  and  well  advanced  towards  the  glans  penis. 
The  median  perineal  raphe  is  caused  by  a  fusion  of  the  tissues  of  the  cloacal 
or  outer  genital  folds  within  the  septum  primarily  formed  by  the  union  of 
the  lateral  perineal  folds.  In  the  female  this  union  of  the  cloacal  folds 
does  not  occur,  and  hence  there  is  no  raphe  on  the  female  perineal  body 
(Wood  Jones).  The  cloacal  folds  remain  separate,  and  form  the  labia 
majora  ;    in  the  male  they  unite,  and  form  the  scrotum. 

By  the  end  of  the  3rd  month  the  process  of  union  which  gives  rise  to  the 
perineal  raphe  extends  to  the  glans,  and  in  this  way  the  primitive  meatus 
is  closed,  the  terminal  parts  of  the  vestibular  folds  forming  the  frenum  of 
the  prepuce.  Thus  the  anterior  parts  of  the  perineal  folds  imite  right  up 
to  the  fraenum  of  the  prepuce,  and  enclose  the  male  urethra.  In  Stage 
IV.  (Fig.  411)  the  margins  of  the  phallic  groove  on  the  glans  have  united  ; 
the  plug  of  epithelium  within  it  breaks  down  as  it  unites  with  the  cloacal 
urethra,  and  the  permanent  terminal  urethra  and  meatus  are  thus  estab- 
lished. In  Stage  IV.  the  preputial  collar  of  skin  is  seen  in  process  of 
formation.  It  is  directly  continuous  with  the  anterior  ends  of  the  folds 
surrounding  the  primary  meatus.  It  does  not  rise  up  as  a  free  fold  ;  ^ 
the  epithelium  on  the  deep  surface  of  the  collar  adheres  to  that  on  the  glans, 
and  hence  when  the  prepuce  is  fully  formed  in  the  4th  month,  the  prepuce 
is  tightly  bound  to  it  until  the  period  of  birth. 

1  Dr.  Berry  Hart  {J own.  Anat.  and  Physiol.  1908,  vol.  42,  p.  50)  and  Dr.  Wood 
Jones  {Brit.  Med.  Journ.  1910,  Jan.  15th)  give  another  interpretation  of  the  manner 
in  which  the  prepuce  is  formed. 


UROGENITAL  SYSTEM 


389 


Hypospadias. — It  is  not  unusual  to  see  cases  in  which  the  process  of 
urethral  development  has  been  arrested.  In  the  female  its  complete 
arrest  is  normal  ;  in  individuals  with  imperfect  differentiation  of  sexual 
glands  (usually  imperfect  males)  the  process  is  also  arrested  at  an  early 
stage.     Fig.  412  represents  three  conditions  of  hypospadias  due  to  arrest 


Fig.  412. — Three  types  of  Hypospadias — A,  in  which  the  Groove  in  the  Glans  (phallic 
groove)  is  open,  and  Urine  passes  by  the  primitive  meatus  ;  B,  in  which  the 
floor  of  the  Phallic  Groove  is  formed,  but  the  primitive  meatus  is  unclosed ; 
C,  in  which  the  Phallic  Groove  is  unformed  or  obliterated  and  the  primitive 
meatus  persists.  1,  Primitive  meatus  ;  2,  fraenum  praeputii ;  3,  phallic  groove 
and  canal ;  4,  permanent  meatus. 

of  development  at  the  terminal  stage.  In  A  the  phallic  groove  is  unclosed  ; 
the  urethra  opens  at  the  primary  meatus  ;  the  folds  bounding  the  meatus 
represent  the  anterior  ends  of  the  urethral  or  perineal  folds.  In  B  the 
primitive  meatus  is  unclosed,  but  the  phallic  groove  is  converted  into  a 
canal ;  in  C,  the  commonest  type,  the  primitive  meatus  is  open  and  the 
phallic  groove  has  remained  uncanaliculized.^ 


-'"'\iA   GENiEMIN: 


HIO  CAV: 
BULSO  CAV: 


rA) 


(B) 


(c) 


Fig.  413. — Stages  in  the  Evolution  of  the  Perineal  Musculature.  (After  Popowsky.) 
A,  sphincter  of  the  cloaca  in  the  2nd  month  ;  B,  its  division  at  the  beginning  of 
the  3rd  month  ;  C,  its  condition  in  the  male  foetus  at  the  end  of  the  3rd  month. 

Perineal  Muscles. — From  what  has  been  said  regarding  the  cloaca, 
the  evolution  of  the  muscles  of  the  perineum  from  the  sphincter  of  the  cloaca 
will  be  readily  understood.  The  sphincter  in  cloaca!  vertebrates  surrounds 
the  part  of  the  cloaca  (perineal  depression)  formed  from  ectoderm  ;  it  is 
a  striated  muscle.  At  the  end  of  the  2nd  month  this  muscle  is  apparent 
in  the  cloacal  fold  of  the  human  foetus  (Fig.  413,  A).  At  the  beginning  of 
the  3rd  month,  when  the  perineal  body  is  formed,  the  sphincter  of  the 

iSee  Ralph  Thompson,  Jo^lrn.  Ayiat.  1919,  vol,  53,  p.  32, 


390      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

cloaca  becomes  divided  into  urogenital  and  anal  parts  ^  (Fig.  413,  B). 
The  sphincters  of  the  urogenital  passage  and  anus  fuse  in  the  perineal  body. 
A  part  of  the  urogenital  sphincter  obtains  an  attachment  to  the  ischium 
and  forms  the  ischio-cavernosus  (erector  penis)  ;  another  strand,  the 
transversus  perinei.  With  the  formation  of  the  urethra  in  the  male,  the 
sphincter  of  the  urogenital  passage  is  carried  forwards  on  the  bulb  and  forms 
the  bulbo-cavernosus  ;  in  the  female  it  remains  as  the  sphincter  vaginae. 
A  deeper  and  older  part  surrounds  the  upper  part  of  the  urogenital  sinus, 
and  becomes  the  constrictor  urethrae. 

Origin  o£  the  Bladder. — In  amphibians  the  bladder  is  a  diverticulum 
of  the  cloaca.  In  the  embryos  of  reptiles,  birds  and  mammals  it  becomes 
modified,  to  form  the  ■  allantois  ;  part  lies  outside  the  body  and  is  lost  at 
birth,  part  remains  within  the  body  to  form  the  urachus  and  all  the  bladder 
except  the  trigone.  By  a  downward  migration  of  the  orifices  of  the 
Wolffian  and  Mlillerian  ducts,  the  upper  part  of  the  urogenital  sinus,  con- 
taining the  insertion  of  the  ureters,  remains  to  form  the  trigone  of  the 
bladder  and  supra-genital  part  of  the  urethra  (Figs.  388,  407). 

The  Urachus. — When  the  body  stalk  becomes  elongated  in  the  forma- 
tion of  the  umbilical  cord,  the  part  of  the  allantoic  cavity  within  it  is 
obliterated.  The  part  of  the  allantois  within  the  abdomen,  stretching 
from  the  umbilicus  to  the  apex  of  the  bladder,  becomes  the  urachus,  a 
fibrous  cord,  in  which  all  trace  of  the  allantoic  cavity  is  lost  (Fig.  416). 
Occasionally  traces  of  the  cavity  may  remain  and  form  urachal  cysts,^ 
or  it  may  remain  open  throughout,  so  that  urine  escapes  from  the  bladder 
by  a  fistula  at  the  umbilicus.  The  urachus  lies  behind  the  linea  alba, 
embedded  in  the  subperitoneal  tissue,  and  flanked  on  each  side  by  the 
umbilical  artery.  In  the  7th  month  the  apical  part  of  the  bladder  is 
attached  by  a  mesentery  to  the  anterior  wall  of  the  abdomen  ;  later  the 
mesentery  disappears  (Broman). 

The  Bladder  at  Birth. — At  birth  the  bladder  is  elongated  and  fusiform 
in  shape,  with  a  small  trigone  (Fig.  416).  The  capacity  of  the  pelvis  is 
relatively  less  than  in  the  adult ;  hence  the  greater  part  of  the  bladder  is 
supra-pubic  in  position. 

Ectopia  vesicae  ^  is  not  easily  explained  on  an  embryological  basis. 
The  condition  is  shown  diagrammatically  in  Fig.  414,  A.  The  anterior 
wall  of  the  bladder  and  roof  of  the  urethra  and  the  parts  of  the  belly  wall 
in  front  of  these  are  absent  ;  the  trigone,  floor  of  the  urethra,  and  posterior 
wall  of  the  bladder  are  flush  and  continuous  with  the  belly  wall.  The 
symphysis  pubis  is  not  formed.  Certain  chambers  in  the  embryo,  such 
as  the  neural  canal  and  pericardium,  are  liable  to  a  dropsy  and  rupture. 
Were  the  cloaca  of  the  embryo  to  become  ruptured  along  its  ventral  wall 
(Fig.  414,  B)  the  condition  of  ectopia  would  be  produced.     Further,  it  is 

1  W.  J.  Otis,  Anat.  Hefte,  1905,  vol.  30,  p.  199  (Dev.  of  Anus  and  External  Sphincter). 

2  See  Alban  Doran,  Proc.  Boy.  Soc.  Med.  April,  1908. 

^  For  current  theories  see  Wood  Jones,  Journ.  Anat.  and  Physiol.  1912,  vol.  46, 
p.  193  ;  Keith,  Brit.  Med.  Journ.  1908,  Dec.  26th  ;  A.  M.  Paterson  and  Emrys-Roberts, 
Journ.  Anat.  and  Physiol.  1906,  vol.  40,  p.  332, 


UEOGENITAL  SYSTEM 


391 


to  be  remembered,  as  Berry  Hart  lias  pointed  out,  that  the  part  of  the 
embryo  on  which  the  primitive  streak  is  sitviated  comes  to  form  the  post- 
umbilical  part  of  the  ventral  wall  of  the  abdomen  (see  page  37).  It  is 
therefore  more  probable  that  the  condition  may  be  due  to  an  unclosed 
condition  of  the  primitive  streak. 

Musculature  of  the  Bladder,  Urethra  and  Rectum. — Seeing  that 
the  rectum,  allantois  and  cloaca  are  continuous  in  the  embryo  one  would 
expect  that  the  musculature  of  the  parts  should  show  traces  of  this  con- 
tinuity. Mr.  F.  Wood  Jones  found  (1)  that  the  band  of  musculature  which 
passes  from  the  front  of  the  rectum  to  be  lost  in  the  tissue  behind  the 
membranous  urethra  is  a  remnant  of  the  recto-cloacal  communication  in 
the  embryo  (Fig.  408)  ;  (2)  that  the  circular  muscular  coat  of  the  urethra 
is  continuous  above  with  the  sphincter  and  circular  coat  of  the  bladder, 


umbilic. 


belly  wall 


post  wall  bladder 


Wolffian  duct 


ureter 


w^^^^ejac.  ducts 


rectum 


memb.  ruptv. 
in  ectopias 


genit.  tub. 
cloaca. 


B. 


Fig.  4U,A. 
B. 


-A  Section  to  show  the  condition  of  parts  of  Ectopia  Vesicae. 
-Section  of  the  Pelvis  of  an  Embryo  (5th  weeli)  to  sliow  how  the  con- 
dition is  probably  produced. 


and  below  it  becomes  continuous  with  the  striated  fibres  (constrictor 
urethrae)  surrounding  the  membranous  urethra.  The  latter,  however, 
are  not  developed  from  the  musculature  of  the  urogenital  sinus,  but  from 
the  sphincter  cloacae  (Fig.  413). 

Neurenteric  Canal. — ^Ano-coccygeal  tumours  are  believed  to  arise  from 
remnants  of  the  neurenteric  canal  as  well  as  from  the  post-anal  gut.  The 
neurenteric  canal,  or  blastopore,  it  will  be  remembered  (p.  38),  is  a 
communication  of  the  cavity  of  the  archenteron  with  the  dorsal  surface 
of  the  embryo.  The  blastopore  opens  at  the  anterior  end  of  the  primitive 
streak,  which  afterwards  is  included  in  the  posterior  end  of  the  neural 
groove  ;  such  a  canal  might  be  represented  by  a  remnant  passing  from  the 
rectum  to  the  sacral  region  of  the  spinal  canal.  A  vestigial  structure, 
which  is  certainly  of  this  nature,  has  not  so  far  been  recognized.  The  part 
of  the  hind-gut  which  is  developed  under  the  tail  of  the  embryo  (post-anal 
gut)  disappears  in  the  2nd  month  of  development,  but  certain  congenital 
tumours  which  arise  between  the  sacrum  and  coccyx  may  spring  from 
remnants  of  the  post-anal  gut  (p.  381).  Seeing  that  the  embryonic  tissue 
which  gives  rise  to  the  caudal  end  of  the  body  lies  in  the  posterior  lip  of 


392      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

tlie  blastopore,  it  is  not  difficult  to  conceive  that  embryological  remnants 
might  persist  in  the  sacro-coccygeal  region  of  the  pelvis  and  give  rise  to 
teratomatous  tumours. 

External  Genitals  of  the  Female. — In  the  female  the  parts  retain 
closely  the  early  foetal  form  represented  in  Fig.  415.  The  genital  tubercle' 
becomes  the  glans  clitoris.  In  the  genital  eminence — of  which  the  tubercle 
is  merely  the  summit — ^the  corpora  cavernosa  develop.  The  vestibular 
or  inner  genital  folds  form  the  labia  minora,  the  prepuce  and  fraenum. 
By  the  junction  of  the  inner  genital  folds  within  the  urogenital  depression 
behind,  the  fourchette  is  formed.  Thus  the  opening  into  the  urogenital 
sinus  (primitive  meatus)  is  bounded  by  the  fourchette,  labia  minora  and 
fraenum  of  the  prepuce.  In  the  lateral  folds,  or  labia  minora,  are  developed 
the  bulbs  of  the  vestibule.  After  the  third  month  the  external  genital 
(cloacal)  folds  become  prominent  around  the  urogenital  depression  and 
form  the  labia  majora.     By  their  anterior  union  they  give  rise  to  the  mons 

genital  tubercle 

outer  fold 

genital  fold  (inner) 

urogenit.  dep. 

raphe 
anal  depression 

Fig.  415. — Diagram  showing  the  terms  usually  applied  to  the  External  Genital 
Parts  of  the  Embryo.  The  outer  genital  represent  the  cloacal  folds  ;  the  inner 
genital  folds  the  anterior  parts  of  the  vestibular  folds  ;  the  urogenital  depression 
or  cleft,  the  primary  meatus  (see  Fig.  411). 

veneris.  Their  posterior  extremities  fade  away  posteriorly  (Fig.  411). 
After  the  3rd  month  the  external  genital  parts  undergo  a  change  directly 
opposite  to  that  which  takes  place  in  the  male.  The  primary  meatus  and 
penile  urethra  open  up  and  form  the  vestibule,  into  which  open  urethra 
and  vagina.  This  change  is  probably  due  to  the  influence  of  the  ovarian 
germinal  tissue. 

External  Genitals  o£  the  Male. — In  the  male,  at  the  end  of  the  2nd 

month,  the  inner  genital  folds  corresponding  to  the  fourchette  and  labia 
minora,  grow  forwards  as  a  crescentic  shelf,  thus  closing  the  urogenital 
cleft  and  forming  the  floor  of  the  penile  urethra.  While  the  floor  of  the 
penile  urethra  is  formed  thus,  its  roof,  corresponding  to  the  vestibule  of 
the  female,  is  derived  from  the  urethral  or  forward  prolongation  of  the 
cloaca  (see  Fig.  407).  The  erectile  tissue  in  the  inner  genital  folds,  which 
forms  the  bulbs  of  the  vestibule  in  the  female,  forms  the  corpus  spongiosum 
in  the  male.  The  corpora  cavernosa  are  formed  m  the  genital  eminence. 
The  anterior  part  of  the  corpus  spongiosum  is  formed  separately  in  the 
apical  part  (glans)  of  the  genital  eminence.  The  corpora  cavernosa  are 
developed  by  the  enlargement  of  capillary  vessels  of  the  body  of  the  genital 
eminence  during  the  4th  month.     The  part  of  the  urethra  within  the  glans 


UROGENITAL  SYSTEM 


393 


is  the  last  j)art  to  be  formed,  and  its  development,  as  we  have  seen,  is 
peculiar  (p.  389).  The  part  of  the  urethra  within  the  glans  becomes  canal- 
iculized  a  short  time  before  birth.  TI13  fossa  navicularis  and  lacuna  magna 
occur  at  the  junction  of  the  part  of  the  urethra  formed  in  the  glans  and  the 
part  formed  from  the  urogenital  sinus. ^ 

The  scrotum  is  formed  during  the  3rd  month  by  the  union  of  the  external 
genital  folds  (labia  majora  of  the  female),  the  raphe  formed  by  their  union 


Ml  it  in  cord 


umchusiallantois) 


bladder  (cloaca) 


ingrowth  of  epiblast 


I  upper  prostatic 
[from  cloaca 

ejac.  duct 
(from  Wolffian  duct) 


[  lower  prostatic  from 
I  urogen.  sinus 


'memb.  from  urogen  sinus 

from  genital  furrow  and 
urogenit  sinus 


Fig.  416. — A  Section  of  the  Male  Bladder  and  Urethra  at  Birth,  showing  the  deriva- 
tion of  parts. 

extending  from  the  fraenum  of  the  prepuce  in  front  to  the  anterior  margin 
of  the  anus  behind  (see  p.  388). 


THE  PROSTATE. 

The  prostate  ^  is  developed  by  outgrowths  of  the  entodermal  lining 
the  upper  part  of  the  urogenital  sinus  and  from  the  mesodermal  tissue 
surrounding  the  sinus.     It  consists  of  glandular  tissue  and  stroma. 

(1)  The  glandular  tissue  is  composed  of  tubular  glands  which  open  into 
the  prostatic  part  of  the  urethra.  They  are  developed  in  the  4th  month, 
as  a  series  of  solid  buds,  about  60  in  number,  from  the  epithelium  lining 

^  For  literature  on  development  of  urethra  see  A.  Lichtenberg,  Anat.  Hefte,  1906, 
vol.  31,  p.  63  ;   J.  E.  Spicer,  Journ.  Anat.  mid  Physiol.  1909,  vol.  43,  p.  195. 

^  For  papers  on  the  development  of  the  prostate  see  E.  J.  Evatt,  Journ,  Anat,  and 
Physiol.  1909,  vol.  43,  p,  314  ;  1911,  vol,  45,  p.  122. 


394 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


bladder 


.ureter. 

Wolffian  duct 
'ateral  lobe  prostate 

rectum 


the  upper  part  of  the  urogenital  sinus  (Fig.  417).  The  buds,  which  soon 
become  tubular  in  form,  arise  from  a  right  and  left  longitudinal  furrow  or 
fold  of  the  wall  of  the  sinus  between  which  the  Wolffian  ducts  open  (Pallin). 

The  prostatic  furrows  reach  up- 
wards above  the  Wolffian  openings 
into  the  purely  urinary  part  of  the 
sinus  and  downwards  into  the  part 
which  serves  as  a  common  passage 
for  the  semen  and  urine.  These 
segments  of  the  sinus  become  the 
upper  and  lower  parts  of  the  pros- 
tatic urethra.  The  buds  from  the 
right  and  left  furrows  form  two 
lateral  masses  or  lobes.  At  first 
the  two  lateral  lobes,  as  in  mam- 
mals generally,  He  separately 
behind  the  urethra.  Other  out- 
growths also  arise  from  the  anterior  or  pubic  side  of  the  sinus — some  of 
these  afterwards  undergo  atrophy — from  the  side  or  lateral  aspect  of  the 
sinus  (Fig.  418).  The  lateral  prostatic  masses  fuse  behind  the  urethra  ; 
in  man  only  do  they  meet  to  form  an  anterior  or  pubic  commissure  over 


genit.  tuber.- 

perineal,  dep. 


Fig.  417. — A  Diagram  to  show  the  position  at 
which  the  Prostatic  Tubules  arise. 


PHALLIC     PART 


COWPERS     GLAND 


Fig.  418. — The  Prostate  and  Urethra  towards  the  end  of  the  4th  month.  (After 
Broman  and  Evatt.)  The  phallic  part  of  the  urethra  ends  posteriorly  at  the 
lacuna  magna.  It  is  developed  in  the  glans.  The  uterus  masculinus  (ut.  mas.) 
is  indicated  diagrammatically  to  show  its  relationship  to  the  common  ejaculatory 
duct. 

it.  The  tubules  of  the  median  or  third  part  arise  from  the  middle  line  of 
the  floor  of  the  sinus  above  the  openings  of  the  Wolffian  ducts  (ejaculatory 
ducts)  (Evatt),  but  the  lateral  lobes  also  fuse  with  this  median  element, 
and  help  to  form  it.     It  may  be  small  or  even  absent.^ 

1  J.  W.  Thomson  Walker,  Journ.  Anat.  and  Physiol.  1906,  vol.  40,  p.   189  ;   0.  S, 
Lowsley,  Amer.  Journ.  Anat.  1912,  vol.  13,  p.  299. 


UROGENITAL  SYSTEM 


395 


Skene's  tubules,  which  may  be  found  opening  into  the  urethra  of  the 
female,  represent  prostatic  tubules.  A  reference  to  Figs.  408,  409  will 
show  that  the  female  urethra  corresponds  to  the  upper  j^rostatic  urethra 
of  the  male,  and  that  the  floor  of  the  vestibule — in  which  rudiments  of 
prostatic  tubules  may  be  formed — represents  the  lower  prostatic  urethra. 

(2)  The  Stroma  of  the  Prostate. — While  the  glandular  tubes  arise  in 
linear  groups  from  the  epithelium  lining  the  urogenital  sinus — the  muscular 
and  fibrous  elements  arise  from  the  mesodermal  tissue  of  the  genital  cord 
in  which  the  terminal  parts  of  the  Wolffian  and  Miillerian  ducts  are  situated 
and  from  the  circular  musculature  of  the  urogenital  sinus  (see  Fig.  419). 
When  the  glandular  elements  grow  out  they  become  embedded  in  and  carry 


coat 
of  bladder 

■rectum 
-prost  tubules 
circ.  coat 


rectal  band, 
memb.  urethra 


Fig.  419. — Diagrammatic  Section  of  the  Bladder  and  Urethra  of  a  6th  month  Foetus 
to  show  (1)  the  development  of  the  Prostate,  (2)  the  relationship  of  the  Prostatic 
Musculature  to  that  of  the  Uretlira  and  Bladder.     (Wood  Jones.) 

before  them  the  circular  musculature  of  the  urogenital  sinus  which  thus 
forms  the  muscular  cortex  or  inner  capsule  of  the  prostate.  Probably 
the  stroma  of  the  genital  cord  also  contributes  to  the  musculature  of  the 
prostate.  The  musculature  of  the  uterus,  which  is  also  developed  from 
the  genital  cord,  like  that  of  the  prostate,  is  liable  to  become  the  seat  of 
fibromyomatous  growths  in  the  later  years  of  adult  life. 
As  regards  the  nature  of  the  prostate  : 

(1)  It  is  purely  genital,  and  develops  only  in  the  rutting  season  in  such 
mammals  as  manifest  a  seasonal  sexual  life.  Its  development  in  the 
female  is  arrested  at  a  very  early  stage — probably  the  result  of  an  ovarian 
influence. 

(2)  It  remains  comparatively  undeveloped  until  puberty.  At  the  age 
of  seven  it  weighs  only  30  grains  ;  after  sexual  life  is  established  it  weighs 
about  300  grains. 


396      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

(3)  The  healthy  prostate  atrophies  if  castration  is  performed,  but  this 
operation  has  no  efiect  on  glands  which  have  become  pathologically 
hypertrophied.i  In  one  man  out  of  three  over  55  years  of  age  the  prostate 
hypertrophies,  both  the  glandular  and  fibro-muscular  elements  partici- 
pating. Hypertrophy  of  the  median  part  causes  a  valvular  elevation 
behind  the  vesical  opening  of  the  urethra. 

Glands  oS  Cowper  and  Bartholin  are  produced  as  solid  buds  from  the 
entodermal  lining  of  the  penile  extension  of  the  urogenital  sinus  (Fig.  418). 
Hence  in  the  female  the  ducts  of  Bartholin's  glands  open  in  the  vulval  cleft 
just  outside  the  hymen  at  each  side  of  the  vagina,  for  the  hymen  marks 
the  junction  of  the  Miillerian  ducts  with  the  urogenital  sinus.  In  the  male 
the  ducts  of  Cowper's  glands  open  in  the  bulbous  part  of  the  urethra 
(Fig.  418).  Their  function  is  unknown,  but  they  are  certainly  sexual  in 
nature.  The  numerous  glands  of  Littre,  like  Cowper's  and  Bartholin's 
glands,  are  produced  by  tubular  outgrowths  during  the  fourth  month 
(Fig.  418).  In  the  male  the  glands  of  Littre  are  produced  most  numerously 
along  the  dorsal  aspect  of  the  urethra. 

Round  the  anus,  and  especially  in  the  furrow  between  the  labium  minus 
and  majus,  groups  of  large  peculiar  sebaceous  glands  are  produced  in  the 
4th  and  5th  months,  corresponding  to  the  anal  and  preputial  glands  of 
mammals.  Occasionally  two  groups  of  sebaceous  glands  occur  on  the 
prepuce  of  the  male,  especially  if  hypospadias  be  present  (Shillitoe). 
Glands  round  the  corona  of  the  glans  penis  (Tyson's  glands)  are  only  very 
exceptionally  present. 

THE  TESTES. 

Descent  and  Development  of  the  Testicle.^ — The  origin  of  the  testis 
on  the  inner  or  mesial  side  of  the  Wolffian  ridge,  and  its  attachment  to  the 
dorsal  wall  of  the  coelom  by  a  mesentery  common  to  it  and  the  Wolffian 
body  have  been  already  described  (see  Figs.  4,  23,  384).  The  position  of 
the  testicle  in  a  foetus  of  the  third  month  is  shown  in  Fig.  420.  Although 
m  the  6th  week  the  genital  ridge  extended  from  the  6th  to  the  12th  thoracic 
segment,  the  testicle,  developed  from  the  hinder  part  of  the  ridge,  is  now 
situated  in  the  iliac  fossa.  The  mesorchium,  a  fold  of  peritoneum,  binds 
its  attached  border  to  the  iliac  fossa.  At  its  outer  side  lies  the  genital 
part  of  the  Wolffian  body  which  forms  the  epididymis.  It,  also,  is  sus- 
pended by  a  mesentery — -the  Wolffian  mesentery.  The  two  mesenteries 
have  a  common  base — the  common  urogenital  mesentery  (see  Fig.  384). 
The  upper  part  of  the  urogenital  mesentery  forms  the  diaphragmatic  fold, 
with  which  the  peritoneal  fold  containing  the  spermatic  artery  becomes 
joined  ;  to  the  combined  fold  is  given  the  name  of  plica  vascularis  (compare 

1  E.  Pittard,  Convpt.  Rend.  Acad.  Sc.  1911,  vol.  152,  p.  1617  (Effect  of  Castration)  ; 
Cuthbert  S.  Wallace,  Trans.  Path.  Soc.  Lond.  190^r  vol.  56,  p.  80  ;  W.  L.  H.  Duck- 
worth, Journ.  Anat.  and  Physiol.  1906,  vol.  41,  p.  30  (Eunuchoid  Man)  ;  R.  H.  White- 
head, Anat.  Bee.  1908,  vol.  2,  p.  177,  A7ner.  Journ.  Anat.  1905,  vol.  4,  p.  193  (Dev.  and 
Nature  of  Interstitial  Cells). 

2  See  Eben  C.  Hill,  Amer.  Journ.  Anat.  1907,  vol.  6,  p.  439  (Dev.  of  Blood  Supply)  ; 
D,  T.  Barry,  Journ.  Anat.  and  Physiol.  1910,  vol.  44,  p.  137  (Differentiation  of  Tubules), 


UROGENITAL  SYSTEM 


397 


Figs.  421,  424).  This  in  the  female  becomes  the  ovario-pelvic  ligament 
(Fig.  5).  A  fold  of  peritoneum,  the  inguinal  fold  or  plica  gubernatrix, 
continues  the  common  urogenital  mesentery  to  the  groin  (Fig.  420).  The 
gubernaculum  testis  is  developed  in  the  plica  gubernatrix  ;  in  the  corre- 
sponding fold  in  the  female  the  round  ligament  of  the  uterus  appears 
(see  p.  371).  The  vas  deferens  (Wolffian  duct)  turns  into  the  pelvis  from 
the  lower  end  of  the  epididymis  (Wolffian  body),  and  within  the  pelvis  lies 
in  the  genital  cord  (Fig.  392).  A  remnant  of  the  Miillerian  duct  lies  along 
the  inner  and  ventral  aspect  of  the  epididymis. 

Seminiferous  Tubules. — The  arteries  for  the  genital  glands  represent 
the  lowest  of  the  vessels  which  originally  supply  the  Wolffian  body  and 
arise  from  the  aorta  at  the  level  of  the  12th  dorsal  vertebra  ;  their  nerve 
supply  comes  from  the  10th  dorsal  segment  of  the  spinal  cord.  The 
testis  is  therefore  developed  in  the  genital  ridge  between  the  10th  and  12th 


rectum 


plica  uascularis 


epididymis 
testicle 
inguinal  fold. 

uas.  deferens. 


bladder 

Fig.  420. — The  Position  of  the  Testis  in  a  Foetus  of  2i  months. 

dorsal  segments.  The  early  development  of  the  testis  is  similar  to  that 
of  the  ovary.  Up  to  the  7th  week,  when  the  embryo  measures  15  mm. 
in  length,  it  is  impossible  to  tell  testicle  from  ovary  ;  both  of  them  at  this 
time  show  a  covering  of  germinal  epithelium  and  deep  central  masses  or 
columns  of  epithelioid  cells  derived  from  the  covering  layer  of  germinal 
epithelium.  In  the  central  masses  are  the  large  primitive  germinal  cells 
(primordial  ova).  At  the  end  of  the  7th  week  two  changes  lead  to  the 
difierentiation  of  a  testis  from  an  ovary  ;  (1)  a  tunica  albuginea  begins  to 
form  under  the  superficial  epithelium,  (2)  the  central  masses  proliferate 
and  form  radiating  cords  which  branch  and  anastomose  as  they  spread 
from  hiluni  to  periphery.  The  cords  become  transformed  into  the  semin- 
iferous tubules  which  are  at  first  solid.  Some  of  the  epithelioid  cells  are 
not  included  in  the  tubes  and  remain  to  form  interstitial  cells.'^  The 
genitaloid  cells  are  included  in  the  epithelial  cords.  The  tubules  become 
separated  into  groups  or  compartments  in  the  6th  month  and  about  the 

1  See  articles  by  Sir  F.  W.  Mott,  Brit.  Med.  Journ.  1919,  vol.  2,  p.  655  ;  T.  Russell 
Godclaid,  Journ.  Aval.  1920,  vol.  54,  p.  173  ;  B.  F.  Kingsbury,  Amer.  Journ.  Anat.  1914, 
vol,  16,  p.  59. 


398 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


same  time  lumina  are  formed  in  them.  The  formation  of  spermatozoa 
has  been  already  described  (p.  9).  The  visceral  layer  of  the  tunica 
vaginalis  on  the  testicle  represents  the  covering  of  flattened  epithelium 
which  remains  after  the  ingrowth  of  the  germinal  epithelium.  The  vasa 
efferentia  and  coni  vasculosi  are  formed  from  the  genital  Wolffian  tubules  ; 
the  rete  testes  and  vasa  recti  from  the  junctional  cords  (p.  361).  Into  the 
rete  open  the  tubuli  seminiferi  formed  in  the  testicle.  The  epididymis 
is  the  elongated  upper  segment  of  the  Wolffian  duct  (Fig.  386).  The 
Wolffian  elements  are  produced  within  the  Wolffian  ridge  (Fig.  390). 

Formation  of  the  Gubernaculum  Testis.^ — There  is  no  trace  of  the 
inguinal  canal  in  the  3rd  month  of  foetal  life  ;  the  various  layers  of  ab- 
dominal wall  are  unbroken,  except  for  a  fine  strand  of  tissue  which  leads 


plica  uascularis 
testis 


uas.  deferens, 
deep  epigastric 


umbilical  artery 
bladder 

gubernaculum 


Fig.  421 


gubernaculum 
scrotum. 


-Showing  the  Position  of  the  Testis  at  the  6th  month,  and  the  Formation 
of  the  Gubernaculum  Testis. 


towards  the  site  of  the  scrotum,  and  evidently  serves  as  a  guide  for  the 
gubernacular  outgrowth.  In  the  fourth  month  the  subperitoneal  layer 
of  non-striated  muscular  tissue  in  the  plica  gubernatrix  and  in  the  mesor- 
chium  takes  on  a  rapid  growth  (Fig.  421).  At  the  same  time  the  tissues  of 
the  abdominal  wall  undergo  a  localized  evagination  towards  the  scrotum. 
They  are  probably  carried  down  by  the  growth  of  the  gubernacular  bud 
which  pushes  its  way  to  the  scrotum.  The  gubernaculum  grows  downwards 
as  a  solid  cellular  mass,  until  it  reaches  the  subcutaneous  tissue  which  at 
that  time  completely  fills  the  scrotum.  Its  attachment  to  the  scrotum 
is  sHght  and  easily  broken  (Fig.  421).  The  gubernaculum,  as  it  grows 
through  the  abdominal  wall,  carries  with  it  : 

(1)  A  process  of  peritoneum  (the  processus  vaginalis)  ;  (2)  The  trans- 
versalis  fascia  (the  infundibuliform  fascia)  ;  (3)  The  internal  oblique  and 
transversalis  muscles  to  form  the  cremaster  ;    (4)  The  spermatic  fascia 

1  See  John  Hunter's  classical  account,  Palmer's  Edition  of  his  Works,  vol.  4,  1837. 
Also  paper  by  Dr.  Berry  Hart,  Trans.  Edin.  Obstet.  Soc.  1909,  vol.  34,  p.  151. 


UROGENITAL  SYSTEM 


399 


from  the  external  oblique  ;  (5)  The  deep  layer  (Scarpa's)  of  the  superficial 
fascia  of  the  groin.  All  these  layers  are  added  to  the  primitive  coverings 
of  the  scrotum,  which  until  then  is  made  up  simply  of  skin  and  superficial 
fascia  (Fig.  421). 

It  will  thus  be  seen  that  the  gubernaculum  testis  is  a  fibro-muscular 
mass  with  an  actively  growing  cellular  cap,  which,  starting  from  the 
muscular  stratum  in  the  mesorchium  and  plica  gubernatrix  in  the  iliac 
fossa,  invades  the  abdominal  wall,  every  layer  of  which  it  carries  as  a  pro- 
longation within  the  scrotum.  It  is  an  invading  army  of  cells.  It  draws 
with  it  into  the  scrotum  the  peritoneum  in  the  iliac  fossa,  on  which  the 
testis  is  dragged  like  a  log  on  a  sledge. 

The  testis  descends  from  the  loins  to  the  iliac  fossa  in  the  3rd  month  ; 
from  the  4th  to  the  7th  month  it  rests  at  the  site  of  the  internal  ring  ; 


peritoneum 


plica  uasculan's. 

testis 

Pouparts  lig.  — '^ 
gubernaculum 


internal  ob. 
peritoneum 

cremaster 

penis 
processus,  uag. 

scrotum 


Fig.  422. — The  manner  in  which  the  Structures  in  the  Wall  of  the  Abdomen  are 
carried  out  so  as  to  form  the  Inguinal  Canal  and  Coverings  of  the  Testis. 

it  spends  the  7th  month  of  foetal  life  in  its  exodus  through  the  abdominal 
wall.  In  the  8th  month  it  leaves  the  inguinal  canal  and  lies  at  the  external 
abdominal  ring.  After  birth  it  reaches  the  fundus  of  the  scrotum.  The 
atrophy  and  contraction  of  the  gubernaculum  pull  it  down.  A  remnant 
of  the  gubernaculum  can  always  be  found  in  the  adult  behind  the  epididy- 
mis and  testicle,  within  the  mesorchium  (Fig.  423). 

Processus  Vaginalis. — The  processus  vaginalis  becomes  occluded  by 
adhesion  or  zygosis  (p.  287)  at  two  points  soon  after  birth,  but  in  a  con- 
siderable proportion  of  individuals  the  process  of  closure  is  delayed  (Fig. 
423).  The  upper  point  of  occlusion  takes  place  at  the  internal  abdominal 
ring  ;  the  lower  a  short  distance  above  the  testicle.  The  part  of  the 
processus  vaginalis  between  the  points  of  occlusion  is  known  as  the  funicular 
process  ;  the  part  surrounding  the  testicle  becomes  the  tunica  vaginalis. 
In  quite  30  %  of  children  the  occlusion  takes  place  at  the  internal  abdominal 
ring  some  considerable  time  after  birth  or  it  fails  to  appear  altogether. 
Occlusion  may  fail  at  the  upper  pointy  at  the  lower  point,  or  at  both.  Or 
it  may  close  at  both  points,  but  the  funicular  process,  instead  of  disappear- 
ing, may  remain  open  and  form  a  cyst. 


400 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


Descent  of  the  testicle  ^  may  be  arrested  at  any  stage  ;  often  in  the 
inguinal  canal ;  more  frequently  at  the  external  abdominal  ring.  Arrest 
of  descent  is  commonly  regarded  as  a  symptom  of  arrest  of  testicular  de- 
velopment. John  Hunter  regarded  arrested  descent  of  the  testicle  as  due 
to  an  imperfection  in  its  development ;  all  recent  observations  support 
his  opinion.  There  can  be  no  doubt  that  in  all  those  mammals  ^  in  which 
the  testis  leaves  the  abdomen  it  does  so  to  escape  the  intra-abdominal 
pressure  to  which  the  abdominal  viscera  are  subjected.  Its  descent  is 
correlated  with  the  evolution  of  the  diaphragm  and  exclusion  of  the  lungs 
from  the  abdominal  cavity.  Violent  respiration  and  flexure  of  the  trunk 
give  rise  to  very  high  degrees  of  tension  within  the  abdomen  ;  from  some 
cause  at  present  not  understood,  a  testicle  atrophies  when  subjected  to 

r  int.  ab.  ring  {upper  point 
lofocc/us.) 

ext.  ab.  ring 
■funicular  process 


lower    point  of 
occlusion 


tunica  vaginalis 


remnant  of  gubernac. 


Fig.  423. — ^A  Diagram  of  the  Processus  Vaginalis. 

this  pressure.  On  the  other  hand,  the  testicle  may  assume  an  ectopic 
position.  The  gubernaculum,  as  it  makes  its  way  towards  the  scrotum, 
may  take  an  eccentric  course,  and  bring  the  testicle  to  rest  in  the  groin, 
root  of  the  penis,  or  over  the  pubis. 

Mesorchium. — The  testis  and  epididymis  were  suspended  within  the 
abdomen  by  the  common  urogenital  mesentery  (Fig.  384).  In  the  course, 
of  the  descent  of  the  testis  this  becomes  shortened  by  the  development  of 
the  gubernaculum,  and  the  testis  and  epididymis  become  thus  firmly  bound 
by  their  posterior  borders  to  the  tunica  vaginalis.  The  digital  fossa, 
situated  between  the  mesorchium  and  mesentery  of  the  Wolffian  body, 
represents  the  recess  which  separated  the  genital  from  the  Wolffian  ridge 
of  the  embryo.  The  mesorchium — the  true  mesentery  of  the  testicle — 
may  assume  the  form  of  an  elongated  fold,  attaching  the  testicle  to  the 
epididymis  (Corner). 

^  D.  Berry  Hart,  Journ.  Anat.  and  Physiol.  1910,  vol.  44,  p.  4. 

2  See  W.  N.  F.  Woodland,  Proc.  Zool.  Soc.  London,  1903,  vol.  1,  p.  319. 


UROGENITAL  SYSTEM 


401 


A  not  unusual  anomaly  of  the  testicle  is  represented  in  Fig.  425.  It 
will  be  observed  that  the  common  mesentery,  in  place  of  becoming  short- 
ened, and  thus  fixing  the  testicle  and  epididymis  widely  to  the  peritoneum, 
becomes  narrow  and  elongated.  Such  testicles  are  usually  arrested  in 
their  descent,  and  are  apt  to  twist  and  become  strangulated.  It  will  also 
be  observed  that  a  gubernaculum  is  present,  but  it  has  seized  and  drawn 
downwards  only  a  loop  of  the  vas  deferens.  The  explanation  is  shown  in 
Fig.  424.  The  inguinal  fold  is  made  up  of  two  parts,  a  lower,  ending  on 
the  vas  deferens  and  corresponding  to  the  round  ligament  of  the  female  ; 
an  upper,  which  continues  the  fold  to  the  epididymis  and  testicle,  and 


m 

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Fig.  424. — To  show  the  Diaphragmatic  Fold  (upper  part  of  the  common  genital 
mesentery),  Vascular  Fold,  and  the  two  parts  of  the  Inguinal  Fold  in  a  Foetal 
Pig.     (Eben.  C.  Hill.) 

Fig.  425.— Elongated  Common  Mesentery  of  a  Testicle  arrested  in  the  course  of  its 
descent. 

which  corresponds  to  the  round  ligament  of  the  ovary.  In  such  cases, 
then,  the  gubernaculum  has  not  extended  to  the  upper  part  of  the  inguinal 
fold. 

Hermaphrodites.^ — A  hermaphrodite — a  human  individual  in  which 
both  testis  and  ovary  are  present — has  never  been  seen.  Dr.  Bulloch 
found  only  five  cases  on  record  in  which,  within  the  same  genital  gland, 
there  were  present  representations  of  imperfect  testicular  and  ovarian 
tissues  (ovario-testis)  ;  spermatozoa  and  ova  were  not  present.  The 
term  is  usually  applied  to  individuals  in  whom  the  genital  glands  are  im- 
perfectly developed.  Usually  they  are  imperfect  males.  It  is  clear  that 
sexual  differentiation  commences  in  the  7th  week  (although  the' sex  is 
probably  determined  at,  or  even  before  the  time  of  fertilization)  ;   by  some 

^  See  Bulloch,  Treasury  of  Human  hiheritance,  London,  1909,  Part  3,  Section  Xa  ; 
Berry  Hart,  Proc.  Roy.  Soc.  Edin.  1909,  vol.  29,  p.  607,  1910,  vol.  30,  p.  230  ;  J.  F. 
Gudernatsch,  Amer.  Journ.  Anat.  1911,  vol.  11,  p.  267  ;  F.  R.  Lillie,  Journ.  Experim. 
Zool.  1917,  vol.  23,  p.  371. 


402 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


means^probably  by  an  internal  secretion — the  ovarian  and  testicular 
tissues  exercise  a  direct  and  opposite  influence  on  tbe  development  of 
genital  structures.  Hence,  if  the  gland  is  imperfect,  development  of  tbe 
genital  parts  is  uncontrolled  and  represents  a  condition  whicli  may  best 
be  described  as  neutral.  There  is  evidence  to  support  the  opinion  that  the 
embryonic  genital  gland  is  composite  ;  the  testicle  develops  within  the 
medullary  or  central  part  of  the  gland  ;  the  ovary  from  its  cortical  or  more 
superficial  parts. 

Development  o£  the  Suprarenal  Bodies. — The  suprarenal  or  adrenal 
bodies  arise  by  the  association  of  two  distinct  embryological  elements 
• — cortical  and  medullary.  In  Fig.  427  is  given  a  diagrammatic  section 
to  show  the  stage  reached  in  the  7th  week  of  development,  the  cortical 
element  then  being  large  and  projecting  at  the  root  of  the  mesentery  and 
in  contact  with  the  genital  and  Wolffian  bodies,  while  the  medullary 


WOLFFIAN    BODY 

GANGLION 


MESO-NFPHRIC 
ARTERIES 


INF  S^ESENT 


GENITAL  GL 

CORTEX 


MESENTERY 


Fig.  426. — Showing  the  distribution  of  the  aortic  chromaffin  bodies  in  the  early 

human  foetus.     (After  Zuckerkandl.) 

Fig.  427. — Section  across  the  suprarenal,  genital  and  Wolffian  bodies  in  an  embryo 

in  the  7th  week,  15  mm.  long.     (After  Zuckerkandl.) 


element  consists  of  indifferent  cells  connected  with  the  blastema  of  the 
sympathetic  system.  The  cortical  element  is  associated  with  the  genital 
system  ;  its  secretion  regulates  the  development  of  certain  sexual  structures 
and  functions.  Like  the  epithelioid  cells  of  the  genital  glands,  the  cortex 
arises  by  an  invagination  of  coelomic  epithelium,  the  suprarenal  ingrowth 
occurring  on  each  side  of  the  root  of  the  mesentery  early  in  the  6th  week. 
The  suprarenals  are  developed  within  the  anterior  'ends  of  the  Wolffian 
bodies,  just  behind  the  pleuro-peritoneal  passages.  The  medulla  arises 
from  groups  of  cells  which  also  form  sympathetic  ganglia  ;  probably 
from  the  primitive  cell  basis  of  the  semilunar  ganglion,  which  is  developed 
by  the  side  of  the  aorta,  close  to  the  pleuro-peritoneal  opening.  Hence 
the  great  plexus  of  nerves  which  passes  from  the  solar  plexus  to  the  medulla 
of  the  suprarenals.  The  medullary  cells  begin  to  migrate  into  the  cortex 
in  the  8th  week  ;  the  process  of  invasion  is  continued  through  the  greater 
part  of  foetal  life.  The  invading  cells,  when  stationed  in  the  cortex, 
give  rise  to  broods  of  chromogenic  cells  and  later  to  sympathetic  nerve  cells. 


UROGENITAL  SYSTEM  403 

By  the  beginning  of  the  4th  month  they  are  arranged  aS  reticulating 
columns  set  on  the  walls  of  branching  venous  sinuses. 

The  cortical  cells  range  themselves  in  rows  between  radiating  blood 
sinuses.  As  the  kidneys  ascend  in  the  3rd  month  they  come  in  contact 
with  the  suprarenal  bodies.  The  suprarenal  is  at  first  larger  than  the 
kidney,  even  at  birth  they  are  nearly  equal  in  size.  The  nerves  and 
arteries  enter  the  bodies  on  their  renal  surface  ;  the  veins  emerge  on  their 
anterior  surface. 

Until  the  3rd  month  the  suprarenal  bodies  are  in  contact  with  the  upper 
pole  of  the  testis  or  ovary.  As  the  genital  glands  descend,  the  diaphragmatic 
fold  is  drawn  from  the  suprarenal  region  and  frequently  carries  with  it 
buds  of  suprarenal  tissue  both  cortical  and  medullary.  It  is  therefore 
readily  understood  how  isolated  parts  of  the  suprarenal  body  (accessory 
suprarenals)  may  occur  in  the  broad  ligament  or  in  the  spermatic  cord 
above  the  testicle.  Such  accessory  bodies  are  probably  derived  from  the 
cortical  element  which  is  developed  within  the  Wolffian  ridge  and  body. 
With  the  descent  of  the  ovary  and  testicle,  which  bring  with  them  the 
Wolffian  body,  adjacent  accessory  suprarenals,  if  such  be  present,  are  also 
brought  down,  and  may  occasionally  give  rise  to  peculiar  tumours. 

Chromaffin  Cells.^ — The  medullary  part  of  the  suprarenals  belongs  to 
a  segmental  series  of  organs.  In  such  fishes  as  the  shark  and  lamprey, 
a  group  of  cells  (a  paraganglion)  is  thrown  off  from  each  ganglion  of  the 
sympathetic  chain  and  comes  into  close  contact  with  the  tributaries  of 
the  cardinal  veins.  These  cells  stain  brown  with  salts  of  chromium — 
hence  their  name  ;  some  of  these  cells  remain  within  the  sympathetic 
ganglia.  Similar  minute  chromaffin  bodies  (paraganglia)  are  also  developed 
in  or  near  all  the  ganglia  of  the  vertebral  chain  of  the  human  foetus.  The 
carotid  body  arises  in  association  with  the  upper  cervical  ganglion.  Other 
collections  of  chromaffin  cells  arise  at  the  sites  of  the  prevertebral  ganglia 
and  plexuses — such  as  the  superior  and  inferior  mesenteric  plexuses. 
The  distribution  of  the  aortic  chromaffin  bodies  is  shown  in  Fig.  426. 
Although  chromaffin  cells  arise  from  the  blastema  of  the  sympathetic 
system  yet  they  are  differentiated  before  the  nerve  cells  of  that  system, 
as  if  they  represented  the  products  of  an  earlier  evolution.  By  their  secretion 
they  assist,  or  serve  as  substitutes  for,  the  vasomotor  sympathetic  cells 
and  for  all  nerve  cells  which  have  to  do  with  regulating  the  action  of  non- 
striated  muscle.  The  medulla  of  the  suprarenal  represents  the  brain 
of  the  chromaffin  system,  but  why  it  should  be  associated  with  a  cortical 
element  has  not  yet  received  an  explanation. 

Coccygeal  Body  ^  is  a  small  mass  of  chromaffin  tissue,  with  rich  blood 
supply,  situated  on  the  ventral  aspect  of  the  coccyx. 

^  For  an  account  of  chromaffin  tissue  see  an  article  by  Swale  Vincent,  Journ.  Anat. 
and  Physiol.  1904,  vol.  38,  p.  34  ;  E.  Zuckerkandl,  Keibd  and  MaWs  Manual  of 
Human  Embryology,  vol.  2,  1912. 

2  J.  Thomson  Walker,  ArcMv.  fiir  3Iil:  Anat.  und  Enlwkld.  1904,  vol.  04,  p.  121 
(Coccygeal  Body). 


CHAPTER  XXV. 

BODY   WALL  AND   PELVIC   FLOOE. 

Stages  in  the  Evolution  of  the  Body  Wall.^ — Behind  the  apparently 
simple  arrangement  of  structures  in  the  body  wall  of  man  lies  a  long 
history,  only  some  of  the  later  stages  being  known  to  us.  Even  in  the 
lowest  vertebrates  the  wall  surrounding  the  pericardial  and  abdominal 
cavities  is  already  muscular.  We  presume,  however,  there  was  a  stage 
in  which  they  were  devoid  of  muscle,  for  in  all  vertebrates  the  musculature 
which  enters  the  somatopleure,  the  lamina  which  forms  the  body  wall 
of  the  embryo,  arises  from  the  muscle  plates  of  the  somites  placed  along 
each  side  of  the  dorsal  median  axis  of  the  embryo  (see  p.  68).  In  fishes 
the  musculature  of  each  side  of  the  body  wall  is  arranged  in  two  systems  : 

(1)  a  vertebral,  lateral  or  oblique  system  in  which  the  ribs  are  embedded  ; 

(2)  a  ventral  or  longitudinal  system  which  extends  from  pharynx  to  tail. 
Both  longitudinal  and  oblique  systems  are  differentiated  from  one  stratum. 
It  is  from  a  simple  system  of  this  nature  that  the  musculature  of  the 
human  body  wall  has  been  evolved  (see  Fig.  445). 

Respiratory  Stage. — With  the  evolution  of  lungs  the  musculature  of 
the  body  wall  assumed  a  respiratory  function.^  In  fishes  its  chief  use — 
if  one  excepts  the  part  it  plays  in  body  movements — is  to  assist  in  the 
circulation  of  the  blood  within  the  body  cavity — to  drive  it  on  towards 
the  heart,  and  to  expand  or  contract  the  cavity  as  the  alimentary  canal 
fills  or  empties.  By  means  of  ribs  embedded  in  the  septa  of  the  lateral 
wall,  the  musculature  of  the  body  cavity  became  capable  not  only  of 
compressing  or  diminishing  the  body  cavity,  but  also  of  expanding  it, 
and  thus  filling  the  lungs  with  air.  In  this  manner  the  body  musculature 
entered  into  the  service  of  the  lungs,  and  the  nerve  centres  (respiratory 
centres)  in  the  hind-bfain,  which  formerly  regulated  the  movements  of 
the  gills  and  pharynx,  came  to  have  an  automatic  dominion  over  muscu- 
lature of  the  body  wall.  The  ribs,  which  served  in  the  simple  economy 
of  the  fish's  body,  became  strengthened  and  firmly  jointed  to  the  vertebrae  ; 
at  the  ventral  ends  of  those  encircling  the  lungs  a  supporting  bar — the 
sternum — was  evolved  ;  the  primitive  sheets  of  musculature  became 
difierentiated  to  act  on  the  ribs.     In  the  latter  part  of  the  2nd  month 

1  See  R.  H.  Paramore's  "  Hunterian  Lectures,"  Lancet,  1910,  May  21st  and  28th  ; 
Prof.  Wood  Jones,  Journ.  Anat.  1913,  vol.  47,  p.  282. 

2  F.  Tourneux,  Compt.  Rend.  Assoc.  Anat.  1902  (Dev.  of  Walls  of  Thorax). 

404 


BODY  WALL  AND  PELVIC  FLOOR  405 

when  the  lungs  and  pleural  cavities  are  undergoing  rapid  development, 
respiratory  transformations,  similar  in  nature  to  those  just  mentioned, 
are  taking  place  in  the  human  embryo. 

Mammalian  Stage. — We  have  already  seen  that  the  lungs  of  mammals 
develop  within  special  cavities,  which  ultimately  surround  the  heart ; 
as  the  pleural  cavities  expand  they  dislocate  from  the  neck  and  depress 
within  the  body  cavity  a  partition  which  completely  divides  it  into  thorax 
and  abdomen.  With  the  evolution  of  the  diaphragm,  and  the  disappear- 
ance of  the  lungs  from  the  abdominal  cavity,  the  body  wall  musculature 
became  further  modified,  so  that  it  can  control  the  thoracic  as  well  as  the 
abdominal  pressure.  The  evolution  of  pleural  cavities  effected  a  trans- 
formation in  the  thoracic  part  of  the  body  wall.  Their  expansion  and  the 
differentiation  of  the  thoracic  wall  are  taking  place  during  the  latter  part 
of  the  2nd  month  of  human  development. 

Orthograde  Stage. — It  -is  believed  by  many  that  the  upright  or  ortho- 
grade posture  is  confined  to  man,  and  that  it  represents  one  of  the  more 
recently  acquired  human  characters.  This  is  certainly  not  the  case  ; 
man  shares  the  orthograde  posture  with  the  group  of  primates  with  which 
he  has  so  many  structural  affinities — namely,  the  anthropoid  apes.  Like 
man,  they  carry  their  bodies  in  an  upright  posture  during  progression. 
The  smallest  and  most  primitive  of  the  anthropoid  apes — the  gibbon — 
is  of  ancient  origin  ;  the  orthograde  posture  is  therefore  an  adaptation 
which  has  been  long  established  in  the  higher  group  of  primates.  With  a 
change  of  posture  to  the  orthograde  the  action  and  fixation  of  the  muscu- 
lature of  the  body  wall  became  greatly  altered ;  the  mechanism  of 
respiration  was  necessarily  altered.  The  chest  became  wide  or  barrel- 
shaped,  the  sternum  broad  ;  the  heart  came  to  rest  on  the  diaphragm. 
The  muscles  of  the  abdominal  wall  had  not  only  to  carry  on  their  respiratory 
function  ;  they  had  also  to  support  the  abdominal  viscera  and  to  assist 
in  emptying  them.  The  mesenteric  adhesions  which  take  place  during 
the  early  months  of  foetal  life  (see  p.  286)  are  designed  to  give  additional 
fixation  to  the  viscera.  The  lower  abdominal  viscera  came  to  rest  on  the 
pelvic  floor  ;  the  muscles  of  the  tail,  which  rise  within  the  pelvis  of 
pronograde  mammals,  were  modified  to  form  a  muscular  hammock  for  the 
support  of  the  viscera  and  the  external  tail  disappeared.  The  caudal 
or  coccygeal  vertebrae  are  more  reduced  in  the  anthropoid  apes  than  even 
in  man.  The  spinal  musculature  and  spinal  column  were  altered  to  meet 
the  new  postural  conditions. 

Plantigrade  Stage. — If  man  shares ,  the  orthograde  posture  with  a 
group  of  higher  primates,  the  power  of  plantigrade  progression  is  peculiarly 
his  own.  Everyone  recognizes  that  the  foot,  the  leg,  the  thigh  of  man 
have  undergone  extensive  structural  alterations,  but  the  fact  is  often 
overlooked  that  the  process  of  adaptation  has  also  led  to  marked  structural 
changes  in  the  body  wall.  The  inguinal  region  especially  has  been  modified. 
The  great  development  and  complete  extension  of  the  thigh  have  altered 
the  musculature  of  the  groin  ;  the  inguinal  (Poupart's)  ligament  has  been 
evolved.     These  structural  adaptations  have  weakened  the  human  groin, 


406 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


and  made  it  tlie  commonest  site  of  hernia.  In  the  normal  human  upright 
posture  the  trunk  is  balanced  on  the  pelvis  ;  the  crest  of  the  ilium  and  the 
external  oblique  have  become  modified  for  this  purpose.  The  muscles 
of  the  abdominal  wall  not  only  support  the  abdominal  viscera,  and  inaintain 
them  during  their  respiratory  excursions,  but  also  take  a  part  in  producing 
and  regulating  the  movements  of  the -body.  Their  functional  value  is 
often  impaired  in  man,  and  hence  he  is  the^subject  of  those  forms  of  slipping 
_,or  dropping  of  the  viscera  which  are  grouped  under  the  name  of  viscerop- 
tosis,    He  is  liable  to  many  other  varieties  of  static  disablements. 

Inguinal  and  femoral  hernia  occur  so  rarely  amongst  mammals  generally 
that^they  may  be  considered  human  peculiarities.     Their  frequency  in 


sacrum 


sacrum 


symphysis. 


A, 


symphysis. 


Fig.  428,  A. — The  Form  of  Pelvis  and  Inguinal  Canal  in  Man. 

,    B. — The  corresponding  forms  in  Pronograde  Primates. 


man  is  due  to  certain  structural  changes  in  his  pubo-femoral  region,  changes 
which  have  resulted  mainly  from  his  adaptation  to  upright  progression. 
His  susceptibility  to  hernia  is  due  to  : 

(1)  The  unique  form  of  Poupart's  ligament  in  man.  It  is  scarcely 
developed  in  any  other  animal  (Fig.  429).  In  the  orang,  for  instance, 
also  an  upright  primate,  the  external  oblique  has  no  attachment  to  the 
crest  of  the  ilium,  and  takes  no  part  in  forming  the  outer  part  of  Poupart's 
ligament  (Fig.  429),  the  aponeurosis  from  the  lower  muscular  idigitations 
terminating  directly  in  the  pillars  of  the  external  abdominal  ring,  thus 
strengthening  the  region  of  the  inguinal  canal.  This  is  the  usual  termina- 
tion in  the  mammalia.  In  man  the  anterior  part  of  the  iliac  crest  has 
grown^into  the  lower  digitations  of  the  external  obHque  and  severed  them 
from  their  tendinous  fibres,  which  now  form  the  main  constituent  of 
Poupart's  ligament.  The  digitations  thus  inserted  to  the  iliac  crest  help 
in  balancing  the  body. 


BODY  WALL  AND  PELVIC  FLOOR 


407 


(2)  The  internal  oblique  and  transversalis  (conjoined  parts)  in  the 
orang,  and  in  all  primates  except  man,  arise  from  the  firm  tubular- sheath 
of  the  ilio-psoas,  also  froni  the  extensive  anterior  border  of  the  ilium, 
and,  arching  over  the  spermatic  cord,  end  in  a  long  insertion  on  the  ilio- 
pectineal  line.  They  act  as  a  powerful  compressor  or  sphincter  of  the 
inguinal  canal,  and  thus  prevent  'hernia  (Fig.  429,  B). 

(3)  The  human  manner  of  walking  and  the  great  head  of  the  human 
child  at  birth  require  a  wide  pelvis.  All  mammals  adapted  to  the  pro- 
nograde  posture  have  a  narrow  pelvis,  and  hence  a  narrow  anterior 
abdominal  wall  (Figs.  428  A  and  B)  through  which  the  inguinal  canal 
passes  very  obliquely.     The  course  of  the  canal  is  more  direct  in  man. 


(tendon  ofent 
I  ob.  cut 


rectus 


bic.  sp. 
uascufar  comp^'-'ypr^^^^^ 


I  conjoined 
■^  I  muscle. 

inguinal  canal. 


Fig.  429,  A. — Poupart's  Ligament  and  the  Crural  Passage  of  Man. 

B. — Poupart's  Ligament,  Crural  Passage,  and  Sphincter-like  Conjojned 
Muscle  of  the  Orang. 


and  therefore  offers  a  greater  facility  to  the  escape  of  the  abdominal- 
contents. 

(4)  Owing  to  the  width  of  his  anterior  abdominal  wall,  the  size  of  the 
space  between  the  edge  of  the  pelvis  and  Poupart's  ligament  (the  crural 
passage)  is  very  much  greater  in  man  than  in  any  other  animal  (Figs.  429 
A  and  B).  In  him,  the  most  internal  part  of  the  passage  is  lef%-amfilleH,. 
and  this  unfilled  space  forms  the  femoral  or  crural  canal  through  which 
femoral  hernia  may  escape.  The  formation  of  the  femoral  canal  has,  ^ 
therefore,  no  embryological  basis  ;  it  is  not  like  the  inguinal  canal  the  site 
of  an  embryological  outgrowth  of  peritoneum.  The  crural  passage  is 
relatively  larger  in  women  than  in  men,  owing  to  the  greater  size  of  the 
female  pelvis,  and  hence  femoral  hernia  is  much  more  common  in  women 
than  in  men.  Some  hint  as  to  the  method  of  treatment  of  hernia  in- man 
may  be  obtained  from  a  consideration  of  the  arrangement  of  structures  ' 
which  prevent  them  in  other  animals. 

(5)  Perhaps  the  most  important  factor  in  the  causation  of  hermajn 
man  is  the  compression  to  which  the  abdominal  contents  are  subjecteti  by 
the   contraction   of  the   musculature   of  the   abdominal   pariet^s  xJuring 


408      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

strenuous  efiorts,  such  as  the  lifting  of  heavy  weights  or  the  carrying  of 
excessive  burdens. 


THE  PELVIC  FLOOR. 

Coccyx. — The  retrograde  changes  undergone  by  the  coccyx  in  the 
evolution  of  the  human  body  are  intimately  connected  with  the  formation 
of  the  pelvic  floor.  The.  coccyx  in  man  is  commonly  comjDOsed  of  four 
vertebrae,  more  or  less  vestigial  in  nature,  which  represent  the  basal 
caudal  vertebrae  of  tailed  mammals.  Evidence  of  their  vestigial  or 
retrograde  nature  is  to  be  found  in  : 

(1)  Only  their  centra  are  developed — with  the  exception  of  the  first, 
which  shows  partial  formation  of  transverse  processes  and  neural  arches 
(superior  cornua)  ; 

.  (2)  Delay  in  the  appearance  of  the  centres  of  ossification.  These, 
instead  of  beginning  in  the  8th  week  as  in  a  typical  vertebra,  commence 
after  birth.  The  centre  for  the  1st  coccygeal  vertebra  appears  in  the 
1st  year,  that  for  the  4th  vertebra  about  the  25th  year  ;  the  2nd  and  3rd 
at  intermediate  periods.  All  four  are  fused  into  one  piece  about  the  30th 
year. 

(3)  Late  in  life,  between  the  40th  and  60th  year,  the  coccyx  unites  with 
the  sacrum. 

The  number  of  coccygeal  vertebrae  varies  ;  four  is  the  normal  number, 
but  there  may  be  three  or  five.  In  the  7th  week  embryo  as  many  as  eleven 
coccygeal  vertebrae  have  been  counted.  The  first  coccygeal  vertebra 
may  join  the  sacrum,  making  six  sacral  vertebrae.  The  coccygeal  verte- 
brae in  anthropoids  are  more  reduced  as  regards  the  development  of  their 
parts  than  in  man. 

The  evidence  of  the  former  existence  of  a  true  tail  in  the  ancestral  human 
stock  consists  of  : 

(1)  From  the  5th  to  the  8th  week  the  coccygeal  region  of  the  spine 
protrudes  (Fig.  430),  and  the  vertebrae  number  from  8  to  11  ;  the  noto- 
chord  is  traceable  beyond  the  vertebral  segments. 

(2)  Vestiges  of  the  extensor  and  flexor  muscles  of  the  tail  are  frequently 
found  (10  %  of  bodies)  on  the  dorsal  and  ventral  aspects  of  the  sacrum 
and  coccyx.  Occasionally  small  nodules  of  bone  are  found  in  front  of  the 
human  coccyx,  spanning  the  continuation  of  the  middle  sacral  (caudal) 
artery  ;  these  nodules  represent  the  chevron  bones  or  haemal  arches  of 
tailed  mammals.  The  depressors  of  the  tail  are  attached  to  the  chevron 
bones  (see  Fig.  431). 

(3)  True  tails,  consisting  of  external  prolongations  of  the  coccygeal 
region,  commonly  fibrous,  rarely  containing  vertebrae,  occasionally 
occur. 

(4)  The  post-anal  pit,  always  to  be  seen  in  the  newly  born  child,  marks 
the  point  at  which  the  coccyx  disappears  below  the  surface  early  in  the 
3rd  month.  In  man  the  coccyx  forms  part  of  the  perineal  floor.  Instead 
of  projecting  far  beyond  the  gut,  as  in  tailed  mammals,  it  terminates 


BODY  WALL  AND  PELVIC  FLOOR 


409 


Pelvic  Floor  is  peculiarly  extensive  in  man,  an  adaptation  to  his  upright 
posture.     The  floor  is  formed  by  the  following  structures  : 

(1)  The  levator  ani  and  its  sheath  (recto-vesical  and  anal  fasciae)  on 
each  side  ;  (2)  The  coccyx  and  coccygeus  muscles  ;  (3)  The  constrictor 
urethrae  and  triangular  ligament ;  (4)  The  pyriformis  and  its  sheath  may 
also  be  included. 

Development  of  the  Pelvic  Floor.^ — The  pelvic  floor  has  been  evolved 
in  man  by  a  transformation  of  the  tail  and  the  caudal  muscles.  The 
arrangement  of  tail  muscles  in  a  four-footed  mammal,  such  as  the  monkey 
or  dog,  is  shown  in  Fig.  431,  A,  and  the  modification  of  this  form  in  anthro- 
poids and  man  in  Fig.  431,   B.     In  mammals,  two  muscles,  the  pubo- 


SEGMENT  20 


Fig.  430. — The  rise  and  retrogression  of  the  caudal  vertebrae  during  the  2nd  month 
of  development.     (After  Kunitomo.) 


coccygeus  and  ilio-coccygeus  act  as  depressors  of  the  tail,  which  in 
four-footed  animals  plays  the  part  of  a  perineal  shutter  ;  in  orthograde 
primates  the  tail  no  longer  helps  to  close  the  perineum,  its  muscles  being 
required  for  the  support  of  the  pelvic  viscera.  In  pronograde  apes  these 
muscles  are  attached  to  the  small  V-shaped  chevron  bones  on  the  under 
surface  of  the  basal  caudal  vertebrae  (Fig.  432).  Another  muscle,  the 
ischio-  or  spino-coccygeus,  acts  as  a  lateral  flexor  of  the  tail.  It  is 
attached  to  the  transverse  processes  of  the  caudal  vertebrae,  and  rises 
from  the  dorsal  border  of  the  ischium.     In  man  the  pubo-coccygeus  and 

1  The  following  are  some  of  the  British  papers  dealing  with  this  subject :  P.  Thomp- 
son, Myology  of  the  Pelvic  Floor,  Manchester,  1899  ;  R.  H.  Paramore,  Lancet,  1910, 
May  21st  and  28th.  In  the  Journal  of  Anatomy  and  Physiology  the  following  papers 
have  appeared  :  P.  Thompson,  1901,  vol.  35,  p.  127  ;  A.  M.  Paterson,  1907,  vol.  41, 
p.  93  ;  D.  Derry,  1908,  vol.  42,  p.  97  ;  G.  Elliot  Smith,  1908,  vol.  42,  p.  198  et  seq.  ; 
J.  Cameron,  1908,  vol.  42,  p.  438. 


410 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


ilio-coccygeus  are  blended  into  one  sheet  and  form  tlie  levator  ani.  The 
shrinkage  of  the  tail  leaves  the  muscle  partly  stranded  on  the  ano-coccygeal 
ligament  (Fig.  431,  B).  Other  fibres  of  the  pubo-coccygeus  lose  their 
primary  insertion  to  the  coccyx,  and  become  attached  to  the  prostate, 
central  point  of  the  perineum,  and  to  the  anal  canal.  Both  muscles, 
especially  the  ilio-coccygeus,  retain  in  part  their  primitive  attachment  to 
the  coccyx  (cauda).  The  spino-coccygeus,  or  coccygeus  muscle,  is  partly 
fibrous  in  man,  its  outer  laminae  forming  the  small  sacro-sciatic  ligament ; 
its  inner  laminae  remain  muscular  and  form  the  coccygeus.     In  man. 


ISCHIO-COCCYGEUS 

RETRACTOR    ANI 


ANUS 
PUBO-COCCYGj 


SVMPH:  PUBIS 


A       PRONOQRADE      FORM 


ISCHIO-COCCYGEOS 

RETRACTOR    ANI 

ANO-COCCYG:  LIG 

ANUS 

LlO-COCCYG'- 
PUBO-COCCYQ: 


B.      ORTHOGRADE      FORM 

Fig.  431. — Diagram  to  show  the  Pelvic  Muscles  of  a  Pronograde  Ape  (A)  and  of  an 
Orthograde  Ape  (B). 

too,  the  origin  of  the  ilio-coccygeus  has  sunk  from  the  pelvic  brim  of  the 
ilium  on  to  the  obturator  fascia  (P.  Thompson)  ;  traces  of  the  primitive 
origin  from  the  pelvic  brim  can  often  be  detected  (Fig.  433).  The  white 
line,  a  structure  peculiar  to  man,  marks  the  new  point  of  origin  of  the 
levator  ani  from  the  obturator  fascia. 

In  fishes  {selachians)  the  levator  ani  is  represented  by  a  backward 
continuation  of  the  rectus  abdominis  (Paramore).  The  pelvic  part  of  the 
rectus  is  attached  behind  to  the  tail ;  anteriorly  it  is  attached  to  the 
movable  pelvic  girdle.  The  cloaca  of  the  dog-fish  passes  out  between 
the  right  and  left  primitive  representatives  of  the  levator  ani,  which  can 
compress  the  cloaca,  not  by  depressing  the  tail  as  in  mammals,  but  by 
pulling  the  pelvis  backwards. 


BODY  WALL  AND  PELVIC  FLOOR 


411 


Pelvic  Fascia  and  Fasciae  in  General. — It  has  been   customary  to 
regard  fasciae  as  sejDarate  structures  forming  distinct  sheets  with  devious 


symph 


sacrum 

Ischio-coccygeus 
ilio-coccygeus 
pubo-coccygeus 
retractor  ani 

tail 


Fia.  432. — The  Pelvic-caudal  Muscles  of  a  Monkey. 

and  complex  courses.     It  is  possible  by  dissection  to  prepare  and  display 
them  according  to  accepted  descriptions,  but  the  structures  so  displayed 


obt.  fascia  \!^ 
ilio-coccyg. 

Symph 


■'^(remnants  of  a  rig. 
iof  ilio-coccygeus 

ischio-coccygeus 
coccyx. 

no-coccyg.  lig. 
central  point 


pubo-coccygeus 


Fig.  433. — The  Pelvic  Muscles  of  Man — corresponding  to  those  shown 
in  Fig.  432. 

are  artificial  and  not  the  true  structures  which  the  surgeon  or  physician 
has  to  deal  with  in  actual  practice.     Embryology  is  the  best  guide  to  their 


412 


HUMAN  EMBRYOLOGY  AND  MORPHOLOaY 


nature.  Take,  for  example,  the  development  of  the  fasciae  seen  on  making 
a  section  of  the  upper  arm  (Fig.  434).  When  the  limb  bud  has  appeared, 
which  it  begins  to  do  about  the  end  of  the  4th  week  of  development,  a 
section  through  it  reveals  a  syncytium  of  mesodermal  cells,  the  blastema 
of  bones,  muscles,  etc.,  surrounded  by  a  covering  of  ectoderm  (Fig.  435). 
Very  soon  the  central  cells  near  the  axis  of  the  bud  are  densely  grouped  and 
form  the  basis  of  the  skeletal  axis.  Others,  derived  from  extensions  of 
the  primary  muscle  plates  (Fig.  435),  arrange  themselves  to  form  the  biceps, 
triceps  and  muscles  of  the  arm  ;  others  become  the  walls  of  vessels  and  the 
sheaths  of  nerves.  After  these  various  groups  of  cells  have  become 
differentiated,  there  is  left  over  a  cellular  residue  in  which  the  highly 
differentiated  cell-groups  are  enmeshed.     The  undifferentiated  mesoderm 


biceps 

artery 

nerue 

septun, 

tendon 


MUSCLE    PLATE 
SKIN    PLATE 


SKELETAL 
BLASTEMA 


MESODERM 


MUSCLE 
BLASTEMA 


periosteum  i  t/ \     p--r>v     limb  bud 

I  ^  BODY   \/VALL 

humerus  deep  fascia  mesentery 

Fig.  434.— Section  across  the  Upper  Arm  to  show  the  continuity  of  its  Fascial 

System. 

Fig.  435. — Section  of  a  Limb-bud  to  show  the  manner  in  which  its  tissues 

become  differentiated.     (After  Kollmann.) 

forms  the  connective  tissue  or  fascial  system  of  the  part.  From  the 
manner  of  its  origin  it  is  evident  that  the  connective  tissue  system — ^the 
fasciae  and  septa — must  form  a  continuous  sponge-work  of  sheaths,  each 
being  in  continuity  with  that  of  every  surrounding  structure.  The  sheaths 
of  the  biceps,  triceps  and  brachialis  anticus,  the  periosteum  of  the  humerus, 
the  deep  fascia,  internal  and  external  intermuscular  septa,  the  sheaths  of 
the  vessels  and  nerves  of  the  arm,  represent  the  mesodermal  tissue  which 
was  left  over  after  the  individual  structure  of  the  brachium  were  differ- 
entiated, and  are,  from  the  manner  of  their  origin,  necessarily  in  contmuity 
(Fig.  434).  They  can  only  be  artificially  separated  from  each  other.  It 
is  more  accurate  and  easier  to  describe  fasciae,  then,  not  as  separate 
structures,  but  as  adjuncts  of  the  structures  which  they  surround  or 
ensheath.  As  to  the  manner  in  which  connective  tissue  is  developed, 
there  are  two  opinions  :  (1)  that  the  substance  of  the  cell  body  elongates 
and  forms  a  fibre  ;    (2)  the  more  probable,  that  fibres  are  formed  in  a 


BODY  WALL  AND  PELVIC  FLOOR 


413 


substance  which  lies  outside  the  cell  body,  but  is  under  the  influence  of 
the  cell.i 

The  Pelvic  Fascia,  which  strengthens  the  pelvic  floor,  is  composed  of 
the  sheaths  of  four  muscles  : 

(1)  Levator  Ani ;  (2)  Obturator  Internus  ;  (3)  Pyriformis  ;  (4)  Con- 
strictor Urethrae  and  deep  Transversus  Perinei. 

The  fibrous  capsules  of  the  following  viscera  also  form  part  of  it : 

(1)  Prostate  and  Vesiculae  Seminales  in  the  male  ;  (2)  Vagina  and 
Uterus  in  the  female  ;  (3)  Bladder  ;  (4)  Rectum.  Under  the  title  of 
pelvic  fascia  these  eight  elements  are  combined.  To  these  must  be  added 
the  important  sheaths  of  the  vessels — especially  of  the  vesical,  uterine  and 
perineal  arteries.^ 

I.  The  Obturator  Fascia  is  the  sheath  on  the  inner  or  pelvic  aspect  of 
the  obturator  internus  ;  the  sheath  on  the  outer  side  of  the  muscle  is 
formed  by  the  periosteum  and  obturator  membrane.  The  obturator 
fascia  is  attached  at  the  circumference  of  the  muscle.  There  it  becomes 
continuous  with  the  periosteum  of  the  os  innominatum.  The  part  above 
the  white  line  (supra-linear)  is  intra-pelvic  ;  the  part  below  (infra-linear) 
is  perineal  and  situated  on  the  outer  wall  of  the  ischio-rectal  fossa. 

II.  Recto- vesical  and  Anal  Fasciae. — The  levatores  ani  form  a  muscular 
floor  for  the  pelvis,  stretching  from  the  white  line  of  one  side  to  the  white 
line  of  the  other.  The  sheath  on  their  under  surface — on  the  inner  wall 
of  the  ischio-rectal  fossa^ — forms  the  anal  fascia.  On  the  upper  surface, 
their  sheath  forms  the  greater  part  of  the  recto-vesical  fascia.  The  pelvic 
viscera  rest  on  the  upper  surface  of  the  levatores  ani  and  the  capsviles  of 
the  viscera  are  continuous  with  the  sheath  on  the  upper  surface  of  the 
muscles.  The  combined  visceral  cap- 
sules and  upper  sheath  of  the  levatores 
ani  form  the  recto-vesical  fascia. 

III.  The  Triangular  Ligament  is 
situated  in  the  neighbourhood  of  the 
constrictor  urethrae  muscle  (Fig.  436), 
but  it  can  scarcely  be  regarded  as 
its  sheath.  It  is  rather  a  fibrous 
septum  for  giving  attachment  to  the 
prostate  on  its  deep  or  pelvic  surface 
and  to  the  bulb  and  root  of  the  penis 
on  its  lower  or  perineal  aspect  (Delbet, 
EUiot  Smith).  The  inferior  transverse 
fibres  of  the  constrictor  form  really  a 
separate  muscle — the  deep  transverse  perineal.  The  apex  of  the  prostate 
rests  on  the  muscle,  its  fibrous  capsule  being  continuous  with  the 
posterior  layer  of  the  muscle  sheath — the  deep  layer  of  the  triangular 
ligament. 

^  For  literature  see  F.  P.  Mall,  Amer.  Journ.  Anat.  1901,  vol.  1,  p.  329  ;  A.  von  Szily, 
Anat.  Hefte,  1907,  vol.  33,  p.  225  ;  J.  S.  Ferguson,  Ainer.  Journ.  Anat.  1912,  vol.  13, 
p.  129  ;  Korff,  Ergebnisse  der  Anat.  1907,  vol.  17,  p.  247. 

-  See  references  on  p.  409  under  the  names  of  Prof.  A.  M.  Paterson  and  Prof.  Elliot 
Smith. 


constrictor  urethrae 

Fig.  43C.— The  Constrictor  Urethrae  Muscle. 


414     HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

IV.  The  inner  sheath  of  the  pyriformis  forms  the  pyriform  fascia.  The 
coccygeus  is  continuous  with  the  levator  ani  and  its  sheath  forms  part  of 
the  recto-vesical  fascia.  The  loose  perirectal  sheath  is  also  continuous 
with  the  tissue  of  the  fascia  pyriformis. 

The  pubo-prostatic  ligaments  and  the  lateral  vesical  ligaments  are. 
strengthened  parts  of  the  fibrous  capsule  of  the  prostate,  which  provide 
the  bladder  with  a  pubic  fixation.  The  vesical  musculature,  in  emptying 
the  bladder,  acts  from  the  pubic  fixation  thus  obtained.  The  great  strains 
to  which  the  pelvic  vessels  are  exposed  when  the  pelvic  floor  and  viscera 
are  depressed  in  forced  muscular  efforts  renders  a  strong  fibrous  protective 
sheath  necessary.  Hence  the  tough  fibrous  coating  round  the  uterine  and 
vesical  vessels.  Alcock's  canal  is  formed  from  the  fibrous  sheath  round 
the  pudic  artery  and  nerve  (Elliot  Smith). 

Cervical  Fascia.^ — From  what  has  been  said  of  the  pelvic  fascia,  the 
nature  and  arrangement  of  the  cervical  fascia  will  be  readily  understood. 
It  is  composed  of  (1)  the  sheaths  of  the  cervical  muscles  (sterno-mastoid, 
etc.)  ;  (2)  of  the  sheaths  of  vessels  (carotid  sheath,  etc.)  ;  (3)  the  sheaths 
of  nerves  (axillary  sheath,  etc.)  ;  (4)  the  fascial  capsules  of  viscera,  such 
as  the  thyroid  body,  salivary  glands,  and  pharynx.  The  carotid  sheath 
and  sheaths  of  the  great  vessels  from  the  base  of  the  skull  to  the  peri- 
cardium within  the  thorax  are  formed  to  a  great  extent  from  mesodermal 
tissue  which  was  developed  within  the  visceral  arches  of  the  pharynx. 
At  first  the  pericardium  lies  beneath  the  mouth  and  pharynx.  With  the 
development  of  the  neck  at  the  end  of  the  2nd  month  of  foetal  life,  the 
cervical  structures  and  their  sheaths  become  stretched,  but  they  maintain 
the  ancient  connection  between  skull  base  and  pericardium. 

The  muscular  sheaths  on  the  inner  aspect  of  the  transversalis,  iliacus 
and  psoas  also  have  been  regarded  as  forming  distinct  fasciae. 

On  the  other  hand,  some  fasciae  are  quite  discrete  structures.  The 
palmar  fascia  is  part  of  the  palmaris  longus  muscle  ;  the  plantar,  part  of 
the  plantaris  muscle  ;  the  vertebral  aponeurosis  or  fascia,  part  of  the 
layer  of  muscle  which  is  represented  by  the  serratus  posticus  superior 
and  inferior  ;  the  epicranial  aponeurosis  is  part  of  the  platysma  sheet. 
The  middle  layer  of  the  lumbar  fascia  represents  a  primary  septum 
developed  between  the  dorsal  and  ventro-lateral  groups  of  musculature 
(see  p.  68). 

Fascial  structures  have  also  a  distinct  relationship  to  the  lymphatic 
system.  Lymphatics  follow  the  septa  and  capsules  of  glands  and  muscles  ; 
the  lymphatics  of  the  lung  collect  in  the  connective  tissue  separating  its 
lobules.  The  most  remarkable  of  all  the  capsular  tissues  of  the  body  are 
those  represented  by  the  membranes  of  the  central  nervous  system ; 
there  the  cerebro-spinal  spaces,  or  clefts,  have  separated  the  cerebral  capsule 
into  three  layers — the  pia  mater,  arachnoid  and  dura  mater. 

Leonard  Hill  has  also  drawn  attention  to  the  part  which  ensheathing 
fasciae  play  in  assisting  the  circulation  of  the  blood.  Every  contraction 
of  the  muscles  of  the  thigh  tends  to  force  the  venous  blood  within  the 
sleeve  formed  by  the  fascia  lata  on  towards  the  heart. 

^  See  Prof.  F.  G.  Parsons,  Journ.  Anat.  and  Physiol.  1910,  vol,  44,  p.  153. 


BODY  WALL  AND  PELVIC  FLOOR 


415 


Body  WaU. — Having  thus  traced  the  evolution  of  the  pelvic  jfloor  and 
discussed  the  nature  of  fasciae  generally  in  connection  with  the  pelvic 
fascia,  we  pass  on  to  consider  the  development  and  nature  of  the  abdominal 
and  thoracic  walls. 

Bilateral  Symmetry  of  the  Body. — From  a  developmental  point  of 
view  the  body  is  made  up  of  two  symmetrical  halves  ;  each  half  of  the 
embryonic  plate,  taking  the  medullary  groove  as  the  line  of  division, 
contributes  equally  to  the  formation  of  the  body.  Each  produces  a  half 
of  the  nervous  system,  each  a  half  of  the  vascular,  muscular  and  alimentary 
systems,  so  that  each  individual  is  in  reality  made  up  of  two  identical  halves, 


i*s} 


Symphysis  menti- 
uent.  line  of  nee  fl- 
uent line  of  thorax— I 

supra-umb.  tinea  alba- 


umbilicus     (j\ 

infra-umb.  linea  alba  - 


symph.  pubis- 


^ 


stomodaeum 


/  uent.  line  of  j 
[pharyn.  region/ 


.  umbilicus 


vulu.  cleft- 
perin.  raphe 


anus 


one 1 

coccyx 


^j^^^enital  part 

LJ perineal  depression 

^  [_l anal  part 


vy- 


coccyx 


Fig.  437.- 
FlG.  438.- 


-Diagram  of  the  Structures  formed  in  the  Median  Ventral  Line  of  the  Body. 
-Tlie  Median  Ventral  Line  in  an  Embryo  of  4  weeks,  to  contrast  with  the 
Corresponding  Line  in  the  Adult. 


right  and  left.  Although  each  side  of  the  body  rises  from  the  same  blasto- 
cyst, yet  each  becomes  specialized  structurally  and  functionally  so  that, 
as  development  goes  on,  there  appears  a  very  remarkable,  asymmetry. 

Ventral  Line  of  the  Body. — The  structures  within  the  right  and  left 
body  walls  become  united  along  the  ventral  line  from  the  mouth  to  the  anus 
(see  Fig.  437).  The  mesoderm,  muscle  plates,  dermatomes,  nerves  and 
cartilaginous  outgrowths,  which  are  produced  on  each  side  of  the  median 
dorsal  line  of  the  body,  meet  on  each  side  of  the  median  ventral  line.  In 
this  line  are  developed  the  symphysis  of  the  lower  jaw,  the  body  of  the 
hyoid  bone  (copula),  the  white  line  of  the  neck  and  angle  of  the  thyroid 
cartilage,  the  sternum,  the  supra-umbilical  part  of  the  linea  alba,  umbilicus, 


416 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


infra-umbilical  part  of  tlie  linea  alba,  symphysis  pubis,  the  septum  of  the 
penis,  and  of  the  scrotum  and  perineal  raphe.  The  ventral  line  is  continued 
forwards  on  the  face  between  the  parts  derived  from  the  mesial  nasal 
processes. 

The  idea  was  at  one  time  prevalent  that  the  whole  of  this  line  was  formed 
by  the  fusion  of  one  somatopleure  with  the  other  ;  the  median  ventral 
line  was  the  suture  formed  by  the  union.  Such  is  not  the  case.  The 
blastoderm,  which  lies  at  first  like  a  cap  on  the  yolk  sac  (Fig.  18),  is  produced 
or  folded  anteriorly  to  form  the  fore-gut  and  the  part  of  the  body  above 
the  umbilicus  ;  it  is  produced  posteriorly  to  form  the  hind-gut  and  the 
part  of  the  body  below  the  umbilicus.     The  blastoderm  grows  out  from  the 


occip.  segs. 


C.    I/. 


D.  II. 


posit  of  stern, 
umb.  cord 


D.  m-\r- 


L    /. 


Fig.  439. — Diagram  of  a  Human  Embryo  (6th  week)  showing  the  Arrangement  and 
Extension  of  the  Mesoblastic  Segments.  (After  A.  M.  Paterson.)  The  first 
and  last  of  each  segment  entering  into  the  formation  of  the  limbs  is  stippled. 
The  position  is  indicated  in  which  the  sternum  is  formed. 


umbilicus  to  form  the  embryo  in  much  the  same  way  as  a  soap-bubble  is 
blown  from  the  bowl  of  a  pipe.  In  an  embryo,  at  the  commencement  of 
the  4th  week,  the  greater  part  of  the  ventral  line  is  occupied  by  the  umbiHcus 
(Fig.  438).  At  that  time  the  umbilicus  is  3  mm.  long,  the  entire  ventral 
line  being  about  4  mm.  At  the  end  of  the  7th  week  the  ventral  line 
measures  15  mm.,  the  umbilicus  retains  its  former  size,  about  3  mm. 

At  first  the  somatopleure  shows  no  trace  of  segmentation.  The  paraxial 
masses  of  mesoderm  become  segmented  early  and  form  the  muscle  plates 
(Fig.  65).  From  each  muscle  plate  of  the  primitive  segments  a  process 
grows  down  into  the  somatopleure  (Fig.  439).  The  somatopleure  thus 
becomes  segmented  secondarily,  the  process  of  segmentation  spreading 
from  the  dorsal  to  the  ventral  side  of  the  plate,  but  along  the  median 


BODY  WALL  AND  PELVIC  FLOOR  417 

ventral  line  of  the  body  wall,  a  band  of  the  primitive  mesodermal  tissue 
remains  unchanged  and  undifferentiated.  In  the  ventral  band  between 
the  left  somatopleure  and  the  right  are  formed  the  sternum  and  the  linea 
alba  (Fig.  437).  In  lower  vertebrates,  in  fishes,  and  to  a  less  marked  extent 
in  amphibians  and  reptiles,  the  myotomic  segments  remain  distinct  from 
end  to  end  of  the  trunk. 

Formation  of  Ribs.^ — Ribs,  like  all  true  skeletal  bones,  pass  through 
three  stages  :  (1)  They  are  represented  by  a  mesenchymatous  or  membran- 
ous basis  in  the  fibrous  tissue  (septa)  between  the  muscular  segments  of 
the  somatopleure  (Fig.  439).  The  condensation  of  the  costal  mesenchyme 
appears  at  the  beginning  of  the  5th  week  as  a  separate  vertebral  element. 

(2)  The  mesenchymatous  basis  or  blastema  of  the  rib  becomes  cartilaginous. 

(3)  Ossification  of  the  cartilage  begins  in  the  8th  week,  but  the  process  of 
ossification  leaves  the  ventral  parts  of  the  costal  segments  untouched  ; 
they  form  the  costal  cartilages  ;  in  lower  forms  they  become  ossified  and 
form  sternal  ribs.  The  process  of  chondrification  begins  at  the  dorsal  end 
of  the  ribs  in  the  6th  week,  and  spreads  ventrally,  thus  repeating  the  order 
in  which  the  blastema  was  laid  down.  The  extension  ventralwards  of  the 
ribs  corresponds  with  the  growth  and  expansion  of  the  lungs  ;  at  the 
beginning  of  the  7th  week  they  scarcely  reach  the  lateral  or  axillary  line 
of  the  body,  but  by  the  end  of  this  week  they  have  effected  a  junction 
with  the  sternal  bars  (Fig.  443).  The  ribs  from  the  1st  to  the  7th  are 
developed  in  the  somatopleure  over  the  pericardium.  In  lower  vertebrates, 
such  as  reptiles,  each  rib  articulates  with  the  neural  arch  of  a  vertebra  by 
two  heads,  dorsal  and  ventral  (Fig.  60).  The  tuberosity  of  a  rib  represents 
its  dorsal  head.  In  man,  with  the  exception  of  the  first  and  last  rib,  or 
in  some  cases,  the  two  last  ribs,  the  costal  head  is  placed  opposite  an 
intervertebral  disc,  for  in  position  the  disc  represents  the  ventral  or  chordal 
part  of  a  primitive  vertebra.  In  the  case  of  the  first  rib  the  head  has 
shifted  backwards  to  the  body  of  the  first  vertebra,  while  in  the  12th  and 
sometimes  the  11th,  the  head  and  tuberosity  are  fused,  and  both  articulate 
with  the  part  of  the  vertebra  which  represents  a  transverse  process. 

The  Sternum. — In  man  and  anthropoids  the  sternum  has  become 
flat  and  highly  modified  with  the  alterations  in  the  shape  of  the  thorax 
(Fig.  372).  With  the  adaptation  to  the  upright  posture  the  thorax  becomes 
flattened  from  back  to  front ;  its  transverse  diameter  is  as  great,  or  greater, 
than  the  autero-posterior.  The  type  of  respiration  is  greatly  altered. 
The  sternum  also  becomes  wider  and  shorter.  To  understand  the  nature 
of  this  change,  it  is  necessary  to  note  the  characters  of  the  sternum  of  a 
pronograde  mammal,  such  as  the  dog  or  ape  (Fig.  440).  In  such,  the 
sternum  is  typically  made  up  of  seven  segments  : 

1.  A  modified  anterior  segment,  the  pre-sternum  ;  2.  Five  narrow, 
cylindrical  segments  or  sternabrae,  forming  the  body  of  the  sternum  ;  3. 
The  ensiform  process,  a  hind  segment,  complex  in  nature  and  ending  in  the 

1  For  development  and  differentiation  of  ribs  see  Charles  R.  Bardeen,  Amer.  Journ. 
Anat.  1905,  vol.  4,  p.  163  ;  also  p.  265  ;  Geddes,  Journ.  Anat.  and  Physiol.  1913, 
vol.  47,  p.  18.  For  ossification  of  ribs  :  Franklin  P.  Mall,  Amer.  Journ.  Anat.  1906, 
vol.  5,  p.  433. 

2d 


418 


HUMAN  EMBRYOLOaY  AND  MORPHOLOGY 


middle  ventral  line.  Tlie  ensiform  process  frequently  bifurcates  and  is 
never  segmented. 

The  chief  changes  in  the  human  sternum  are  : 

1.  Each  segment  has  become  flat  and  wide  ;  2.  The  segments  of  the  body 
fuse  together  during  the  years  of  adolescence,  the  fusion  beginning  behind 
and  passing  forwards  ;  3.  The  4th  sternabra  of  the  body  is  usually  vestigial 
and  is  probably  made  up  of  two  or  more  fused  segments. 

In  low  primates  8  or  9  pairs  of  ribs  may  reach  the  sternum,  six  or  more 
sternabrae  being  then  present.  In  man  the  number  has  been  reduced  to 
7  pairs,  the  sternal  ends  of  the  7th  pair  lying  in  front  of  the  fourth  sternabra. 
It  is  not  uncomimon  to  find  the  8th  rib  reaching  the  sternum,  especially 


1st  rib 

2nd  rib 
3rd  rib 
4th  rib 
5th  rib 
6th  rib 

7th  rib 
8th  rib 
9th  rib 

FlQ.  440- 
FiG.  441.- 


.     supra-sternal 


1st  seg. 

f:s^2nd  seg. 
3rd  seg. 
4th  seg. 

5th  seg. 
6th  seg. 
7th  seg. 

ensiform 


clau. 


Thterart.  cart 


4th  seg: 
5th  seg. 
6th  seg. 


centres  of  ossific. 
r    of  3rd  seg. 

(uestigial  of  5th 
y  &  6th  segments) 


ensiform 


-The  Form  of  Sternum  in  a  Pronograde  (quadrupedal)  Mammal. 
-The  Form  of  Sternum  m  a  Mammal  adapted  to  the  Orthograde  (upright) 
Posture.     The  Points  of  Ossification  are  also  shown. 


on  the  right  side  ;  it  is  rare  to  find  the  7th  pair  fail  to  reach  the  sternum. 
The  more  frequent  presence  of  an  8th  sternal  rib  on  the  right  side  is  due  to 
right-handedness  (Cunningham),  or,  as  seems  more  probable,  to  give  a 
more  secure  origin  to  the  right  costal  fibres  of  the  diaphragm,  which  have 
a  greater  resistance  to  overcome  during  inspiration,  than  those  of  the  left 
side.  In  man  and  the  anthropoid  apes  a  new  feature  appears  in  the  lower 
costal  cartilages.  The  5th,  6th  and  sometimes  the  7th  throw  out  processes 
which  articulate  with  the  cartilage  below.  When,  during  inspiration, 
the  diaphragm  raises  the  chest,  these  articulations  permit  it  to  elevate 
the  5th  and  6th  pairs  of  ribs  as  well  as  the  7th  pair. 

Morphology  of  the  Sternum.^ — In  amphibia  the  ventral  parts  of  the 
shoulder  and  pelvic  girdles  develop  towards  the  ventral  median  line.     In 

^  The  account  given  by  Paterson  (Hunterian  Lectures,  1903)  has  been  followed 
with  some  modifications.     For  an  introduction  to  the  more  recent  literature  see 


BODY  WALL  AND  PELVIC  FLOOR 


419 


the  median  line  a  rod  of  cartilage  is  formed  between  them  (Fig.  442). 
The  median  rod  is  differentiated  as  right  and  left  bars  from  the  ventral 
parts  of  the  limb  girdles.  The  right  and  left  bars  fuse  to  form  the  median 
cartilage.  The  median  rod  between  the  shoulder  girdles  becomes  the 
sternum  ;  it  is  divided  into  three  parts — anterior,  which  projects  in  front 
of  the  girdle  (omo-sternum  or  supra-sternum)  ;  posterior,  behind  the  girdle  ; 
and  the  middle,  with  which  the  shoulder  girdle  articulates  (Fig.  442,  A). 
The  sternum  affords  a  basis  from  which  muscles  act  on  the  shoulder  girdle, 
and  also  a  ventral  basis  for  the  articulation  of  the  shoulder  girdle.  In 
all  classes  of  vertebrates,  the  sternum  is  developed  over  and  shields  the 
heart.  The  median  cartilage  of  the  pelvic  girdle  is  similarly  divided 
into  anterior,  middle  and  posterior  parts  (Fig.  442,  B). 

mid.  uent.  line^  i — mid.  vent,  line 


episternum 
clou. 


i—epipubis 


corac. 
sternum 

xiphistem 


symphysis 
— pubis 

^—ischium 

hy poise  ilium 


B. 


Fig.  442. — The  Cartilages  developed  on  each  side  of  tlie  Median  Line  between  the 
Shoulder  and  Pelvic  Girdles.  A,  the  shoulder  girdle  of  the  frog  ;  B,  the  pelvic 
girdle  of  sphenodon.  (The  term  "  epi-sternum  "  is  wrongly  applied  in  Fig.  A  ; 
it  should  be  omo-sternum  or  supra-sternum.  There  is  now  a  general  agreement 
that  the  term  epi-sternum  should  be  applied  to  the  membrane  bone  formed 
between  the  clavicles.) 

The  evolution  of  a  costal  type  of  respiration  in  reptiles  leads  to  a  further 
stage  of  development.  Some  of  the  costal  processes  of  the  vertebrae 
grow  towards  the  median  ventral  line,  some  of  them  reaching  and  articu- 
lating with  the  middle  part  of  the  bar  between  the  shoulder  girdles  ;  this 
part  now  serves  as  a  fulcrum  or  sternum  for  both  ribs  and  girdle.  Such  a 
condition  is  also  seen  in  birds  and  monotremes  (Fig.  466).  In  the  higher 
mammals,  the  ventral  part  of  the  shoulder  girdle  retains  only  its  ventral 
connection  with  the  sternum  through  the  clavicle  ;  it  still  serves  as  the 
basis  of  origin  for  muscles  which  act  on  the  shoulder  girdle  and  on  the  arm. 
Its  chief  purpose  has  become  respiratory.  In  the  human  sternum  the  three 
parts  of  the  primitive  sternum  can  be  recognized  :  the  supra-sternal  bones 
(Fig.  441),  which  are  only  rarely  separated  from  the  presternum,  represent 

Whitehead  and  Waddell,  Amer.  Jovrn.  Anat.  1912,  vol.  12,  p.  89  ;  F.  B.  Hanson,  Anat. 
Rec.  1920,  vol.  17,  p.  1  ;  Amer.  Journ.  Anat.  1919,  vol.  26,  p.  41. 


420 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


the  anterior  part  (omo-sternum) ;  tlie  manubrium  and  body,  the  middle 
part  of  the  shoulder  girdle  sternum  ;  and  the  ensiform  process,  the  posterior 
part. 

Development  of  the  Sternum. — In  Fig.  443  four  stages  in  the  develop- 
ment of  the  human  sternum  are  represented.  Stage  A  shows  the  extent 
to  which  the  ribs  have  become  chondrified  at  the  end  of  the  6th  week  ; 
the  cellular  costal  blastema,  into  which  the  process  of  chondrification  is 
spreading,  is  not  shown.  In  the  following  week  (Stage  B)  the  process  of 
chondrification  has  reached  the  middle  line  in  the  region  of  the  presternum . 


ENSIFORM 


(c).  23mm. 


3rd 

7'} 


(d).  31-  MM. 

Fig.  443. — Four  Stages  in  the  Chondrification  of  the  Human  Ribs  and  Sternum  and 
showing  the  Fusion  of  the  Sternal  Bars.  (After  Charlotte  Miiller.)  A,  end  of 
6th  week  ;  B,  end  of  7th  week  ;  C,  end  of  8th  week  ;  D,  end  of  10th  week. 

The  ventral  ends  of  the  ribs  are  now  joined  together  by  a  ventral  or  lateral 
sternal  bar.  The  sternal  bars  in  the  region  of  the  presternum  have  begun 
to  fuse  together  across  the  middle  line.  At  their  anterior  extremities 
they  are  joined  by  the  ventral  cartilaginous  end  of  the  clavicle.  In  the 
presternum  there  is  thus  an  element  apparently  derived  from  the  ventral 
end  of  the  clavicle.  In  Stage  C,  about  the  end  of  the  8th  week,  the  process 
of  fusion  is  advanced,  but  the  projection  of  the  foetal  heart  and  liver  at 
this  time  (see  Fig.  45),  tends  to  keep  them  apart.  Each  sternal  bar  has 
now  7  ribs  continuous  with  it,  and  its  posterior  end  is  free.  Early  in  the 
3rd  month  (Stage  D)  the  process  of  fusion  is  complete,  the  cartilaginous 
basis  of  the  sternum  has  been  formed  by  the  fusion  of  right  and  left  bars. 


BODY  WALL  AND  PELVIC  FLOOR 


421 


At  the  end  of  the  2nd  month  the  diaphragm  is  descending  to  its  final 
position,  the  pleural  cavities  are  rapidly  forming,  and  the  liver  is  assuming 
a  more  abdominal  position.  Charlotte  Miiller,^  whose  illustrations  are 
represented  here,  found  that  the  mesenchymal  sternal  bars  were  chondrified 
as  direct  extensions  from  the  ribs. 

The  sternum  is  thus  developed  in  the  median  ventral  line  over  the 
pericardium  and  between  the  mandible  in  front  and  the  umbilicus  behind 
(Figs.  437,  439).  The  mesoderm  condenses  during  the  5th  week  on  each 
side  of  this  part  of  the  median  line  to  form  the  right  and  left  mesenchymal 
halves  of  the  sternum,  which  anteriorly  are  continuous  with  the  bases 
of  the  ventral  part  of  the  shoulder  girdle  (Fig.  444).  These  two  halves, 
the  right  and  left  mesenchymal  sternal  bars,  fuse  gradually  in  the  middle 


epi  coracoid  element 


sternal  bar 
(mesobl.) 


basis  of  clavicle 

2nd  rib  {chondrified) 
centres  of  chondrification 

sternal  bar 


ventral  median  line 

Fig.  444. — The  Sternal  Bars  in  an  Embryo  of  7  weeks.    (After  Paterson.) 

line,  the  process  of  fusion  commencing  at  the  presternum  and  spreading 
backwards. 

The  sternum  is  regarded  by  Paterson  as  a  structure  rising  independently 
of  the  ribs  on  each  side  of  the  median  ventral  line.  This,  however,  is  not 
the  commonly  accepted  view.  Ruge's  researches  led  him  to  the  conclusion 
that  the  segments  of  the  sternal  bars  were  produced  as  buds  from  the 
ventral  ends  of  the  ribs.  The  evidence  of  comparative  anatomy  and  the 
difference  in  the  type  of  the  cartilage  cells  in  the  costal  and  sternal  elements 
negative  Ruge's  interpretation. 

In  its  development  the  sternum  passes  through  three  stages — fibrous, 
cartilaginous  and  bony. 

1.  Fibrous  or  mesenchymal  Stage. — In  the  7th  week  (Fig.  444)  the 
costal  cartilages  are  already  chondrified.  The  mesoderm  on  each  side 
of  the  median  line,  in  which  they  end,  has  become  condensed,  and  forms 

^MorpTi.  Jahrb.  1906,  vol.  35,  p.  591. 


422      HUMAN  EMBRYOLOaY  AND  MOEPHOLOGY 

the  membranous  basis   of  the  two   sternal  bars   (Paterson).     The  bars 
begin  to  fuse  together  anteriorly. 

2.  Cartilaginous  Stage. — The  blastema  of  each  sternal  bar  begins  to 
chondrify  in  the  intervals  between  the  ends  of  the  costal  cartilages.  The 
process  of  chondrificatidn  and  fusion  proceed  apace,  and  by  the  commence- 
ment of  the  third  month  the  segments  of  each  side  have  united  to  form  the 
cartilaginous  sternal  bars  (Paterson).  Fibrous  joints  are  subsequently 
formed  between  the  presternum  and  mesosternum  and  between  the 
mesosternum  and  ensiform  process.  A  fibrous,  and  then  synovial  joint, 
is  also  developed  at  the  union  of  the  costal  cartilages  with  the  sternum, 
except  in  the  case  of  the  first  pair,  where  a  synovial  joint  is  only  occasionally 
present. 

3.  Ossification. — A  centre  appears  for  each  sternabra  ;  those  for  the 
third  and  fourth  of  the  mesosternum  are  frequently  double,  one  being 
placed  on  each  side.  The  centres  for  the  4th  mesosternal  segment  are 
frequently  absent.  The  centre  for  the  presternum  (there  may  be  two 
or  even  more)  appears  about  the  4th  month  ;  the  centres  behind  appear 
in  the  6th  and  7th  month  ;  that  for  the  4th  sternabra  of  the  mesosternum 
appearing  about  the  time  of  birth  ;  that  for  the  ensiform  four  or  five  years 
after  birth.  The  process  of  fusion  of  segments  begins  behind  about 
puberty  ;  the  segments  of  the  mesosternum  are  united  together  by  the  25th 
year.  Occasionally  a  median  foramen  may  be  seen  in  the  sternum  ;  it 
is  due  to  imperfect  union  of  the  sternal  bars. 

Sterno-Manubrial  Joint  ^  becomes  of  great  functional  importance  in 
man  and  those  primates  adapted  to  the  upright  posture.  Even  in  old  age 
this  joint  is  rarely  ossified  (8  per  cent.,  Paterson).  In  man  a  considerable 
respiratory  movement  occurs  between  the  manubrium  and  body  of  the 
sternum.  The  manubrium  moves  in  continuity  with  the  ventral  ends  of 
the  first  pair  of  ribs  ;  the  body  of  the  sternum  follows  the  excursion  of  the 
3rd  to  the  7th  pairs  of  sternal  ribs.  As  a  rare  abnormality  (commoner  in 
black  than  in  white  races)  this  joint  is  formed  between  the  first  and  second 
segments  of  the  mesosternum. 

Presternum. — Clear  evidence  of  the  origin  of  the  sternum  from  the 
shoulder  girdle  is  to  be  seen  in  the  presternum.  In  the  earlier  develop- 
mental phases,  it  is  continuous  with  the  precoracoid  element  in  the  ventral 
end  of  the  clavicle  (Figs.  442,  443,  444).  It  is  separated  from  this  element 
by  the  development  of  the  sterno-clavicular  joints  and  meniscus.  In 
over  80  per  cent,  of  bodies  the  upper  border  of  the  human  manubrium 
sterni  shows  traces  of  the  supra-sternal  bones  which  represent  the  anterior 
parts  (omosternum,  epicoracoids)  of  the  primitive  sternum.  Very  rarely 
these  bones  are  separate  (Fig.  441)  ;  commonly  they  are  present  as  eleva- 
tions or  nodules  on  each  side  of  the  suprasternal  notch  (Paterson).  The 
interclavicular  ligament,  which  represents  the  interclavicle  of  birds 
(episternum),  reptiles  and  monotremes  (Fig.  466),  is  attached  to  the 
presternum. 

^  Keith,  Further  Advances  in  Physiology,  edited  by  Leonard  Hill,  1909  (Arnold)  ; 
Journ.  Anat.  and  Physiol.  1896,  vol.  30,  p.  275. 


BODY  WALL  AND  PELVIC  FLOOR 


423 


Linea  Alba. — The  separation  of  the  sternal  bars  is  not  a  reproduction 
of  an  ancestral  phase,  but  is  simply  due  to  an  embryological  convenience 
to  accommodate,  first  the  yolk  sac  and  later  the  large  heart  and  liver  of 
the  embryo.  In  Fig.  445  is  shown  the  early  condition  of  the  linea  alba — 
from  the  classical  research  by  Bardeen  and  Lewis. ^  The  umbilical  cord  is 
still  distended  by  a  loop  of  intestine,  and  the  two  recti  are  wide  apart, 
separated  by  the  mesial  ventral  membrane — the  primitive  linea  alba. 
The  two  sternal  bars  are  also  held  apart  by  the  condition  of  the  umbilical 
structures ;  indeed,  the  primitive  linea  alba  is  not  only  wide,  but  also 
extends  from  the  neck  to  the  perineum.     In  the  10th  week  the  intestines 


MUL.TIF10US    SPIN^ 

LONG  .  OORSI: 

IL\0-COSTAUIS 

QUADRATUS     Ll 
NERVE. 


TRANS  (diOPh) 


CAUDA 


Fig.  445. — The  Primitive  Linea  Alba  in  a  Human  Foetus  in  the  8th  week — 20  mm. 
long.  (After  Bardeen  and  Lewis.)  Only  the  right  half  of  the  body  is  shown ; 
the  rectus  abdominus  is  lateral  in  position,  it  and  the  sternal  bars  being  kept 
from  the  mesial  ventral  line  by  the  structures  in  the  neighbourhood  of  the 
umbilicus. 

Fig.  446. — Transverse  Section  of  the  Thoracic  Wall  of  a  Lizard  to  show  the  Primitive 
Arrangement  of  the  Muscular  Strata  of  the  Body  Wall. 

return  from  the  umbilical  cord  to  the  abdomen,  the  chest  wall  expands 
before  the  growing  lungs  and  the  mesial  ventral  line  becomes  gradually 
closed. 

In  Fig.  446  a  transverse  section  is  shown  of  the  muscular  layers  in  the 
anterior  or  thoracic  body  cavity  of  a  lizard,  and  which  also  represents  a 
stage  in  the  evolution  of  the  musculature  of  man's  body  wall.^  It  will 
be  seen  that  there  are  three  layers  :  an  outer  represented  by  the  rectus  and 


^  C.  R.  Bardeen  and  W.  H.  Lewis,  Aryier.  Journ.  Anat.  1901,  vol.  1,  p.  1  ;    Amer. 
Journ.  Anat.  1901,  vol.  1,  p.  145  (Nerves  of  Abdominal  Wall). 

^  Keith,  Journ.  Anat.  and  Physiol.  1905,  vol.  39,  p.  243  ;   Kazzander,  Anat.  Hefte, 
1904,  vol.  23,  p.  541  (Dev.  of  Rectus  Abdominis). 


424      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

external  oblique  ;  an  inner  by  the  transversalis,  and  a  middle  double 
layer — the  internal  and  external  intercostals.  In  the  abdomen  they  are 
combined  in  one  layer — the  internal  oblique.  The  three  layers  are 
functionally  different ;  the  transversalis  is  a  constrictor  of  the  body  cavity  ; 
the  middle  layer  is  mainly  respiratory  in  its  action  ;  the  outer  is  also 
respiratory,  but  chiefly  concerned  in  body  movements. 


CHAPTEK  XXVI. 
DEVELOPMENT  AND  DIFFEEENTIATION  OF  THE  LIMB  BUDS. 

Evolution  of  the  Limbs.^ — The  nature  of  the  primitive  structures 
from  which  limbs  were  evolved  is  still  a  much  debated  question.  The 
manner  of  their  development  in  vertebrate  embryos  makes  it  certain 
that  they  were  not  outgrowths  from  the  vertebral  system  ;  in  every 
case  they  sprout  out  from  the  somatopleure,  which  encloses  the  body 
cavity,  and  are  always  supplied  by  the  nerves  of  that  lamina — the  ventral 
branches  of  the  spinal  nerves.  We  are  also  certain  that  the  limbs 
correspond  to  the  pectoral  and  pelvic  fins  of  fishes.  It  is  clear  that 
when  land-living  vertebrates  were  evolved,  the  slight  structures  which  were 
equal  to  the  balancing  and  finer  movements  of  an  animal  suspended  in 
water,  had  to  undergo  great  modifications  in  order  to  become  capable 
of  moving  and  supporting  the  body  on  a  solid  medium.  It  was  with  the 
evolution  of  pulmoniferous  land-living  vertebrates  that  a  very  definite 
type  of  limb  made  its  appearance.  In  all  cases  the  limb  of  a  primitive 
Tetrapod  is  built  on  the  same  plan  ;  it  is  made  up  of  a  basal  segment  or 
girdle,  with  a  free  part  divided  into  proximal,  middle  and  distal  segments. 
The  distal  segment  carried  5  digits. 

Although  man  has  departed  greatly  from  the  primitive  mammalian 
type  in  the  structure  of  his  brain  and  trunk,  yet  in  the  elements  which 
enter  into  the  formation  of  his  limbs  he  has  retained  more  of  the  ancestral 
mammalian  features  than  many  other  mammals.  He  retains  the  original 
number  of  digits  ;  the  bones  of  his  hand  and  foot  are  much  less  specialized 
than  those  of  the  horse.  It  is  true  that  the  skeleton  of  his  lower  extremity 
has  been  extensively  modified  for  his  plantigrade  posture,  yet  under  all 
the  adaptational  features  one  can  see  very  clearly  the  outlines  of  a  most 
primitive  form.  He  comes  of  a  stock  which  led  an  arboreal  existence 
almost  from  the  dawn  of  the  mammalian  type. 

Embryonic  Limbs.^ — The  limbs  begin  to  appear  at  the  end  of  the  4th 
week.     A  slight  elevation  or  ridge  is  then  seen  to  run  along  the  dorsal 

^  The  following  papers  will  give  those  interested  a  clue  to  the  extensive  literature 
on  this  subject :  Osbum,  Ainer.  Journ.  Anat.  1907,  vol.  7,  p.  171  ;  Goodrich,  Quart. 
Journ.  Mic.  Science,  1906,  vol.  50,  p.  333  ;  E.  Miiller,  Anat.  Hefte,  1909,  vol.  39, 
p.  469  ;  D.  M.  S.  Watson,  Journ.  Anat.  1918,  vol.  52,  p.  1  ;  W.  K.  Gregorv,  Annals 
N.  Y.  Acad.  Sc.  1915,  vol.  26,  p.  317  ;  Prof.  Wood  Jones,  Arboreal  Man,  1915  ;  Principles 
of  Anatomy,  1920. 

*  Development  and  Differentiation,  see  Bardeen's  Monographs,  A77ier.  Journ.  Anat. 
1905,  vol.  4,  pp,  163,  265  ;    vol,  6,  p.  259  (muscles  and  nerves  of  lower  extremity)  ; 

425 


426 


HUMAN  EMBEYOLOGY  AND  MORPHOLOGY 


border  of  the  somatopleure,  at  some  distance  from  the  row  of  primitive 
segments  formed  in  the  paraxial  mesoblast  (Fig.  447).  The  limb  buds 
spring  from  this  ridge  as  flat  processes  with  an  upper,  dorsal  or  extensor 
surface,  and  a  lower,  ventral  or  flexor  surface.  The  two  borders  are 
anterior  or  cephalic  and  posterior  or  caudal.  It  is  generally  held  that  the 
lateral  ridge,  of  which  the  limb  buds  are  specialized  parts,  represents  a 
continuous  row  of  lateral  fins.  If  this  view  is  right,  then  the  fore  and  hind 
limbs  represent  highly  specialized  fin-rays. 

A  section  shows  each  bud  to  be  composed  of  undifferentiated  mesoderm, 
with  a  covering  of  ectoderm  (Figs.  435,  454).  It  represents  in  structure 
a  process  of  the  undifferentiated  mesoderm  of  the  somatopleure  or  body 
wall ;  hence  the  limbs  are  to  be  regarded,  not  as  structures  developed 
from  the  axis  of  the  embryo,  but  as  processes  of  the  body  wall.     Extensions 


4-^  VISC:  ARCH 
BAD ; BORDER 

HEART 


5?*?  C: SEGMENT. 


SOMATOPLEURE 


FIB:  BORDER 


TIBIAL    BORDER 


LEG   QUO 


IZ'*D. 


5';!'L: 


Fig.  447. — Lateral  View  of  a  Human  Embryo  at  the  28th  day,  showing  the  Limb 
Buds,  Lateral  Ridges,  and  Primitive  Segments. 

grow  into  each  limb  bud  from  the  muscle-plate  and  skin-plate  (dermatome) 
of  the  segments  which  are  situated  opposite  the  origin  of  the  bud.  Each 
corresponding  segment  of  the  spinal  cord  also  sends  to  the  limb  bud  a 
nerve  process.  At  least  seven  body  segments  contribute  to  the  formation 
of  each  limb  (Fig.  439).  Outgrowths  from  the  myotomes  into  the  limbs 
have  been  observed  only  in  the  embryos  of  lower  vertebrates  ;  their 
occurrence  in  higher  vertebrates  is  inferred.  When  the  arm  musculature 
becomes  apparent  as  a  mass  in  the  6th  week,  it  shows  no  signs  of  separate 
segmental  origin. 

Changes  in  External  Conformation. — In  the  5th  week  (Fig.  448) 
the  limb  buds  are  unsegmented  ;  in  the  6th  a  constriction  marks  the  hand 
off  ;  the  position  of  the  elbow  being  indicated  in  the  same  week.  In 
the  7th  week  the  fingers  appear  as  thickenings  in  the  webbed  hand,  the 
middle  digit  being  indicated  first.     They  become  free  at  the  end  of  the  8th 

also  Bardeen  and  Lewis,  1901,  vol.  1,  p.  1  ;  Keibel  and  MalVs  Manual  of  Embryology, 
vol.  1,  1910. 


LIMB  BUDS 


427 


week  ;  occasionally  tlicy  remain  attached,  the  child  being  born  with 
its  fingers  in  a  syndactylous  condition.  The  shoulder  remains  buried 
in  the  body  wall  ;  the  skeletal  structures  of  the  arm  and  thigh  are  the 
first  to  be  differentiated  ;  those  of  the  forearm  and  leg  precede  the  cartil- 
aginous differentiation  of  the  shoulder  and  pelvic  girdle.  In  all  the 
embryological  changes  the  upper  extremity  is  nearly  a  week  ahead  of  the 
lower. 

The  Internal  Differentiation   of  Tissues   begins    at  the  basal   part    of 
the  limb  and  spreads  towards  the  digits,  the  terminal  phalanges  being  the 


basisofam  f''"'"" 

rad.  border  hand      hand 


ulnar  border 


forearm 


Fig.  448.- 


-Four  stages  in  the  development  of  the  Upper  Limb- 
and  8th  weeks.     (After  His.) 


-at  the  5th,  6th,  7th 


last  of  the  skeletal  parts  to  become  difierentiated  (8th  week).  The  meso- 
derm or  mesenchyme  becomes  condensed  in  the  axis  of  the  bud  and  forms 
the  cellular  basis,  or  blastema,  of  the  limb  bones  early  in  the  6th  week. 
The  skeletal  basis  is  continuous,  but  where  joints  are  to  be  formed  there 
occur  opener  formations  in  the  arrangement  of  the  cells.  Centres  of 
chondrification  appear  in  the  skeletal  blastema  of  the  arm  late  in  the  6th 
week  (shaft  of  humerus)  and  the  leg  in  the  7th  week  (shaft  of  femur). 


basis  of  leg    fyot '^9 .    , 
tib.  border;^-^  ,  ,  •'   V-C\>^,    foot 


fib.  border 


Fig.  449.- 


cauda 


-Four  stages  in  the  development  of  the  Lower  Limb — at  the  5th,  6th,  7th 
and  8th  weeks.     (After  His.) 


The  condition  of  the  skeletal  blastema  of  the  arm  of  a  human  foetus  in  the 
7th  week  of  development  is  shown  in  Fig.  450.  The  centres  of  chondrifica- 
tion have  appeared  for  the  humerus,  radius,  ulna  and  certain  of  the  carpal 
bones  ;  the  centres  for  the  phalanges  have  not  yet  begun.  The  scapula, 
acromion  and  clavicle  (outer  part)  are  continuous  ;  a  common  centre 
appears  for  scapula  and  acromion,  the  outer  clavicular  blastema  is 
chondrified  separately.  Before  the  end  of  the  2nd  month  the  cartilaginous 
bases  of  all  the  arm  bones  have  ajDpeared.  Centres  of  ossification  begin  to 
appear  in  the  latter  part  of  the  2nd  month,  and  correspond  generally  to 
the  centres  of  chondrification.  During  the  3rd  month  the  skeletal  blastema 
between  the  chondrified  bases  of  the  bones,  by  a  process  of  vacuolation 
within  and  between  the  cells,  opens  out  into  a  cavity  and  forms  the  synovial 


428 


HUMAN  EMBRYOLOaY  AND  MORPHOLOGY 


membranes  of  the  joints  (Fig.  473).     By  the  end  of  the  6th  week  the 
proximal  muscles,  vessels  and  nerves  are  appearing  ;    a  week  later,  they 

are  also  apparent  in  the  distal  parts 
of  the  limbs.  The  tissue  left  over, 
not  included  in  these  structures, 
forms  their  sheaths,  and  the  fasciae 
and  connective  tissue  of  the  limb. 
The  processes  of  the  nerve  cells  to 
form  the  nerve-fibres,  and  of  the 
muscle  plates  to  form  the  muscle- 
fibres,  grow  in  very  early  (see  Fig. 
454).  The  blood  vessels  appear  first 
as  a  capillary  plexus  surrounding 
the  ingrowing  nerve  buds  ;  in  some 
mammals  (the  lemur,  etc.)  this  em- 
bryonic plexus  persists  and  forms 
the  plexus  mirabile.  The  limb 
vessels  spread  outwards  from  the 
segmental  vessels. 

^     ..„    rp,    ci,  , .  iTj,  .        .  .1,  TT  Skeletal  Blastema  of  Lower  Ex- 

fig.  450. — The  Skeletal  Blastema  of  the  Upper    ,  ..  .,       ,   ,,  i      c  ,i      n,i 

Extremity  of  a  Human  Embryo  in  the  7th    tiemity. — About  the  end  01  the  7  th 

drm^atLn^MiTn'dkated!'^*^(w!H':^Lewi?)°'^'  week  the  blastema  of  the  ilium  be- 
comes joined  to  the  costal  masses  of 
the  1st,  2nd  and  3rd  sacral  vertebrae  (Fig.  451).    The  scapula,  which  at  the 
beginning  of  the  2nd  month  lies  opposite  the  4th,  5th,  6th,  7th  cervical 
vertebrae,  retains  its  freedom  (Fig.  450).     By  the  end  of  the  7th  week  the 


i"c 


^X_y^,L.UM 


^ 

/  ,-■ 

•;  \    ^-^ 

V^^/_  HEAD 

^ 

.■•\".;'-"-v" 

Tj —  FEMUR 

^—i— SMALL    TROC 

J 

f     /if 

/—TIBIA 

1 FEMUR 

Cuboid    y   \^ 

v^..H 

r ASTRAG! 

V~-SCAPH: 

ir 

V  V'  ■ 

■■ .'  f;  "'"•- 

/^ 

UIZARD 

m     II 

Fig.  451. — The  Skeletal  Blastema  of  the  Lower  Extremity  of  a  Human  Embryo  in 
the  7th  week — 14  mm.  long.  (Bardeen.)  Inset  is  the  outline  of  the  upper  part 
of  the  lower  extremity  of  a  lizard.     (Parsons.) 

cartilage  centres  have  appeared  for  the  majority  of  the  bones  of  the  lower 
extremity  (Fig.  451).  The  centres  for  some  of  the  tarsal  and  for  the 
phalanges  are  formed  before  the  end  of  the  2nd  month,  the  terminal 


LIMB  BUDS 


429 


phalanges  being  the  last.  The  acetabulum  develops  at  the  site  of  union 
of  the  iliac,  ischial  and  pubic  cartilages  at  the  end  of  the  2nd  month.  At 
that  time  the  femur  has  no  neck — a  condition  seen  in  reptiles  (Fig.  451). 
The  neck  begins  to  form  early  in  the  3rd  month.  In  the  3rd  month  the 
symphysis  pubis  is  formed. 

Torsion  and  Rotation  of  the  Limbs. — As  the  limbs  are  developed, 
the  extensor  surfaces  of  the  knee  and  elbow  are  directed  upwards.  If 
the  body  of  an  adult  were  placed  in  the  prone  position,  it  would  be  necessary, 
in  order  to  restore  the  limbs  to  their  embryonic  position,  (1)  to  draw  them 
out  at  right  angles  to  the  axis  of  the  body,  (2)  to  rotate  the  leg  outwards 
so  that  the  extensor  surface  of  the  knee  is  directed  upwards,  with  the  great 
toe  in  front  and  the  little  toe  behind.     (3)  The  arm,  on  the  other  hand, 


epiphysis  of  crest=supm-  scapula 


D. 

fax.  border) 

reot  fern.  (B) 


acetab.  (A)       J/  Subscap. 
J}ub:o/au. 


long  head 
triceps  (B) 


glenoid  (A) 


ischium=coracoid 

Fia.  452. — The  Corresponding  Points  {A,  B,  C,  and  D)  in  the  Ilium  and  Scapula. 

would  require  to  be  rotated  inwards  to  bring  the  elbow  (extensor  surface) 
into  the  dorsal  position.  The  rotation  which  brings  the  embryonic  limbs 
into  the  adult  position  appears  to  occur  at  the  junction  of  the  limb  girdle 
with  the  trunk. 

Rotation  at  the  Limb  Girdle. — To  understand  the  extent  of  this 
rotation  it  is  best  to  compare  the  scapula  and  ilium  and  pick  out  their 
corresponding  points.  The  extensors  of  the  thigh  and  arm  may  be  taken 
as  guides.  The  long  head  of  the  triceps  and  rectus  femoris  of  the  quadriceps 
manifestly  correspond  ;  their  points  of  origin — the  anterior  border  of  the 
ilium  and  axillary  border  of  the  scapula — may  be  regarded  as  homologous 
points.  The  other  corresponding  points  are  shown  in  Fig.  452.  The 
sacral  articular  surface  of  the  ilium  corresponds  to  part  of  the  supra-spinous 
fossa.  To  restore  the  limb  girdles  to  their  primitive  and  corresponding 
positions,  the  scapula  has  to  be  rotated  so  that  its  axillary  or  posterior 
border  comes  to  occupy  the  position  of  its  spine,  while  the  ilium  has  to  be 
placed  at  right  angles  to  the  spine  and  its  anterior  border  rotated  outwards 
until  it  occupies  a  position  corresponding  to  the  axillary  border  of  the 
scapula.  The  free  edge  of  the  spine  represents  a  former  border  of  the 
scapula  ;  the  supra-spinous  blade  of  the  scapula  appears  first  in  mammals. 


430 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


There  is  a  manifestly  spiral  twist  in  the  humerus,  but  it  is  doubtful  if 
this  be  in  any  way  due  to  the  torsion  which  the  limb  undergoes. 

Professor  Parsons  ^  and  Sir  A.  Geddes  ^  have  shown  that  although 
there  is  a  direct  correspondence  in  the  elements  of  the  upper  and  lower 
extremity,  the  correspondence  is  a  reversed  one — ^the  right  ilium  represent- 
ing a  "  mirror-image  "  of  the  right  scapula  which  certainly  is  true.  There 
is  no  evidence  of  a  rotation  of  the  elements  of  the  limb-girdles  during 
development.  A  reference  to  Fig.  453  will  show  that  there  is  a  correspond- 
ence between  the  structures  on  the  distal  border  of  the  fore-limb  and  on  the 
proximal  border  of  the  hind-limb.  The  subscapularis,  teres  major  and 
latissimus  dorsi  {A),  derivatives  of  a  common  flexor  mass,  correspond  to 


LEG 


ARM 


!FiG.  453. — Diagram  of  the  Fore  and  Hind  Limbs  of  the  same  side  to  show  the  "  Mirror- 
image  "  Relationship  between  their  Constituent  Parts.  The  vertical  line  passing 
through  the  umbilicus  is  regarded  as  the  centre  from  which  the  two  limbs  have 
become  differentiated.     (After  Parsons  and  Geddes.) 

the  ilio-psoas — also  the  derivative  of  a  common  flexor  mass  {A^).  The 
triceps  and  quadriceps  (C,  C^)  also  agree  ;  so  do  the  olecranon  and  ulna 
with  the  patella  and  tibia.  The  specialization  of  the  proximal  digit  of  the 
hand  to  form  a  pollex,  and  of  the  first  of  the  foot  to  form  a  hallux,  occurs 
only  in  primates.  The  mirror-image  theory  particularly  applies  to  the 
distribution  of  nerves.  To  explain  this  peculiar  relationship,  which 
exists  between  the  fore  and  hind  limbs  of  the  same  side  in  vertebrates, 
one  is  tempted  to  suppose  that  they  represent  anterior  and  posterior 
halves  of  a  single  primitive  locomotory  appendage  ;  the  line  of  separation 
is  represented  by  the  adjacent  borders  of  the  limbs.  On  such  a  theory 
the  adjacent  borders  should  be  constituted  alike. 

Segmental  Nature  of  the  Limbs. — The  nerves  of  the  limbs,  probably 
also  the  muscles,  vessels  and  skin,  are  derived  from  a  number  of  the 

1  F.  G.  Parsons,  Journ.  Anat.  and  Physiol.  1908,  vol.  42,  p.  388. 

^  Sir  Auckland  C.  Geddes,  Journ.  Anat.  and  Physiol.  1912,  vol.  46,  p.  350. 


LIMB  BUDS 


431 


primitive  body  segments.  The  4th  cervical  to  the  2n(i  dorsal  contribute 
to  the  formation  of  the  upper  extremity  ;  the  1st  lumbar  to  the  3rd  sacral 
to  the  lower,  but  even  in  man  the  extent  to  which  the  most  anterior  and 
most  posterior  of  each  of  these  contributes  to  the  limb  varies  considerably. 
Since  the  processes  of  the  skin  and  muscle  plates  of  these  segments  retain 
in  the  limbs  (so  we  infer  from  the  study  of  limbs  of  lower  vertebrates) 
their  original  nerve  supply,  it  is  evident  that  the  muscles  and  skin  of  the 
human  limbs  may  be  assigned  to  their  original  body  segments  by  a  study 
of  the  distribution  of  the  nerves.  Such  a  study  has  been  carried  out  by 
a  great  number  of  men  during  the  two  last  decades.^  The  primitive 
simple  arrangement  of  muscle  segments  may  be  seen  in  the  fins  of  certain 


neural/canai 


^ost  root  gang. 

muscle  plate 
sp.  nerue 

basis  of  head 
of  humerus 

■nerue  of  exten.  asp. 
nerues  of  flex.  asp. 

[:}  nerue  to  somatopleure 
somatopleure 


Pig.  454. — Section  of  the  Arm  Bud  of  a  Human  Embryo  at  the  end  of  the  5th  week. 
(Alex.  Low.) 

fishes,  but  in  man  these  segments  have  been  divided  and  combined  and 
special  muscles  formed  from  them  ;  yet  the  primitive  arrangement  can  be 
recognized. 

Nerve  Supply  of  the  Limbs.  The  Arm. — It  is  important  to  note 
that  the  limb  buds  arise  from  the  ventro-lateral  aspect  of  the  trunk  (Fig. 
454)  near  the  junction  of  the  somatopleure  with  the  paraxial  mesoderm. 
Therefore  the  nerves  of  the  limbs  are  the  nerves  of  the  ventro-lateral  zone 
— the  lateral  cutaneous  branches  of  the  typical  segmental  nerves  (Fig.  455). 
The  muscles  are  derived  from  the  ventro-lateral  sheet,  which  gives  rise  to 
all  the  muscles  of  the  body  wall.  As  soon  as  the  limb  buds  appear,  bundles 
of  fibres  from  the  anterior  and  posterior  nerve  roots  of  the  corresponding 

^  For  references  see  Geddes,  Jovrn.  Anat.  and  Physiol.  1912,  vol.  46,  p.  350;  also 
the  researches  of  Bolk,  Morph.  Jahrb.  from  1894  to  1898  ;  A.  T.  Kerr,  Amer.  Journ. 
Anat.  1918,  vol.  23,  p.  285. 


432 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


body  segments  enter  them  and  keep  time  with  their  growth.  The  limb 
nerves  are  at  first  so  large  in  comparison  with  the  size  of  the  limb  bud 
that  they  are  crowded  together  and  already  form  a  plexus  (Figs.  456,  457). 
As  they  enter  the  bud,  the  nerves  encounter  the  condensed  skeletal  blastema 
at  its  base  and  divide  into  a  dorsal  or  extensor  set  and  a  ventral  or  flexor 
set  (Figs.  454,  455). 

The  relationship  of  the  segmental  nerves  to  the  arm  bud  in  the  6th  week 
of  development  is  shown  in  Fig.  456 — a  drawing  taken  from  Professor 
Streeter's  research.^  The  base  of  the  arm  is  then  situated  in  the  cervical 
region  ;  the  hypoglossal  nerve  issues  almost  at  its  anterior  border.  The 
arm  descends  tailwards  during  the  2nd  month,  the  nerves  consequently 


dorsal  spinal  musc.j^. 
(epi-axial)        "^^f^ 


becomes  trapezius,  etc. 
post.  prim.  diu. 


dorsal  limb  muse. 

dorsal  ramus 

axial  structures 
of  limb 


uentro-lateral  muscles  fin 
somatopleure) 


uent.  ramus 
ventral  limb  muse. 


Fig.  455. — Schematic  Section  to  show  the  primitive  grouping  of  the  Nerves  and 
Musculature  of  Limbs.     (After  Kollmann.) 

undergoing  an  elongation.  The  ventral  divisions  of  the  spinal  nerves 
from  the  5th  cervical  to  the  1st  dorsal  have  entered  the  bud,  and  already 
the  chief  nerves  can  be  traced.  The  brachial  plexus  is  formed  ;  the 
interlacing  of  fibres  does  not  arise  owing  to  a  compression  of  the  nerves  due 
to  a  lack  of  room,  but  represents  a  physiological  or  functional  adaptation. 
Professor  Goodrich  ^  found  that  in  fishes  only  the  posterior  root  or  sensory 
fibres  entered  into  the  plexiform  arrangement — the  motor  or  ventral 
fibres  proceed  into  the  limb  without  exchanging  fibres.  By  the  beginning 
of  the  3rd  month  all  the  muscles  and  nerves  are  differentiated.  In  Fig. 
455  the  distinction  between  the  nerves  of  the  extensor  and  flexor  aspects 
of  the  limb  is  shown. 

In  Fig.  457  the  bud  of  the  hind  limb  of  the  same  embryo  is  represented. 
It  will  be  seen  that  the  stage  of  development  is  less  advanced  than  in  the 
arm.     The  crescentic  base  of  the  limb  is  in  relationship  with  the  spinal 


^  Geo.  L.  Streeter,  Amer.  Journ.  Anat.  1908,  vol.  8,  p.  285. 
^  See  reference  on  p.  425. 


LIMB  BUDS 


433 


nerves  from  the  1st  lumbar  to  the  3rd  sacral.  The  crural  and  sciatic 
plexuses  are  continuous  ;  their  separation  occurs  in  the  7th  week,  when 
the  ilium  becomes  attached  to  the  costal  processes  of  the  sacrum. 

The  nerve  supply  assists  to  indicate  the  body  segments  from  which  the 
arm  is  developed  (Fig.  458).  The  4th  cervical  is  the  most  anterior,  the  2nd 
dorsal,  sometimes  it  is  the  3rd,  is  the  most  posterior  segment.  Hence 
the  arm  is  produced  from  seven,  or  more  commonly  eight,  segments  in 
all.  Each  segment  contributes  from  its  nerve,  its  muscle  plate  and  probably 
also  its  artery  (see  p.  250).  The  typical  distribution  of  a  segmental  nerve 
to  the  limb  bud  is  shown  diagrammatically  in  Fig.  455.  Each  segmental 
nerve,  as  is  the  case  with  the  typical  lateral  cutaneous  nerves,  divides  into 
a  dorsal  division  for  the  extensor  muscles,  and  ventral  for  the  flexor  muscles. 
The  nerves  to  the  extensor  muscles  form  the  dorsal  divisions  and  cord  of 
the  brachial  plexus  ;  the  nerves  to  the  flexor  muscles  form  the  ventral 
divisions  and  the  outer  and  inner  cords.     The  processes  to  the  limbs  from 


LUMBAR 


ULNAR 
ME.DrAM 
MUSC  :  SP: 
MU5C:   CUT; 


UPPER    EXTREMITY 


LOWER    EXTREMITY 


Fig.  456. — The  Arm  Bud  and  its  Nerves  in  a  Human  Embryo  in  the  6th  week  of 

development.     (After  Streeter.) 

Fig.  457. — The  Bud  of  the  Lower  Extremity  with  its  Relationship  to  Spinal  Nerves 

in  a  Human  Embryo  in  the  6th  week  of  development.    (After  Streeter.) 

the  skin  plates  and  muscle  plates  are  also  divided  into  dorsal  and  ventral 
sets  ;  the  one  set  making  up  the  extensor  aspect  of  the  limb  ;  the  other, 
the  flexor  aspect. 

Clinical  and  experimental  research  have  shown  that  each  of  the  seven 
or  eight  segments  contributes  to  the  cutaneous  supply  of  the  limb.  The 
classical  investigations  of  Sherrington  ^  into  the  segmental  distribution  of 
the  sensory  nerves  in  the  limbs  of  apes,  showed  that  they  are  arranged  in 
a  definite  and  orderly  manner  (Fig.  459).  The  sensory  distribution  of  the 
spinal  nerves  in  the  human  arm  is  shown  diagrammatically  in  Fig.  458. 
The  distribution  of  the  motor  nerves  of  each  segment  is  fully  described 
in  anatomical  text-books. 

Only  three  anomalous  points  in  the  arrangement  of  nerves  in  the  upper 
limb  require  attention  :  (1)  The  segments  which  supply  nerves  for  the  arm 
are  nearly  constant.  The  extent,  to  which  the  4th  cervical  and  3rd  dorsal 
contribute,  varies  ;  the  degree  of  variation  is  markedly  less  than  in  the 
lower  limb.     (2)  A  part  of  the  musculo-cutaneous  nerve  frequently  joins 

1  Sherrington,  Journ.  of  Physiol.  1892,  vol.  13,  p.  621. 
2  E 


434 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


the  median  below  the  insertion  of  the  coraco-brachialis  ;  this  communica- 
tion is  frequently  seen  in  lower  primates  ;  its  meaning  is  not  known.  (3) 
A  communication  between  median  and  ulnar  in  the  forearm  is  also  common 
and  is  seen  constantly  in  some  primates.  The  communicating  branch 
passes  with  the  deep  branch  of  the  ulnar  nerve  to  the  palm.  It  is  also 
manifest  that  there  is  a  correspondence  between  the  musculo-spiral  nerve 

posl.  root  ganglia.-,:.^  '>IL_/)  a 


Fig.  458.- 


-The  Distribution  of  the  Posterior  Roots  of  the  Spinal  Nerves  on  the  Flexor 
Aspect  of  the  Arm. 


on  the  proximal  border  of  the  arm  and  the  sciatic  on  the  distal  border  of 
the  leg. 

The  Formation  of  Nerve  Plexuses  ^  depends  on  the  following  factors  : 
(1)  Each  skin  segment  is  supplied  not  only  by  its  own  nerve,  but  by  the 
nerve  of  the  segment  in  front  of  it  and  behind  it.  (2)  A  muscle  segment, 
such  as  may  be  seen  in  the  rectus  abdominis,  is  supplied  by  its  own  and  the 
two  adjacent  nerves,  the  fibres  forming  a  plexus  before  entering  the  muscle. 
(3)  Each  muscle  is  formed  by  the  combination  of  parts  of  two  or  more 

1  H.  Braus,  Verhand.  Anat.  Gesellsch.  1910,  p.  14  (Origin  of  Nerve-Plexuses). 


LIMB  BUDS 


435 


segments,  and  therefore  its  nerve  rises  from  two  or  more  spmal  nerves. 
(4)  The  muscles  of  the  limbs  have  migrated  from  their  original  positions 
and  carried  their  nerves  with  them.  (5)  Most  important  of  all,  the  afferent 
or  sensory  fibres  from  a  muscle  have  to  be  linked  to  the  centres  of  all  the 
muscles  which  act  as  its  antagonists  or  coadjutors.  All  these  influences 
have  led  to  the  nerve  fibres  being  assorted  into  definite  cords  at  their  first 
outgrowth. 

Nerve  Supply  o£  the  Lower  Limb.- — Usually  ten  segments  contribute 
to  the  nerve  supply  of  the  lower  limb — the  12th  dorsal  to  the  4th  sacral 
(Fig.  460).  The  sensory  nerves  are  derived  from  these  segments  ;  the  motor 
nerves  begin  at  the  1st  lumbar  segment  and  end  at  the  3rd  sacral.  There 
is  a  considerable  variation  in  the  number  of  body  segments  or  vertebrae 
to  which  the  lower  limb  is  attached  ;  usually  it  is  the  25th  vertebra  which 
becomes  the  1st  sacral,  but  it  may  be  the  26th  or  24th  (p.  55).     Of  these 


uent.  line  — : 


Fig.  459. — Diagram  to  show  the  typical  manner  in  which  the  Posterior  Nerve  Roots 
are  distributed  in  the  Lower  Limb  (based  on  Sherrington's  researches  into  the 
Sensory  Distribution  of  the  Limb  Nerves  of  Apes). 

three  forms,  the  first  is  the  normal  type  (25th)  ;  the  second  the  post-fixed 
type  (26th) ;  the  third  the  prefixed  type  (24th).  There  is  even  a  greater 
variation  in  the  segments  which  contribute  nerves  to  the  limb  ;  the  normal 
motor  segments  are  the  1st  lumbar  to  the  3rd  sacral ;  in  the  post-fixed  type 
(more  common  than  the  next)  the  motor  segments  commence  at  the  2nd 
lumbar  and  cease  at  the  4th  sacral ;  in  the  pre-fixed  type  the  motor  segments 
commence  at  the  12th  dorsal  and  end  at  the  2nd  sacral.  The  spinal 
nerve  which  bifurcates  and  joins  both  lumbar  and  sacral  plexuses  is  known 
as  the  nervus  furcalis.  In  the  normal  type  it  is  the  4th  lumbar  ;  in  the 
pre-fixed  type  it  is  the  3rd  lumbar  ;  in  the  post-fixed  type  the  5th 
lumbar. 

The  nervus  bigeminus,  normally  the  4th  sacral,  may  also  vary  in  a 
corresponding  manner. 

The  nerves  to  the  extensor  surface  of  the  lower  limb,  the  anterior  crural 
(femoral),  external  popliteal  (common  peroneal),  etc.,  represent  the  dorsal 
divisions  of  lateral  cutaneous  nerves  (Fig.  455).  The  nerves  to  the  adductor 
and  flexor  aspects,  the  obturator  and  internal  popliteal  (tibial),  represent 


436 


HUMAN  EMBEYOLOGY  AND  MORPHOLOGY 


the  ventral  divisions.  In  a  considerable  number  of  individuals,  tlie  dorsal 
division  (external  popliteal)  and  ventral  (internal  popliteal)  of  tlie  great 
sciatic  separate  in  the  pelvis,  tlie  external  popliteal  perforating  the 
pyriformis. 

The  segmental    distribution   of   the    motor    nerves   in  the   lower   ex- 
tremities is  given  at  length  in  text-books  on  anatomy.     The  muscular 

impost  root  gangl. 

K 

fc    crest  of  ilium 

D.I  J 

-X.D.12 


Fig.  460. — Flexor  Aspect  of  the  Lower  Limb,  showing  the  Sensory  Distribution 
of  the  Segmental  or  Spinal  Nerves. 


segments  correspond  approximately  in  their  distribution  with  those  of 
the  skin. 

It  will  be  remembered  that  the  perineal  region  is  developed  behind  the 
limb  buds  of  the  lower  extremities  (Fig.  439)  ;  hence  its  nerve  supply 
from  the  most  posterior  nerve  segments  (3rd  and  4th  sacral). 

Sherrington  found  that  the  posterior  roots  of  the  limb  nerves  were 
distributed  in  a  regular  and  simple  manner  in  apes.     His  results  are  applied 


LIMB  BUDS  437 

to  the  lower  limb  of  a  human  foetus  in  Fig.  459.  The  actual  distribution 
in  man,  which  has  been  partially  worked  out  by  clinicians,  varies  con- 
siderably from  what  might  be  expected  from  Sherrington's  results  (compare 
Figs.  459  and  460). 

In  the  human  leg  and  foot  there  is  a  tendency  for  the  nerve  fibres 
destined  for  the  outer  digits  to  proceed  in  the  external  saphenous  (sural) 
nerve  instead  of  by  the  musculo-cutaneous  (superficial  peroneal).  The 
external  saphenous  nerve  may  supply  the  4th  and  5th  digits  (the  ancestral 
form)  in  a  manner  similar  to  the  ulnar  nerve  in  the  hand  ;  more  frequently 
it  is  confined  to  the  outer  side  of  the  5th  digit.  The  outgrowing  fibres  of 
the  obturator  nerve  may  be  divided  into  ventral  and  dorsal  parts  by  the 
blastema  of  the  pubis.  In  such  a  case  the  more  ventral  fibres  cross  the 
ramus  of  the  pubis  and  form  the  accessory  obturator  nerve. 

Vessels  of  the  Limbs. — When  the  limb  buds  are  being  formed  in  the 
5th  week  they  are  permeated  by  a  capillary  network,  which  in  the  case 
of  the  arm  is  chiefly  fed  by  the  artery  of  the  7th  cervical  segment,  while 
in  the  case  of  the  leg  bud  the  chief  axial  artery  arises  from  a  pelvic  arterial 
plexus — soon  connected  with  the  internal  iliac  (hypogastric)  artery. 
During  the  6th  week  the  main  arteries  of  the  limbs  are  being  evolved  from 
pathways  in  the  primary  capillary  plexuses  ;  by  the  end  of  the  8th  week, 
all  the  important  arterial  channels  have  been  laid  down.  Every  student 
knows  how  frequently  the  arteries  of  the  leg  and  arm  depart  from  the 
arrangement  which  is  regarded  as  normal.  Comparative  anatomy  and 
embryology  throw  light  on  these  arterial  anomalies. ^ 

In  Figs.  461,  462,  the  upper  and  lower  limbs  have  been  placed  in  corre- 
sponding positions — the  extensor  surfaces  being  directed  upwards  and  a 
scheme  of  their  arteries  depicted  in  relationship  to  their  skeletal  elements. 
In  each  limb  bud  there  is  developed  a  main  or  axial  artery,  certain  parts 
of  which  are  suppressed  in  the  8th  week  while  other  accessory  vessels  are 
developed.  The  axial  artery  of  the  upper  limb  persists  as  the  subclavian, 
axillary  and  brachial  trunks,  but  in  the  lower  limb  the  corresponding 
trunk  (Fig.  362)  is  suppressed,  as  Professor  Senior  has  shown,  during  the 
8th  week  of  development — save  for  the  sciatic  branch  of  the  internal 
iliac  artery  and  the  anastomotic  chain  along  the  sciatic  nerve  which  links 
together  branches  of  the  sciatic  and  popliteal  arteries.  In  the  flexor 
aspect  of  the  elbow,  as  in  the  corresponding  space — the  popliteal — of  the 
lower  limb,  the  axial  artery  undergoes  a  degree  of  suppression.  In  the 
popliteal  space,  as  we  know  from  Professor  Senior's  investigations,  the 
axial  artery  passes  deep  to  the  popliteus  muscle  ;  the  part  which  lies  deep 
to  the  muscle  becomes  reduced  during  the  8th  week  and  a  new  vessel 
develops  superficial  to  the  muscle.  The  part  of  the  popliteal  artery 
proximal  to  the  popliteus  muscle  is  derived  from  the  axial  vessel ;  the  part 
lying  on  the  popliteus  from  the  new  trunk.  In  the  anticubital  space  the 
corresponding  axial  vessel  disappears,  the  terminal  part  of  the  brachial 

^  For  development  of  arteries  see  Prof.  H.  D.  Senior,  Journ.  Anat.  1919,  vol.  53, 
p.  1.31  ;  Amer.  Journ.  Anat.  1919,  vol.  25,  p.  55  ;  Erik  Miiller,  Anat.  Hefte,  1903,  vol.  22, 
p.  377.  For  comparative  anatomy  of  vessels  in  limbs  of  primates  see  articles  by  Dr. 
Manners-Smith,  Journ.  Anat.  and  Physiol.  1910,  1911,  1912,  vols.  44,  45,  46;  E. 
Goeppert,  Ergebnisse  der  Anat.  1904,  vol.  14,  p.  170. 


438 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


artery  with  its  divisional  trunks — the  ulnar  and  radial  arteries — repre- 
senting later  channels. 


^       X 


.S  Sf  o 


■Ms    <o 


In  the  fore-arm  the  axial  vessel  is  represented  by  the  anterior  (volar) 
interosseus,  continued  into  the  hand  to  give  ofi  the  palmar  interosseus 


LIMB  BUDS  439 

vessels — the  primary  blood  supply  of  the  hand.  On  the  extensor  or  dorsal 
aspect  of  the  interosseus  membrane  of  the  fore-arm  develops  the  dorsal 
interosseus  artery  of  the  fore-arm  fed  by  branches  of  the  axial  artery  which 
perforate  at  the  proximal  and  distal  ends  of  the  membrane  (Fig.  461). 
In  the  leg  the  axial  artery  disappears,  save  its  distal  part,  which  is  in- 
corporated in  the  peroneal  artery  (Fig.  462).  As  in  the  fore-arm,  perforating 
branches  pass  to  the  dorsal  aspect  of  the  interosseus  membrane  to  form 
the  anterior  tibial  artery. 

Having  thus  traced  the  fate  of  the  axial  artery  in  each  limb  we  now 
turn  to  the  origin  of  the  great  secondary  channels.  The  external  iliac 
artery  and  its  continuation,  the  femoral  artery,  open  up  a  new  channel 
to  the  lower  limb  along  the  course  of  the  anterior  crural  or  femoral  nerve. 
The  channel  arises  from  the  umbilical  artery  proximal  to  the  origin  of  the 
internal  iliac  (the  axial  vessel)  and  by  the  end  of  the  7th  week  has  effected 
a  union  with  that  part  of  the  axial  vessel  which  lies  in  the  popliteal  space 
(Fig.  462).  In  the  upper  limb  there  is  no  corresponding  arterial  trunk, 
although  communications  between  the  suprascapular  (transverse  scapular), 
circumflex  and  superior  deep  branch  of  the  brachial  artery  may  represent 
it.  In  both  the  leg  and  fore-arm  more  superficial  secondary  channels  are 
formed — the  ulnar  and  posterior  tibial  arteries  and  their  branches  which 
end  in  the  superficial  palmar  and  plantar  arches  (Figs.  461,  462).  In  all 
primates  with  the  exception  of  man,  the  femoral  artery,  before  piercing  the 
adductor  magnus,  gives  off  a  large  branch — the  saphenous  artery — which 
accompames  the  long  saphenous  nerve  and  turns  to  the  extensor  aspect 
of  the  leg  above  the  internal  malleolus  where  it  becomes  the  dorsal  artery 
of  the  foot.  At  no  stage  of  human  development  does  the  saphenous 
artery  serve  as  a  main  channel,  but  the  superficial  branch  of  the  anasto- 
motica  magna,  which  represents  this  vessel  in  man,  is  more  highly  developed 
at  the  8th  week  than  it  is  at  any  subsequent  period.  The  saphenous 
artery  corresponds  to  the  radial  of  the  upper  limb. 

Vas  Aberrans. — At  a  very  early  stage  (7th  week)  there  is  developed 
along  the  superficial  aspect  of  the  median  nerve  an  arterial  anastomotic 
channel  fed  by  a  succession  of  branches  which  spring  from  the  axial 
brachial  vessel  (Fig.  461).  This  channel  frequently  opens  up  in  part,  or 
even  in  its  whole  extent  and  gives  rise  to  the  greater  number  of  arterial 
anomalies  met  with  in  the  arm.  The  vas  aberrans  may  replace  the  main 
artery,  being  known  from  the  normal  brachial  artery  by  the  fact  that  it 
lies  superficial  to  the  median  nerve,  whereas  the  usual  vessel  is  deep  to  that 
nerve.  The  first  ramus  of  the  vas  appears  between  the  heads  of  the 
median  nerve  (Fig.  461).  The  ulnar  or  radial  artery  frequently  arises 
from  the  brachial  artery  in  the  lower  third  of  the  arm  ;  in  such  cases  the 
upper  part  of  the  radial  or  ulnar  vessels  will  be  found  to  be  formed  out  of 
the  anastomotic  channel.  In  the  fore-arm  the  median  artery  may  be  of 
large  size,  ending  in  the  superficial  palmar  arch  ;  it,  too,  is  formed  out  of 
the  arterial  anastomosis  which  is  laid  down  in  foetal  life,  superficial  to  the 
median  nerve. 

Superficial  Veins. — During  the  6th  week  the  terminal  margin  of  the 
limb  buds  is  fringed  with  a  venous  plexus  which  becomes  broken  up  by  the 


UO  HUMAN  EMBEYOLOGY  AND  MORPHOLOGY 

outgrowth  and  differentiation  of  tlie  digits.  The  terminal  plexus  is  drained 
by  a  vein  which  passes  upwards  on  the  fibular  or  peroneal  margin  of  the 
limb,  this  marginal  vessel  becoming  the  superficial  ulnar  and  basilic  veins 
in  the  arm  and  the  external  saphenous  vein  in  the  leg.  Later,  radial  and 
tibial  marginal  venous  channels  are  formed,  becoming  the  radial  and 
cephalic  veins  in  the  upper  limb  and  the  long  or  internal  saphenous  in  the 
lower.  The  cephalic  vein  originally  crosses  the  clavicle  and  terminates 
in  the  external  jugular  vein  as  is  the  rule  in  apes,  but  later  ends  in  the 
axillary  vein,  below  the  clavicle.  In  man  only  does  the  long  saphenous 
vein  terminate  at  the  groin  by  piercing  a  hiatus  in  the  fascia  lata  ;  in  all 
other  primates  it  ends  above  the  internal  (mesial)  condyle  of  the  femur 
by  joining  the  femoral  veins  in  Hunter's  canal. 


CHAPTER  XXVII. 

MOEPHOLOGY  OF  THE   LIMBS. 

In  the  previous  chapter  the  chief  events  connected  with  the  development 
of  the  limb  buds  in  the  human  embryo  have  been  noted  and  incidentally 
certain  points  relating  to  the  morphology — primitive  structure — of  the 
limbs  have  been  alluded  to.  In  the  present  chapter  we  propose  to  deal 
with  the  more  important  problems  relating  to  the  pectoral  and  pelvic 
girdles,  to  the  bones  of  the  hand  and  foot,  to  the  origin  of  joints  and  to  the 
significance  of  certain  muscular  modifications. 

Congenital  Elevation  o£  the  Shoulder.^ — We  have  already  seen  that 
the  arm  of  the  human  embryo  is  cervical  in  position — in  this  respect 


1^"^  DORSAL 


OMO-VERTEBRAl. 
BONE 


Fig.  463. — The  Omo-vertebral  Bone  in  a  Case  of  Congenital  Elevation  of  the  Shoulder. 

resembling  the  pectoral  fins  of  fishes.  It  descends  during  the  2nd  month, 
reaching  its  final  position  over  the  ribs  in  the  3rd  month.  Its  descent  is 
not  only  accompanied  by  an  elongation  of  the  brachial  nerves,  but  also 
by  a  downward  migration  of  certain  muscles — originally  placed  in  the  neck 
— trapezius,  serratus  magnus,  latissimus  dorsi  and  pectoral  muscles. 
The  descent  may  be  arrested.  The  condition,  which  is  not  rare  in  children, 
is  often  accompanied  by  irregularities  in  the  formation  of  the  cervical 
vertebrae — for  the  elongation  of  the  cervical  region  to  form  a  neck  is 
related  to  the  descent  of  the  shoulder,  of  the  heart,  and  of  the  diaphragm 
— and  with  the  appearance  of  a  skeletal  element  of  the  shoulder  girdle 
which  is  present  in  certain  fishes  (dipnoi  and  selachians).  This  omo- 
vertebral  element  is  represented  in  Fig.  463 — from  the  classical  case  of 

^  The  condition  is  often  spoken  of  as  SprengeVs  shoulder.  See  H.  A.  T.  Fairbank, 
Brit.  Med.  Journ.  1911,  ii.  p.  1533.  For  recent  literature  see  D.  M.  Greig,  Edin.  Med. 
Journ.  1910,  vol.  5,  p.  236  ;  1911,  vol.  6,  p.  242. 

441 


442      HUMAN  EMBEYOLOGY  AND  MORPHOLOGY 

Willet  and  Walsham  (1880).  In  fishes  this  bone  joins  the  supra-scapula 
to  the  occiput ;  when  it  appears  in  man  it  is  usually  fixed  to,  or  articulates 
with,  the  spinous  processes  of  the  lower  cervical  vertebrae.  Man's  upright 
posture  has  thrown  the  duty  of  constantly  supporting  the  shoulder  on  the 
trapezius.  Under  certain  circumstances  it  gives  way,  the  shoulders  then 
drooping.  Symptoms  may  then  arise  from  pressure  of  the  nerves  against 
the  1st  rib — or  a  cervical  rib.^ 

Pelvic  and  Shoulder  Qirdles. — In  the  basal  part  of  each  limb  bud  a 
cartilaginous  arch  is  developed.  It  consists  of  a  dorsal  and  ventral  part, 
the  joint  cavity  for  the  articulation  of  the  limb  being  situated  at  the  junction 
of  the  two  parts.     Fishes  retain  this  simple  primitive  form  of  girdle. 

The  pelvic  girdle  has  undergone  less  modification  from  the  primitive 

type  than  the  shoulder  girdle.     The  primitive  type  of  pelvic  girdle,  such 

as  is  seen  in  the  crocodile  or  lizard,  and  of  which  the  mammalian  type  is 

a  derivative,  is  shown  diagrammatically  in  Fig.  464.     For  comparison  the 

.  human  girdle  in  the  7th  week  foetus  is  shown  in  Fig.  465. 

^ventral  median  line 

ilium 


cost  proc. 

acetabulum 
\ 
ischium 

.cloaca 

Fig.  464. — Diagram  of  the  Pelvic  Girdle  of  a  Lizard. 

The  dorsal  element  consists  of  the  ilium  ;  it  is  attached  by  ligaments 
to  the  costal  process  of  one  or  more  sacral  vertebrae.  In  the  ventral 
portion  of  the  mesenchymal  arch  are  developed  two  cartilaginous  elements 
the  pubes  and  ischium,  both  of  which  take  part  in  the  formation  of  the 
acetabulum  (Fig.  464).  Both  reach  the  ventral  median  line  in  which  a 
median  bar  of  cartilage  is  developed  (see  Fig.  442). 

In  man  the  following  changes  may  be  noted  :  (1)  The  costal  processes 
of  the  sacral  vertebrae  (2|  usually)  have  fused  together  to  form  the  lateral 
sacral  mass  ;  with  these  the  ilium  articulates  (Fig.  451)  ;  (2)  the  vertebral 
border  (crest)  has  become  enormously  elongated  and  gives  attachment  to 
abdominal  muscles,  cutting  ofi  the  fibres  of  insertion  of  the  external  oblique 
which  form  the  chief  part  of  Poupart's  ligament ;  (3)  the  ischium  does  not 
reach  the  ventral  line.  In  most  birds,  neither  ischium  nor  pubis  reaches 
the  ventral  line.  The  pubes  fail  to  meet  in  cases  of  ectopia  vesicae,  just 
as  the  sternum  is  cleft  in  cases  of  ectopia  cordis.  The  symphysis  pubis 
is  formed  in  the  ventral  line  during  the  3rd  month.  The  cotyloid  bone — 
OS   acetabuli — is  formed  in  the   Y-shaped   cartilage   between  the  three 

1  Prof.  T.  W.  Todd,  Anat.  Anz.  1912,  vol.  41,  p.  385. 


MORPHOLOGY  OF  THE  LIMBS 


443 


elements.  It  ossifies  in  the  ISth  year.  Professor  Howes  has  pointed  out 
that  it  is  this  ossification  which  forms  the  pubic  part  of  the  acetabulum, 
and  that  it  is  really  part  of  the  pubes. 

The  median  pelvic  bar  ^  corresponds  to  the  sternum,  and  like  it  is  of 
bilateral  origin.  In  reptiles  (Fig.  442)  it  is  divided  into  anterior,  middle 
and  posterior  parts.  The  anterior  parts  form  the  cartilaginous  epiphysis 
of  the  pubic  crest,  which  represent  the  marsupial  bones,  and  correspond 
to  the  supra-sternal  ossifications  ;  the  middle  parts  become  the  cartil- 
aginous surfaces  of  the  symphysis  ;  the  posterior  parts  (the  hypoischium 
of  reptiles)  form  the  epiphyses  on  the  pubic  arch  and  ischial  tuberosity 
(Parsons). 

Congenital  Dislocation  o£  the  Hip  Joint. — Under  this  title  two  quite 
different  groups  of  cases  are  included  :    (1)  cases  in  which  there  has  been 


ant  crur.  nerve 
ubes 


1st  sac 


median  ventral  line 
obt  nerve 
acetabulum 
ischium 

gr.  sc.  nerve 


coccyx 

Fig.  465. — The  Pelvic  Girdle  of  a  Human  Foetus  at  the  7th  week.     (After  KoUmann.) 

an  arrest  of  development  of  the  parts  entering  into  the  formation  of  the 
hip  joint ;  (2)  cases  which  are  produced  during  the  act  of  birth.  It  is 
only  the  first  group  which  is  referred  to  here.  In  the  8th  week — when 
the  foetus  is  about  20  mm.  in  length,  the  three  cartilaginous  elements 
— ilium,  ischium  and  pubis — meet  in  a  Y-shaped  acetabular  suture,  the 
pubic  element  being  later  in  chondrifying  than  the  other  two.  In  the  9th 
week  the  hip  joint  is  formed  by  (1)  the  appearance  of  a  synovial  cavity, 
(2)  cartilaginous  outgrowths  from  all  three  elements,  but  especially  from 
the  iliac — ^to  form  the  acetabular  cup  ;  (3)  the  separation  of  the  head  from 
the  shaft  of  the  femur  by  the  formation  of  the  neck  (see  p.  429).  The 
joint  is  completely  formed  early  in  the  3rd  month.  The  synovial  lining 
of  the  joint  arises  from  an  ingrowth  of  peripheral  cells  into  the  blastemal 
tissue  between  the  acetabulum  and  head  of  the  femur  (Jenkins  ^).  The 
outgrowth  of  the  acetabular  brim  may  be  arrested  at  the  reptilian  stage 
reached  in  the  2nd  month  ;  congenital  dislocation  of  the  femoral  head, 
which  is  fully  formed,  results.     In  the  cases  of  cleft  palate  and  imperforate 

^Prof.  T.  W.  Todd,  Amer.  Journ.  Physic.  Anthrop.  1920,  vol.  3,  p.  285  (age  changes 
in  pubic  bone). 

"  G.  T.  Jenkins,  Brit.  Med.  Journ.  1906,  vol.  2,  p.  1702. 


444 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


anus  (and  tMs  is  a  similar  case)  human  development  is  arrested  at  a 
reptilian  stage.  The  condition  has  an  obscure  relation  to  the  development 
of  the  female  sexual  characters  ;   90  %  of  cases  occur  in  female  infants. 

Shoulder  Girdle. — The  duckbill  (ornithorhynchus)  shows  the  most 
generalized  type  of  mammalian  shoulder  girdle  ;  it  resembles  closely  the 
primitive  reptilian  type  ;  from  such  a  form  the  various  types  of  mammalian 
shoulder  girdle  were  probably  evolved. 

The  dorsal  part  of  the  arch  consists  of  (1)  scapula,  (2)  supra-seapula 
(Fig.  466).  The  supra-scapula  is  represented  in  man  by  the  cartilage 
along  the  vertebral  border  ;  it  ossifies  in  the  early  years  of  manhood. 
The  supra-spinous  part  of  the  scapula  appears  first  in  higher  mammals  ; 
it  is  produced  late  in  the  development  of  the  scapula  (in  the  3rd  month 
of  foetal  life)  by  the  upgrowth  of  the  supra-spinous  blade  of  the  scapula  ; 
it  is  not  represented  in  the  pelvic  girdle.     The  dorsal  segment  of  the 


spme 


clavicle  (dermal) 

inter  clavicle  (dermal) 


spine 


supra-sc. 


ma  nub.       ^^^yT'i^ 2nd  rib 

Fig.  466. — The  Shoulder  Girdle  of  Ornithorhynchus. 


pelvic  girdle  becomes  fixed  to  the  costal  processes  ;  the  corresponding  part 
of  the  scapula  remains  free. 

In  the  typical  reptilian  shoulder  girdle,  as  in  the  pelvic  (Fig.  464), 
two  elements  are  formed  in  the  ventral  part  of  the  arch — a  posterior  part 
— the  coraeoid,  corresponding  to  the  ischium,  and  an  anterior — the  pre- 
coracoid,  corresponding  to  the  pubes.^  Both  elements  reach  the  ventral 
median  line  in  which  the  sternum  is  developed  (p.  419).  In  ornithorhynchus 
the  coraeoid  element  is  represented  by  two  bones — the  coraeoid  and  epicora- 
coid — the  second  of  which  is  formed  from  the  anterior  end  of  the  sternal 
bar  and  therefore  corresponds  to  the  suprasternal  ossification  of  man. 

^  I  have  repeated  the  statement  made  in  former  editions,  but  the  reader  will  perceive 
if  the  mirror-image  correspondence  is  true  (p.  430),  that  the  ischium  on  the  distal 
side  of  the  pelvic  girdle  corresponds  with  the  coraeoid  on  the  proximal  side  of  the 
shoulder  girdle — as  stated  above — but  the  representative  of  the  pubis  should  be  on 
the  distal — not  on  the  proximal  side  of  the  shoulder  girdle  as  the  clavicle  is  placed. 
I  am  convinced  that  there  is  no  pubic  representative  in  the  shoulder.  Developmental 
phenomena  show  that  the  clavicle  is  a  new  formation.  Dr.  D.  M.  S.  Watson  (reference 
p.  425)  has  shown  that  in  the  evolutionary  history  of  the  shoulder  girdle  the  precoracoid 
is  the  first  element  to  reach  the  mid  ventral  or  sternal  line  and  that  later  it  is  supplante  d 
by  the  coraeoid  element.  He  regards  the  epicoracoid  of  ornithorhynchus  as  corre- 
sponding to  the  precoracoids  of  amphibia. 


MOEPHOLOGY  OF  THE  LIMBS 


445 


The  dorsal  extremity  of  the  coracoid  helps  to  form  the  glenoid  cavity  ; 
its  ventral  articulates  with  the  presternum.  In  man  and  all  higher 
mammals,  in  which  mobility  of  the  fore  limb  is  of  advantage  for  speed  and 
free  movement,  the  coracoid  element  is  much  reduced.  It  forms  merely 
a  process  on  the  scapula,  which  it  joins  in  man  about  the  15th  year.  It 
still  enters  into  the  formation  of  the  glenoid  cavity,  the  articular  part 
(supra-glenoid)  having  a  separate  centre  of  ossification  which  appears  in 
the  12th  year.  It  is  possible  that  the  costo-coracoid  ligament  may  be 
derived  from  the  ventral  part  of  the  coracoid  element — the  part  which 
articulates  with  the  sternum  in  the  duckbill.  The  precoracoid  in  the 
shoulder  girdle  of  a  lizard  corresponds  to  the  pubic  element  in  the  pelvis. 
The  precoracoid,  which,  like  all  the  primitive  elements  of  the  pelvic  and 
shoulder  girdle,  is  formed  in  cartilage,  has  been  partly  or  entirely  replaced 

inter-clau.  lig.  (inter-clauicle) 
^represent  pre-or  epi-coracoid 

memb.  clauicle  (pn'mitiue  clau. ) 
f  cartilage 

acrom. 

coraco-clau.  figs, 
coracoid 
glenoid 


post.  asp.  mamtb. 


right  scap.  fr.  behind 


Fig.  467. — The  Parts  in  the  Shoulder  Girdle  of  a  Human  Foetus  which  correspond 
with  those  of  Ornithorhynchus. 

in  all  mammals  by  the  development  over  it  of  the  clavicle,  a  dermal  or 
membrane-formed  bone,  the  first  of  all  the  bones  to  ossify.  There  is  thus 
no  true  representative  of  the  clavicle  in  the  pelvis.  The  interclavicle  so 
strongly  developed  in  the  ornithorhynchus  and  in  the  "  merry-thought  " 
of  the  fowl  is  also  a  dermal  bone.  It  is  represented  in  man  by  the  inter- 
clavicular ligament. 

In  order  to  give  greater  mobility  and  speed  to  some  four-footed  mammals, 
the  clavicle  has  been  reduced  to  a  ligamentous  band,  except  at  its  ex- 
tremities (rabbit,  dog,  etc.).  In  climbing  animals,  and  those  in  which  the 
power  of  grasping  or  embracing  is  highly  developed,  the  clavicles  are 
fully  developed. 

Clavicle.^ — At  the  beginning  of  the  7th  week  the  clavicle  is  represented 
by  a  cellular  or  blastemal  bar  passing  from  the  neighbourhood  of  the 
acromial  process  of  the  scapula  to  end  ventrally  in  the  anterior  end  of  the 
sternal  blastema.     Professor  Fawcett  found  that  during  the  7th  week, 

^  For  development  of  clavicle  :  Prof.  E.  Fawcett,  Journ.  Anat.  1913,  vol.  47,  p.  225  ; 
Pr.  N.  C.  Rutherford,  ibid.  1914,  48,  p.  355  ;  for  sexual  characters  Prof.  F.  G.  Parsons, 
ibid.  1917,  vol.  51,  p.  71. 


446 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


when  the  embryo  is  15  mm.  long,  two  centres  of  chondrification  appear  in 
the  clavicular  blastema,  quite  close  to  each  other,  one  corresponding  to 
the  termination  of  fibres  of  the  sternomastoid,  the  other  to  the  ending  of 
fibres  of  the  trapezius.  Before  proper  cartilage  has  had  time  to  form 
centres  of  ossification  appear  in  the  adjacent  margins  of  the  two  precartil- 
aginous  masses,  the  two  ossific  centres  uniting  in  a  few  days.  From  this 
double  centre  ossification  spreads  during  the  8th  week  towards  the  sternum 
and  towards  the  acromion,  ossification  being  preceded  by  a  true  formation 
of  cartilage. 

There  is  a  malformation  of  the  clavicle  which  throws  light  on  its  double 
nature.  In  the  remarkable  disorder  of  growth  known  by  the  cumbersome 
name  of  cleido-eranial  dysostosis  ^  the  clavicle  is  made  up  of  two  parts — an 
outer  and  inner,  united  by  a  fibrous  band  which  may  form  only  a  short 
ligament,  or  may  even  represent  the  middle  two-thirds  of  the  bone.  In 
such  cases  all  the  bones  of  the  skeleton  which  are  formed  in  membrane — 


Fig.  468.— The  Carpal  Bones  of  a  Tortoise. 

especially  those  of  the  cranial  vault — are  imperfectly  ossified.  The 
condition  in  this  disease  suggests  that  the  clavicle  is  a  compound  bone 
made  up  of  outer  and  inner  elements,  and  that  arrest  has  occurred  before 
the  two  elements  have  become  joined.  In  cleido-eranial  dysostosis  some 
condition  occurs  which  arrests  the  union  of  the  two  ossific  centres  ;  as 
ossification  in  cartilage  proceeds  normally  in  such  cases  we  may  presume 
that  in  spite  of  the  appearance  of  cartilage  in  the  clavicle,  it  was  originally 
a  membrane  bone. 

The  acromion  process  is  ossified  from  several  centres  which  appear  in 
the  years  of  adolescence  ;  the  epiphysis  so  formed  may  be  united  to  the 
spine  by  fibrous  tissue  only.  This  occurs  in  over  8  %  of  subjects  (Syming- 
ton), and  may  be  mistaken  for  a  fracture  of  the  process.  The  coraco- 
clavicular  ligaments  may  be  derived  from  the  precoracoid  element. 

Hand  and  Foot. — The  hand  and  foot  of  man,  as  is  the  case  in  all 
primates,  retain  the  primitive  arrangement  of  elements  much  more  closely 
than  do  most  other  mammalian  orders.  The  primitive  type  of  hand  or 
foot,  out  of  which  the  various  forms  found  in  mammals  have  been  modified, 
are  seen  in  such  reptiles  as  the  lizard  or  tortoise  (Fig.  468).  In  the  hand 
1  D.  Fitzwilliams,  Lancet,  1910,  vol.  2,  p.  1466. 


MORPHOLOGY  OF  THE  LIMBS  447 

of  man  the  same  bones  are  to  be  seen  as  in  the  tortoise,  and  in  the  same 
order  of  arrangement,  with  some  exceptions.  The  elements  in  the  foot 
of  a  typical  lizard  resemble  closely  the  arrangement  seen  in  its  hand  ; 
the  same  elements  are  present  even  in  the  highly  modified  human  foot. 
The  hand  and  foot  bones  have  undergone  great  specialization  in  most 
mammals.  In  the  evolution  of  the  horse,  for  instance,  one  lateral  digit 
after  another  has  become  vestigial,  leaving  the  central  digit  enormously 
enlarged  and  specialized  to  form  the  lower  part  of  the  extremities.  In 
ruminants  the  3rd  and  4th  digits  have  become  predominant ;  the  rest  of 
the  digits  have  become  reduced  until  only  traces  of  them  are  left ;  in 
rodents  the  hallux  is  vestigial.  The  hallux  and  pollex  are  the  mammalian 
digits  most  liable  to  undergo  retrogression.  In  man,  on  the  other  hand, 
the  hallux  and  pollex  find  their  greatest  development. 

/^>j,___      ^^os  calcis  (fibulare) 


.  OS  trigonum  {intermedium) 
.astrag.  (tibiale) 


cuboid  (4.  5  ^am/e)_J^^f^^— ^^«^^^^'^  {centrale) 

Fig.  469. — The  Os  Trigonum  and  other  Bones  of  the  Tarsus. 

Comparison  of  the  Tarsus  and  Carpus.^ — Both  are  the  derivatives 
of  such  a  typical  form  as  is  shown  in  Fig.  468.  In  the  typical  tarsus  or 
carpus  there  occur  the  following  bones  : 

1.  Badiale  or  Tibiale  forms  the  scaphoid  in  the  hand  and  astragalus  in 
the  foot. 

2.  Intermedium  forms  the  semilunar  in  the  hand  ;  in  the  foot  it  is 
much  reduced  and  usually  unites  with  the  astragalus  to  form  the  external 
tubercle  of  that  bone.  It  may  remain  separate  and  form  the  os  trigonum 
(Fig.  469). 

3.  Ulnare  becomes  the  cuneiform  in  the  hand,  the  os  calcis  in  the  foot. 
During  the  cellular  and  early  cartilaginous  stages  in  the  development  of 
the  human  tarsus,  the  os  calcis  is  in  contact  with  the  fibula.  In  the  hand 
the  ulnare  and  intermedium  are  bound  by  fibrous  bands  to  the  ulna  (Fig. 
468)  ;  these  bands  assist  to  form  the  triangular  fibro-cartilage  ;  in  the 
ankle  the  corresponding  bands  form  the  middle  and  posterior  fasciculi 
of  the  external  lateral  ligament. 

4.  Carpale  or  Tarsale  I.  becomes  the  trapezium  in  the  hand,  the  internal 
cuneiform  in  the  foot.  In  the  prehensile  foot  of  apes,  the  hallucial  articular 
surface  is  directed  inwards  for  the  movable  big  toe.     This  is  also  the  case 

1  See  note  on  p.  444  and  Fig.  453.  Papers  on  carpal  and  tarsal  bones  :  see  R.  B.  S . 
Sewell,  Journ.  Anat.  and  Phynol,  1906,  vol.  40,  p.  152  (Astragalus)  ;  T.  Manners- 
Smith,  Journ.  Anat.  and  Physiol.  1907,  vol.  41,  p.  255  (Navicular  of  Foot) ;  H.  M. 
Johnston,  Journ.  Ayiaf.  and  Physiol.  1907,  vol.  41,  p.  59  (Scaphoid  of  Hand). 


448     HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

during  the  foetal  development  of  the  human  foot  (Leboucq).  At  no 
period  of  development  is  the  hallux  of  man  directed  inwards  and  separated 
from  the  other  toes.  In  man  the  great  toe  resumes  a  primitive  position, 
and  its  metatarsal  lies  in  line  with  the  metatarsal  series. 

5.  Carpale  or  Tarsale  II.  forms  the  trapezoid  in  the  hand,  the  middle 
cuneiform  in  the  foot. 

6.  Carpale  or  Tarsale  III.  forms  the  os  magnum  in  the  hand,  the  external 
cuneiform  in  the  foot. 

7.  Carpale  or  Tarsale  IV.  and  V.  have  united  in  both  hand  and  foot  to 
form  the  unciform  and  cuboid.  This  union  occurs  in  all  mammals.  The 
unciform  process  has  a  separate  centre  of  chondrification  (Lewis).  In 
the  cat  and  carnivores  the  scaphoid  and  semilunar  unite  together,  a  union 
which  may  occur  in  man.  In  the  foot  an  intimate  union  persists  at  the 
junction  of  the  os  calcis,  cuboid  and  scaphoid  until  late  in  the  3rd  month  ; 
in  the  cartilage  of  union  a  separate  tarsal  element  may  develop.^ 

The  Os  Centrale  is  situated  between  the  first  and  second  rows  of  the 
carpal  or  tarsal  bones  (Fig.  468).  In  the  foot  it  forms  the  scaphoid — a 
bone  which  plays  an  important  part  in  the  formation  of  the  plantar  arch 
— but  is  yet  remarkably  late  in  beginning  to  ossify,  viz.  about  the  3rd 
year.  It  appears  at  the  end  of  the  6th  week  as  a  separate  cartilage  element 
of  the  human  carpus,  but  at  the  end  of  the  2nd  month  it  has  joined  the 
dorsal  and  distal  aspect  of  the  scaphoid  of  the  hand.  It  may  be  occasion- 
ally detected  as  a  tubercle  on  the  dorsal  aspect  of  the  scaphoid,  or  even 
as  a  separate  bone.  It  is  a  separate  bone  in  the  carpus  of  all  primates 
except  the  gorilla,  chimpanzee  and  man.  There  are  two  centralia  in 
lower  vertebrate  forms.  The  styloid  process  at  the  base  of  the  3rd  meta- 
carpal bone  may  occur  as  a  separate  ossification  {os  styloideum). 

The  Pisiform  (ulnare  laterale  of  Forsyth  Major)  is  of  doubtful  nature. 
It  is  possible  that  in  a  very  early  stage  of  the  evolution  of  mammals  there 
were  more  than  five  digits — one  behind  the  little  finger — post  minimi 
digiti ;  and  another  on  the  radial  side  of  the  hand — a  prehallux.  Super- 
numerary digits,  when  they  appear,  are  commonly  situated  on  the  radial 
side  of  the  thumb  or  ulnar  side  of  the  little  finger,  but  they  may  represent 
merely  a  fission  of  the  normal  pollex  or  little  finger.  The  pisiform  has 
been  regarded  as  the  vestige  of  a  post-minimal  digit ;  the  sesamoid  on  the 
trapezium,  in  which  a  slip  of  the  extensor  ossis  metacarpi  pollicis  ends,  as  a 
remnant  of  a  prehallux.  It  is  possible  also  to  regard  the  pisiform  as  a 
sesamoid  developed  in  the  tendon  of  the  flexor  carpi  ulnaris — for  that 
muscle  is  originally  a  flexor  of  the  metacarpus  and  ends  on  the  5th  meta- 
carpal— the  pisimetacarpal  ligament  representing  the  terminal  part  of  the 
tendon.  The  pisiform,  however,  is  developed  with  the  rest  of  the  carpal 
bones  and  before  the  tendon  of  the  flexor  carpi  ulnaris.  In  mammals 
generally,  but  not  in  man,  the  pisiform  articulates  with  the  ulna  as  well  as 
the  cuneiform,  and  its  synovial  facet  opens  into  the  wrist  joint.  It  may 
be  represented  in  the  foot  by  the  heel  epiphysis  of  the  os  calcis.  The 
gastrocnemius,  which  represents  the  flexor  carpi  ulnaris  in  the  leg,  is  also 

^  For  extra  carpal  and  tarsal  bones  see  Pfitzner,  Morphol.  Arbeiten,  1896,  vol.  6, 
p.  245. 


MORPHOLOGY  OF  THE  LIMBS  449 

primitively  a  flexor  of  the  metatarsus  ;  tlie  long  plantar  ligament,  from 
which  it  is  separated  by  the  growth  of  the  heel,  represents  the  continuation 
of  its  tendon. 

Eversion  o!  the  Foot  and  Development  of  the  Arch.— The  human 
foot  has  been  highly  modified  for  upright  progression.  The  chief 
modifications  are  : 

(1)  Gradual  eversion  of  the  foot,  so  that  the  sole  can  be  applied  to  the 
ground.  Even  at  birth — and  for  some  time  after — and  always  up  to  and 
before  the  7th  month  of  foetal  life,  the  soles  of  the  feet  are  inverted,  so  that 
when  the  foetal  limbs  are  in  their  natural  position  they  are  directed  towards 
the  belly  of  the  child.  In  club  foot  the  natural  process  of  eversion  does 
not  take  place.     The  ape's  foot  is  kept  normally  in  the  inverted  position, 

right  astrag.  at  birth 

nech  long  and.     Z'  .•••■'■\ -, 

bent  inwards  \    >^fx      \  X-  '''9^^  astrag.  of  adult 

tib.  artic.  surface-^j 
\ 


K.J 


Fia.  470. — The  Foetal  and  Adult  (in  dotted  outline)  Forms  of  the  Astragalus 
contrasted. 

an  adaptation  for  prehension.     The  following  factors  assist  in  producing 
eversion  : 

(a)  The  neck  of  the  astragalus  (Fig.  470),  which  in  the  foetal  foot  is 

long  and  directed  downwards  and  inwards  at  an  angle  to  the  axis 
of  its  body,  becomes  relatively  shorter  and  directed  more  in  line 
with  the  axis  of  the  articular  surface  of  its  body  (Fig.  470).  Further, 
the  lateral  border  of  the  tibial  articular  surface  of  the  astragalus 
is  prominent  in  the  foetus  ;  the  mesial  border  is  much  the  lower  ; 
a  growth  upwards  of  the  mesial  border  causes  the  astragalus  and 
foot  to  rotate  outwards  (Lazarus). 

(b)  The  bones  on  the  inner  side  of  the  foot,  particularly  the  scaphoid 

and  internal  cuneiform,  grow  more  rapidly  than  those  on  the  outer 
side  of  the  foot — especially  after  birth.  This  tends  to  evert  the 
foot  and  also  to  produce  the  longitudinal  arch. 

(c)  A  special  evertor  of  the  foot  is  produced — the  peroneus  tertius — 

a  muscle  peculiar  to  man.  It  is  developed  from  the  outer  and 
lower  fibular  fibres  of  the  extensor  longus  digitorum  and  represents 
part  of  the  tendon  of  that  muscle  to  the  5th  toe.  The  peroneus 
brevis  and  longus  may  also  assist,  especially  the  latter,  which 
in  apes  is  a  grasping  muscle,  acting  as  a  flexor  of  the  metatarsal 
bone  of  the  hallux. 

(2)  The  tarsal  bones  of  the  human  foot — -especially  the  astragalus  and 
OS  calcis — are  of  great  size  when  compared  with  the  tarsal  bones  of  other 


450     HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

primates  ;  while  the  digital  or  phalangeal  elements,  except,  in  the  case 
of  the  great  toe,  which  is  relatively  of  great  size,  have  undergone  retro- 
gression. This  is  especially  the  case  in  the  human  little  toe  ;  some  of  its 
muscles  are  not  infrequently  fibrous,  and  the  terminal  phalanx  may  not 
be  separated  from  the  middle  phalanx.  The  terminal  phalanx  is  the  last 
to  be  differentiated  in  development  of  the  fingers  and  toes  (in  3rd 
month). 

(3)  The  plantar  arches,  both  longitudinal  and  transverse,  are  produced. 
The  arch  of  the  foot  is  a  human  character.  At  birth  the  child  is  flat- 
footed  when  the  weight  of  the  body  rests  on  its  feet ;  the  head  of  the 
astragalus  touches  the  ground.  When  the  muscles  are  removed  by 
dissection  the  foot  of  the  newly  born  child  shows  a  well-developed  arch 
(RusseU  Howard).  The  arch  becomes  stable  as  the  child  learns  to  walk. 
The  chief  factor  in  its  production  is  the  growth  of  the  tarsal  bones — 
especially  of  the  scaphoid  and  internal  cuneiform — and  1st  metatarsal 
and  the  co-ordinated  action  of  the  muscles.  Hence  in  rickets,  where  the 
normal  tarsal  growth  is  disturbed,  the  occurrence  of  flat  foot.  Amongst 
the  structures  which  help  to  maintain  the  arch  are  : 

(a)  The  growth  of  the  os  calcis  to  form  a  prominent  heel  separates 
the  tendon  of  the  plantaris  from  its  prolongation  in  the  sole — the 
middle  part  of  the  plantar  fascia,  which  assists  in  maintaining  the 
arch.  In  lower  primates  the  two  parts  are  continuous,  the  tendon 
of  the  plantaris  plying  across  the  os  calcis  in  a  cartilage-lined 
groove. 

(6)  The  internal  lateral  ligament  of  the  ankle  (anterior  part)  and  the 
inferior  calcaneo-scaphoid  ligaments  undergo  great  development 
in  man. 

(c)  The  flexor  brevis  digitorum  which  in  lower  primates  arises  principally 

from  the  long  flexor  tendons  in  the  sole,  has  its  origin  completely 
transferred  to  the  os  calcis  in  man.  It  can  thus  act  more  powerfully 
in  maintaining  the  arch.  The  flexor  accessorius,  a  detached  part 
of  the  flexor  longus  hallucis,  is  specially  well  developed  and  helps 
to  maintain  the  arch  of  the  foot. 

(d)  The  tibialis  posticus,  originally  a  flexor  of  the  metatarsus,  correspond- 

ing to  the  flexor  carpi  radialis  in  the  hand,  obtains  a  secondary 
attachment  to  the  scaphoid.  The  tibialis  anticus,  which  answers 
to  the  extensor  ossis  metacarpi  poUicis,  becomes  permanently 
inserted  into  the  internal  cuneiform  and  metatarsal.  Both  of  these 
muscles,  thus  modified,  help  to  maintain  the  arch  of  the  foot.  So 
does  the  tarsal  part  of  the  tendon  of  the  tibialis  posticus. 

(e)  The  long  plantar  ligament,  originally  a  part  of  the  tendon  of  insertion 

of  the  gastrocnemius — also  assists  to  maintain  the  arch. 

(4)  The  development  of  the  great  toe  and  the  peculiar  arrangement  of 
its  muscles  must  also  be  regarded  as  adaptations  in  the  foot  to  upright 
posture  and  progression. 


MORPHOLOGY  OF  THE  LIMBS  451 


CERTAIN  FEATURES  OF  THE  MUSCULATURE  OF  HUMAN  LIMBS. 

Muscles  of  the  Pollex  and  Hallux. — The  extensor  ossis  metacarpi 
poUicis  corresponds  to  the  tibialis  anticus.  The  thumb  muscle  has  com- 
monly a  carpal  insertion  as  well  as  metacarpal.  The  extensor  brevis  or 
primi  internodii  pollicis  is  constant  in  man  only  ;  it  is  a  segment  of  the 
extensor  ossis  metacarpi  pollicis.  The  extensor  brevis  hallucis  is  not 
represented  in  the  thumb. 

Muscles  of  the  Second  Digit. — In  the  lower  primates  each  finger  has 
two  extensors — a  deep  and  superficial.  The  deep  in  the  second  digit 
becomes  the  extensor  indicis  ;  in  the  little  finger  it  forms  the  extensor 
minimi  digiti.  The  deep  extensor  muscles  have  disappeared  in  man  from 
the  3rd  and  4th  digits,  but  occasionally  reappear.  In  the  leg  the  deep 
extensors  have  migrated  to  the  foot,  and  form  the  extensor  brevis  digitorum. 
That  for  the  little  toe,  however,  has  not  descended  ;  it  is  always  vestigial, 
if  present.  It  runs  beneath  or  with  the  peroneus  brevis,  and  is  known 
as  the  peroneus  quartus  or  peroneus  quinti  digiti.  If  the  mirror-image 
theory  is  true  it  represents  the  extensor  brevis  pollicis. 

Flexors  and  Extensors  of  the  Metacarpus.^ — These  have  retained 
their  primitive  insertions  in  the  hand  ;  their  modifications  in  the  foot 
have  been  already  mentioned.  Both  at  the  knee  and  elbow  joint  the 
origins  of  these  muscles  have  undergone  much  shifting  and  migration. 

Migration  of  Muscular  Attachments. — Many  of  the  human  muscles 
acquire  during  development  attachments  to  segments  at  a  distance  from 
those  from  which  they  are  developed.  The  serratus  magnus  arises  from 
5th,  6th,  7th  cervical  segments  ;  its  attachment  has  extended  backwards 
from  the  1st  rib  until,  in  man,  it  reaches  the  8th  rib  ;  the  trapezius,  origin- 
ally situated  in  the  neck,  migrates  backwards,  and  in  the  7th  week  obtains 
an  insertion  to  the  shoulder-girdle,  and  before  the  end  of  the  3rd  month 
its  origin  has  reached  as  far  backwards  as  the  12th  dorsal  spine  along  the 
median  dorsal  line.  The  latissimus  dor  si  migrates  to  the  median  dorsal 
line  over  the  spinal  musculature  and  reaches  the  spine  and  crest  of  the 
ilium.  The  diaphragm,  which  arises  in  the  neck  (4th  and  5th  segments) 
comes  to  be  attached  in  the  floor  of  the  thorax.  The  facial  musculature 
takes  its  origin  in  the  hyoid  arch.  The  sub  vertebral  (hypaxial)  muscula- 
ture is  a  migrated  part  of  the  transversalis  sheet.  The  omo-hyoid  is 
attached  at  first  to  the  sternum  ;  it  migrates  along  the  clavicle  and  reaches 
(often  it  fails  to  reach)  the  upper  border  of  the  scapula.  The  migration 
of  the  subclavius  has  been  in  an  opposite  direction  ;  originally  it  reached 
to  the  humerus.  The  case  of  the  extensor  brevis  digitorum  of  the  foot 
has  just  been  mentioned.  The  flexor  accessorius  is  a  part  of  the  flexor 
longus  hallucis  which  has  migrated  to  the  sole  of  the  foot.  The  opponens 
of  the  thumb  and  of  the  little  finger  is  a  separated  part  of  the  short  flexor 
muscles  of  these  digits.     These  are  only  a  few  of  the  more  striking  examples 

1  J.  P.  McMurrich,  Amer.  Journ.  Anat.  1906,  vol.  6,  p.  407  (Plantar  Musculature) ; 
J.  P.  McMurrich,  Amer.  Journ.  Anat.  1904,  vol.  4,  p.  33  (Musculature  of  Thigh). 


452 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


of  the  migration  of  tlie  attachment  of  muscles,  but  the  mechanism  which 
brings  about  migration  and  the  biotactic  influence  which  is  at  work  are 
unknown. 

Vestigial  and  Abnormal  Muscles  of  the  Limbs  and  Trwik.— (1)  The 

muscles  of  the  human  ear  and  scalp  may  be  described  as  vestigial 
when  compared  to  the  development  in  other  mammals.  Although  their 
action  on  the  ear  and  scalp  is  feeble,  yet  they  serve  as  most  important 
bases  into  which  certain  psychological  states  are  reflected. 

(2)  The  levator  claviculae  (omo-trachelian)  is  a  muscle  which  passes 
from  the  upper  transverse  cervical  processes  to  the  outer  end  of  the  clavicle 
or  acromion  process.     It  is  well  developed  in  climbing  primates.     It  is 


scapula 


long  head  triceps 
teres  major 

latis.  dorsi 

^uestigial  lat.  condyl.  of  man 
-lat-condyl.  of  primates 
inner  head  triceps 

-internal  intermusc.  sept 
-triceps 
^internal  condyle. 
-  olecranon 

Fig.  471. — Latissimo-condyloideus  Muscle. 

not  a  common  muscle  in  man.     It  can  be  recognized  during  life  in  the 
posterior  triangle  of  the  neck. 

(3)  The  latissimo-condyloideus  (dorsal  epitrochlearis),  a  climbing  muscle, 
is  always  represented  in  man,  commonly  by  a  fibrous  bundle  between  the 
tendon  of  the  latissimus  dorsi  and  the  long  head  of  the  triceps  (Fig.  471). 
The  bundle  may  be  occasionally  muscular.  In  apes  it  passes  from  the 
latissimus  dorsi  at  the  axilla  to  the  inner  aspect  of  the  elbow  and  arm, 
which  it  retracts  in  climbing.  It  belongs  to  the  same  sheet  as  the  coraco- 
brachialis.  The  ligament  of  Struthers — a  strip  of  fibrous  tissue  over  the 
internal  intermuscular  septum,  above  the  internal  condyle — represents 
part  of  the  tendon  of  this  muscle.  The  muscular  slips  occasionally  found 
crossing  the  brachial  or  axillary  artery  from  the  latissimus  dorsi  to  the 
coraco-brachialis  or  biceps  are  derivates.  Other  slips  found  crossing  the 
fioor  of  the  axilla,  between  the  adjacent  borders  of  the  pectoralis  major 
and  latissimus  dorsi,  are  parts  of  the  muscular  sheet  out  of  which  these 
two  muscles  are  developed. 

(4)  The  pectoralis  externus  arises  from  the  4-5-6  ribs  and  costal  cartilages, 
beneath  the  axillary  border  of  the  pectoralis  major.     This  is  its  normal 


MORPHOLOGY  OF  THE  LIMBS  453 

condition  in  most  mammals,  but  in  man  it  is  commonly  fused  with,  and 
forms  part  of,  the  pectoralis  major. 

(5)  The  sternalis  is  a  remnant  of  the  primitive  rectus  sheet  (Fig.  445). 
The  pectoralis  major  is  formed  from  the  same  ventral  longitudinal  sheet  as 
the  rectus  abdominis  and  sterno-mastoid.  The  fibres  of  the  sternalis,  which 
lie  along  the  sides  of  the  sternum,  superficial  to  the  origin  of  the  pectoralis 
major,  represent  a  persistent  part  of  the  primitive  longitudinal  sheet. 

(6)  In  the  sterno-mastoid  four  elements  are  recognized  :  sterno-mastoid, 
sterno-occipital,  cleido-mastoid,  cleido-occipital.  The  cleido-occipital 
fibres,  which  form  part  of  the  same  sheet  as  the  trapezius,  are  often  absent. 
On  the  other  hand,  the  cleido-occipital  fibres  may  be  continuous  with  the 
trapezius.  The  sterno-mastoid  and  trapezius  muscles  are  developed  in 
the  occipital  segments  and  are  originally  connected  with  gill  arches. 

(7)  The  pectoralis  minor  is  sometimes  inserted  to  the  capsule  of  the 
shoulder  and  great  tuberosity  of  the  humerus  as  is  the  case  in  primates 
generally.  The  coracoid  insertion,  which  is  the  usual  one  in  man  and  also 
in  the  gorilla,  is  usually  regarded  as  a  secondary  attachment,  but  Miss 
K.  Lander  ^  has  shown  that  it  is  also  found  in  primitive  types  of  mammals. 
When  the  pectoralis  minor  is  inserted  to  the  coracoid,  the  former  fibres 
of  insertion  become  fused  with,  and  form  part  of,  the  coraco-humeral 
ligament,  which,  however,  is  a  distinct  structure,  and  represents  a  special- 
ized part  of  the  capsule  of  the  shoulder  joint. 

(8)  In  some  apes  (such  as  the  Gibbons)  the  biceps  has  four  heads — the 
two  usual,  the  long  and  short,  and  two  others,  one  from  the  inner  border 
of  the  humerus  and  one  from  the  bicipital  groove.  These  two  extra 
heads  appear  frequently  in  man. 

(9)  The  epitrochleo-anconeus  is  frequently  present.  It  crosses  the 
ulnar  nerve  from  the  internal  condyle  to  the  olecranon. 

(10)  The  palmaris  longus  and  its  homologue  in  the  leg,  the  plantaris, 
are  vestigial,  aberrant  in  form,  and  often  absent.  The  plantar  and  palmar 
fasciae  represent  their  divorced  tendons.  The  plantaris  and  palmaris 
undergo  retrograde  changes  in  the  primates  with  the  transformation  of 
claws  to  nails. 

(11)  Each  digit  (fingers  and  toes)  in  lower  primates,  such  as  monkeys, 
is  provided  with  three  short  muscles  which  arise  from  the  carpus  or  tarsus. 
The  three  muscles  are  (Fig.  472)  :  (1)  a  short  flexor  on  the  radial  side  of 
the  digit ;  (2)  a  short  flexor  on  the  ulnar  side  ;  (3)  a  contrahens  or  adductor 
muscle  (always  absent  in  the  middle  digit).  The  ten  short  flexor  muscles 
form  a  deeper  sheet  than  the  four  contrahentes.  Of  this  form  the  arrange- 
ment of  the  short  muscles  in  the  human  hand  is  a  derivative.  The 
remnants  in  the  human  hand  and  foot  of  the  contrahentes  are  :  (1)  The 
adductors  of  the  1st  digit  (poUex  or  hallux)  ;  (2)  fibrous  remnants  of  the 
others  occur  over  the  deep  plantar  or  carpal  arch  (Fig.  472).  The  short 
flexors  in  man  have  become  (1)  the  seven  interossei  ;  (2)  the  flexores  breves 
(ulnar  and  radial)  and  opponens  of  the  first  digit  ;  the  flexor  brevis  and 
opponens  of  the  fifth  digit  (see  Fig.  472).  The  ulnar  flexors  of  the  thumb 
and  great  toe  are  absent  or  fibrous. 

1  Journ.  Anat.  1918,  vol.  52,  p.  292. 


454 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


(12)  The  pyramidalis  is  often  absent  in  man  or  vestigial.  It  is  the 
tensor  of  the  linea  alba. 

(13)  Remnants  of  the  extensors  and  flexors  o!  the  tail  may  occur  between 
the  sacrum  and  the  coccyx  (p.  410). 

(14)  The  coccygeus  is  vestigial ;  its  superficial  part  forms  the  small 
sacro-sciatic  ligament. 

(15)  Fibres  of  the  biceps  of  the  thigh  may  be  followed  into  the  great 
sacro-sciatic  ligament.  This  ligament,  which  is  almost  peculiar  to  man 
— in  other  primates  it  is  quite  thin  and  slender — may  contain  fibres  derived 
from  the  caudo-femoral  group  of  muscles,  such  as  the  tenuissimus,  a  long 
strap-like  muscle  which  passes  from  the  coccyx  to  the  femur  and  leg  in 
lower  mammals.  The  sacro-sciatic  ligament  is  mainly  derived  from  the 
great  median  sheet,  out  of  which  the  middle  layer  of  the  lumbar  fascia  is 
also  formed.     Parsons  regards  the  short  head  o£  the  biceps  as  a  derivative 


pi.  inter.  3 
opponensand    j  flor.  inter.  4    dor.  3  pfj 

pi.  2  I     dor.,  II 


Iflex.  breu.  mm. 
V    digiti 


tib.  head  Hex. 
breu.  hal. 


abd.  rain.  dig. 


^deep  plant,  nerue 


abd.  halluo 


Fig.  472. — The  Morphology  of  the  Short  Muscles  of  the  Digits.  The  muscles  shaded 
are  those  of  the  ape's  hand  or  foot ;  the  positions  of  the  corresponding  muscles 
in  the  human  hand  or  foot  are  indicated  by  dotted  outlines. 

of  the  tenuissimus,  while  others  regard  it  as  part  of  the  muscular  sheet 
which  forms  the  peroneal  muscles.  Amongst  primates,  the  short  head 
is  found  only  in  man,  the  anthropoids,  and  some  South  American  apes. 
It  corresponds  to  the  brachialis  anticus  in  the  arm,  and  is  supplied  by  the 
external  popliteal  nerve. 

(16)  The  psoas  parvus  is  also  vestigial.  It  acts  primarily  as  a  flexor 
of  the  pelvis  on  the  spine.  It  begins  to  disappear  in  those  primates  which 
assume  the  erect  posture. 

(17)  The  scansorius  is  a  separated  segment  of  the  gluteus  medius  and 
minimus.  It  rises  from  the  anterior  border  of  the  ilium  and  passes  to  the 
great  trochanter.  It  corresponds  to  the  teres  minor.  It  is  not  constant 
in  any  animal. 

(18)  The  flexor  brevis  digitorum  to  the  little  toe  and  the  adductor  trans- 
versus  of  the  great  toe  are  often  fibrous. 

The  Supra-condylar  Process  ^  is  well  developed  in  lemurs,  the  lowest 
primates,  and  in  mammals  of  many  orders.  Its  function  is  unknown. 
It  occasionally  appears  in  man.  Dr.  Rutherford  found  it  represented  in  a 
human  foetus  in  the  9th  week  of  development.     It  is  developed  from  the 

1  T.  Dwight,  Amer.  Journ,  Anat.  1904,  vol,  3,  p.  221. 


MORPHOLOGY  OF  THE  LIMBS  455 

humerus  about  two  inches  above  the  internal  condyle  as  a  hook-like 
process  of  bone.  It  lies  in  front  of  the  internal  intermuscular  septum,  and 
when  well  developed  the  brachial  artery  and  median  nerve  may  pass 
beneath  it,  as  they  do  in  such  animals  as  the  squirrel  and  cat. 

Development  of  Joints. — Each  limb  bone  is  formed  from  a  centre  of 
chondrification  which  appears  in  the  2nd  month  within  the  unjointed 
skeletal  blastema  of  the  limb  bud.  At  these  centres  the  mesodermal  cells 
assume  the  characters  of  cartilage  cells  ;  growth  proceeds  most  rapidly 
at  the  periphery  of  the  cartilage  centres  ;  as  the  growing  centres  approach 
each  other,  part  of  the  original  blastema  is  left  between  them.  This 
tissue,  which  may  be  named  the  interchondral  disc,  forms  the  first  basis 
of  a  joint  (Fig.  473).  The  cells  in  the  peripheral  part  of  the  blastema 
condense  and  form  a  perichondrium — a  membrane  which  surrounds  growing 
cartilages.     In  the  8th  and  9th  weeks,  joints  begin  to  appear  in  the  inter- 

^gSj^Sa^^-^    _  NAIL  FOLO 

:.W  ■■'■'■    -^^^^^^^^^=s=  EPrTRlCHIUM 

-ii^i^^>J5t-W-i'^^^^^V^^^^^;Il  SOLE  PLATE 


4jffji=S^      "^^^^^^^^^^53^       T£:/7M .  PHALANX 

\  \^      PERICHONDRIUM 

\  CAPSULE 

PERICHONDRIUM 

Fig.  473. — Sagittal  section  of  Terminal  Joint  of  Finger  of  Foetus  in  10th  week  of 
development.     (After  Nicolas.) 

chondral  discs,  the  more  important  before  the  less  important.  The 
manner  of  formation  is  the  same  for  all  joints  ;  in  the  periphery  of  the 
interchondral  disc,  the  mesenchymal  cells  begin  to  disappear,  giving  rise  to 
the  synovial  space  which  spreads  towards  the  centre  of  the  developing  joint 
— the  central  part  being  the  last  to  form.  The  perichondrium  is  continued 
from  segment  to  segment  over  the  interchondral  discs  and  thus  becomes  the 
basis  of  the  capsular  ligament.  At  first  the  ends  of  the  cartilages  pro- 
jecting into  joint  cavities  are  also  covered  by  an  extension  of  the  peri- 
chondrium. Peripheral  cells  of  the  interchondral  disc  line  the  capside 
and  form  the  synovial  membrane,  the  cells  of  which,  even  in  the  adult, 
show  by  their  structure  that  they  are  cartilaginous  in  nature.  In  certain 
pathological  conditions,  the  synovial  villi  give  rise  to  cartilaginous  nodules. 

Interarticular  Fibro-cartilages. — In  every  developing  joint  fringes  of 
synovial  membrane,  representing  remnants  of  the  interchondral  disc 
(intermediate  plate),  project  in  the  gap  between  the  articular  margins  of 
bones  (Fig.  473).     In  the  elbow  joint  they  are  present,  even  in  the  adult  ; 


456      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

in  the  hip  and  shoulder  joint  they  form  the  cotyloid  and  glenoid  ligaments. 
In  the  knee  joint  they  are  much  better  marked,  forming  the  semilunar 
cartilages.  At  the  wrist  joint  the  interchondral  disc  forms  the  triangular 
fibro-cartilage,  but  here  it  is  possible  that  certain  other  elements  are 
included.  A  nodule  of  cartilage,  which  may  ossify,  is  present ;  within  it 
certain  ligaments  which  united  the  radius  and  ulna,  and  these  two  bones 
with  the  semilunar  and  cuneiform,  have  been  included  (Parsons  and  Corner). 
This  cartilage  is  complete  in  man  only  ;  it  plays  a  part  in  the  mechanism 
of  pronation  and  supination.  In  the  sterno-clavicular  joint  two  synovial 
cavities  are  formed,  one  on  either  side  of  the  interchondral  disc.  In  this 
case  it  is  only  in  the  higher  primates  that  a  complete  interarticular 
disc  is  present.  Two  synovial  cavities  are  also  formed  in  the  temporo- 
mandibular joint,  the  meniscus  separating  two  joints,  which  are  functionally 


-patella 


round  lig. 


V 


artic.  surface 
reflected  lig, 

,         ,  capsule 

I  'crucial  ligaments 

post  lig. 

Fig.  474. — Showing  the  Origin  of  the  Crucial  Ligaments  of  the  Knee. 

Fig.  475. — Showing  the  Origin  of  the  Ligamentum  Teres  and  Reflected  Bundle  of  the 
Capsular  Ligament. 

difierent.     The  upper  is  for  gliding  movements,  the  lower  for  hinge-like 
movements. 

Capsular  Ligaments. — Certain  parts  of  the  capsule  of  every  joint 
become  thickened  and  specialized  according  to  the  strains  to  which  the 
joint  is  subjected.  Parsons  found  that  it  is  the  middle  gleno-humeral 
ligament  of  the  shoulder  joint  which  becomes  enlarged  and  projects  within 
the  joint  of  prono grade  mammals.  In  man,  the  coraco-humeral  ligament 
is  by  far  the  strongest.  The  anterior  part  of  the  capsule  of  the  hip  joint 
in  man  has  to  withstand  the  strain  of  the  body  when  the  thigh  is  extended 
in  the  upright  posture.  Part  of  it  becomes  specialized  to  form  the  ilio- 
femoral or  Y-shaped  ligament.  In  the  knee  joint  the  posterior  part  of 
the  capsule  is  strengthened  to  prevent  over-extension.  The  development 
of  the  condyles  of  the  femur  towards  the  popHteal  space  isolates  a  posterior 
part  of  the  capsule  which  thus  comes  to  lie  within  the  joint  and  form 
the  crucial  ligaments  (Fig.  474).  The  ligamentum  teres,  the  best  example 
of  an  intra-articular  ligament,  appears  in  the  human  foetus,  as  part  of  the 
capsule  of  the  joint ;  in  reptiles  this  foetal  form  is  retained.  The  round 
ligament  is  isolated  by  the  development  of  the  head  of  the  femur,  which 
expands  as  a  wing  on  each  side  of  the  ligamentum  teres,  and  by  the  fusion 
of  the  wings  isolates  it  from  the  capsule  (Fig.  475).     The  reflected  ligament. 


MOKPHOLOGY  OF  THE  LIMBS 


457 


on  the  under  surface  of  the  neck  of  the  femur,  is  the  part  of  the  capsule 
with  which  the  ligamentum  teres  was  continuous.^ 

Knee  Joint. — In  Fig.  476  is  given  a  diagrammatic  representation  of 
the  posterior  aspect  of  the  knee  joint  as  seen  in  a  primitive  mammalian 
type.  Three  interarticular  discs  are  shown  ;  an  internal  tibio-femoral, 
an  external  tibio-femoral  and  a  fibulo-femoral.  When  the  fibula  became 
excluded  from  the  knee  joint,  the  fibulo-femoral  disc,  from  which  fibres 
of  the  popliteus  took  origin,  was  included  in  the  tendon  of  that  muscle 
(Carl  Ftirst).  The  popliteus  originally  passes  from  the  fibula  to  the  tibia 
like  the  pronator  quadratus  in  the  forearm.  The  upper  fibres  migrate  to 
the  capsule  and  to  the  fibulo-femoral  disc,  and  through  the  disc  and  its 
ligaments  gain  an  attachment  to  the  femur.  Thus,  instead  of  rotating 
the  tibia  on  the  fibula,  the  popliteus  muscle  now  rotates  the  tibia  on  the 
femur.  Occasionally  the  cavity  of  the  human  knee  joint  communicates 
with  the  superior  tibio-fibular  joint  through  the  synovial  diverticulum 
beneath  the  tendon  of  the  popliteus.     The  upper  end  of  the  fibula  is  being 


femur. 

int.  fat  lig 
int.  s.  cart, 
ext.  s.  cart: 
tibia 


^^i^sesamoid 


ext.  lat.  lig, 
fibulo.-fem, 
interart.  cart 
'fibula 
tib.-fib.  muscle 


Fig.  476. — Scheme  of  a  Primitive  Mammalian  Knee-joint  to  show  (1)  the  Articulation 
of  the  Fibula  with  the  Femur  ;  (2)  the  Fibulo-fermoral  Interarticular  CartUage 
which  becomes  included  in  the  Tendon  of  the  Popliteus  ;  (3)  the  Tibio-fibular 
Muscle  out  of  which  the  Popliteus  is  evolved  ;  (4)  the  division  of  the  Tibio- 
femoral Interarticular  Cartilage  into  external  and  internal  Semilunar  Cartilages. 
(Carl  Fiirst.) 

excluded  from  the  knee  joint  during  the  8th  week.  There  are  five  separate 
synovial  cavities  developed  in  this  joint — one  between  the  patella  and 
femur,  two  between  the  femoral  condyles  and  the  primitive  semilunar 
cartilages,  and  two  between  the  cartilages  and  the  upper  extremity  of  the 
tibia.  The  five  joints  become  continuous  in  the  4th  month,  the  crucial 
and  alar  ligaments  being  derived  from  the  primary  septa  between  the 
cavities  (Bardeen).^  The  external  semilunar  cartilage  is  circular  in  form 
and  continuous  with  the  posterior  crucial  ligament  in  primates,  in  which 
the  power  of  rotation  at  the  knee  is  highly  developed  ;  in  man  the  circular 
form  of  the  cartilage  is  lost  and  it  only  retains  part  of  its  continuity  with 
the  posterior  crucial  ligament  (Parsons).  The  ligamentum  mucosum,  which 
in  many  mammals  separates  the  knee  joint  into  three  compartments — 
two  condylar  and  a  patellar — is  much  reduced  in  man. 

Ossification  of  Bones. — The  simplest  and  most  primitive  manner  in 
which  bones  pass  from  the  cartilaginous  to  the  osseous  stage  is  seen  in  the 

^  See  Walmsley,  Journ.  Anat.  1917,  vol.  51,  p.  61, 
"  See  reference,  p.  425, 


458 


HUMAN  EMBEYOLOGY  AND  MORPHOLOGY 


carpus  and  tarsus  (Fig.  477).  Bone  is  entirely  deposited  witMn  the 
cartilage  by  a  process  of  endochondral  ossification.  The  various  stages 
in  this  process  may  be  grouped  as  follows  :  (1)  calcification  of  the  inter- 
cellular matrix  in  the  centre  of  the  bone — a  temporary  phase  in  human 
ossification,  but  a  permanent  one  in  some  fishes  ;  (2)  an  invasion  of  vaso- 
formative and  osteoblastic  cells  which,  commencing  at  a  point  beneath 
the  perichondrium,  reach  the  middle  of  the  central  area  of  calcification 
and  form  a  centre  of  ossification  (Fig.  477).  The  osteoblasts  and  their 
accompanying  vessels,  when  the  cartilage  cells  are  absorbed,  deposit  bone 
in  the  spaces  of  the  calcified  matrix.  A  section  through  an  ossifying  and 
growing  carpal  bone  shows  (1)  a  centre  of  ossification  ;  (2)  a  surrounding 
narrow  area  of  calcification  ;  (3)  a  peripheral  area  of  actively  growing 
cartilage  ;  (4)  a  covering  membrane  or  perichondrium.  The  processes 
of  growth  and  ossification  cease  when  the  cartilage  beneath  the  peri- 


cuboid 

pen'chond, 

cartil: 
calci'f.  cart 
bone 


int.  cun. 

caph. 
blood  ues. 

rtic  cartil 
calcif.  cart 
bone 


OS.  calc.    astrag. 


Fig.  477. — Section  of  the  Tarsus  at  the  3rd  year  of  development  to  show  pure  Endo- 
chondral Formation  of  Bone. 

chondrium  is  completely  transformed  to  bone.  Not  only  are  the  tarsal 
and  carpal  bones  formed  thus,  but  so  are  the  epiphyseal  ends  of  all  long 
bones. 

In  the  shafts  of  long  bones,  to  the  process  of  endochondral  ossification, 
another — the  ectochondral — is  added  (Fig.  478,  A,  B,  C,  D).  An  endo- 
chondral centre  is  formed  as  in  the  tarsal  bones,  and  from  this  centre  the 
process  extends  rapidly  in  every  direction.  Some  of  the  osteoblasts, 
instead  of  invading  the  cartilage,  form  a  layer  beneath  the  perichondrium, 
which  surrounds  the  cartilaginous  substance  of  the  bone.  The  perichon- 
drium now  becomes  periosteum  ;  the  deposit  of  periosteal  bone  leads  to  an 
increase  in  the  thickness  of  the  shaft  (Fig.  478,  C)  ;  the  extension  of  the 
endochondral  ossification  into  the  growing  cartilaginous  ends  of  the  bone 
leads  to  an  increase  in  the  length  of  the  shaft.  As  the  periosteal  bone  is 
deposited,  the  endochondral  bone  within  the  shaft  is  absorbed  and  a 
medullary  cavity  is  formed,  in  which  red  marrow  begins  to  appear  in  the 
6th  month  (Fig.  478,  D).  The  cartilaginous  parts  of  the  bone,  at  each 
extremity  of  the  shaft,  form  the  epiphyses.  When  the  endochondral 
centres  appear  and  grow  within  the  epiphyses,  a  line  of  growing  cartilage 
is  gradually  isolated  between  them  and  the  endochondral  centre  of  the 
shaft  (Fig.  478,  D).     At  the  epiphyseal  line  the  bone  grows  in  length, 


MORPHOLOGY  OF  THE  LIMBS 


459 


the  addition  being  made  solely  at  the  shaft  or  diaphyseal  side  of  the  line. 
These  g^o^vth  discs  should  therefore  be  named,  not  epiphyseal  but  dia- 
physeal lines.  By  the  formation  of  epiphyses  at  the  ends  of  long  bones, 
the  growing  line  of  cartilage  is  sheltered  from  the  friction  and  stress  to 
which  it  would  be  exposed  if  situated  on  the  articular  ends  of  the  bones. 
All  the  cartilage  of  a  bone,  except  that  on  the  articular  surfaces,  is  ossified 
when  the  body  is  fully  grown.  The  evidence  at  our  disposal  points  to 
both  the  absorption  of  the  cartilage  and  the  deposition  of  bone  as  being 
regulated  by  secretions  derived  from  the  thyroid,   pituitary  and  other 


capsule 
cartil: 

nut  art. 
bone 
calcif.  cart; 


cartil. 

prolifer  zone 
■calcif.  cart, 
bone 
■prolif.  zone 

cartil. 


capsule 
epiph.  centre 
■perichondrium 
calcif.  cartil. 
■periost  bone 
med.  cauity 
-periosteum 

epiphy.  line 
epiphy.  c 
ic.  cartil. 


cart; 
prolif.  zone, 
periosteum 
periost.  bone 
cart,  bone 
prolif.  zonej^ 
cartil. 
capsule- 


FiG.  478. — Ossification  of  a  Long  Bone  by  Endochondral  and  Ectocliondral  Ossifica- 
tion.    (After  Nicolas.) 

A,  Ossification  within  the  cartilage  of  the  shaft. 

B,  Complete  ossification  of  the  middle  part  of  the  shaft. 

C,  Formation  of  bone  in  the  shaft  outside  the  cartilage  by  osteoblasts  lying  beneath 
the  perichondrium  (now  named  periosteum). 

D,  Complete  absorption  of  the  endochondral  bone  of  the  shaft ;  formation  of  a 
medullary  cavity ;  appearance  of  endochondral  centres  in  the  epiphyses ; 
formation  of  the  epiphyseal  lines. 

glands  of  internal  secretion.^  In  the  growth  of  a  long  bone,  such  as  the 
humerus,  the  proximal  and  distal  diaphyseal  lines  take  an  unequal  share. 
Digby  2  found  that  while  the  proximal  line  added  4  parts  to  the  length 
of  the  humerus  the  distal  line  contributed  only  1  part.     The  chief  nutrient 

1  Keith,  Lancet,  April  15th,  1911  ;    Journ.  Anat.  1913,  vol.  47,  p.  189  ;    ibid.  1920, 
vol.  54,  p.  101  ;    Lancet,  1913,  vol.  1,  p.  305. 

2  Kenelm  Digby,  Journ.  Anat.  1916,  vol.  50,  p.  186. 


460 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


canal  of  the  shaft  of  a  long  bone  points  to  the  centre  at  which  endochondral 
ossification  commenced. 

Nature  of  Epiphyses.^ — Epiphyses  are  of  three  kinds  :  (1)  pressure 
epiphyses,  forming  the  articular  extremities  of  long  bones  (Fig.  479,  B)  ; 
(2)  traction  epiphyses,  which  form  processes  for  the  insertions  of  muscles 
(Fig.  479,  B)  ;  (3)  atavistic  epiphyses,  formed  by  the  union  of  an  element 
which  formerly  existed  as  a  separate  bone  (Fig.  479,  A). 

The  upper  extremity  of  the  femur  affords  typical  examples  of  pressure 
and  traction  epiphyses.  By  the  extension  of  the  ossification  of  the  shaft 
within  the  cartilage  of  the  upper  extremity  of  the  femur,  the  pressure  and 
traction  epiphyses  become  widely  separated  to  form  the  head  and  tro- 
chanters. Pressure  epiphyses  are  the  first  to  ossify,  their  centres  appearing 
in  the  order  of  their  functional  importance  ;  they  are  always  fitted  to  the 
shaft  by  a  species  of  dovetailing  to  withstand  dislocating  forces.     The 


isch.  tub 


A. 


B. 


0. 


Fig.  479,  A. — The  Epiphyseal  Cartilage  of  the  Pubis  and  Ischium,  which  arises  from 
the  Median  Cartilage  of  the  Pelvic  Girdle.     (Parsons.) 
B. — Traction  and  Pressure  Epiphyses  on  the  upper  extremity  of  the  Femur. 
C. — The  Epiphyses  of  the  Olecranon  :    a,  the  usual  Epiphyses  ;    b,  occasional 
Epiphyses  :  both  a  and  b  may  be  present.     (Fawcett.) 

upper  extremity  of  the  shaft  of  the  humerus  projects  as  a  three-sided  pyra- 
mid within  the  epiphysis  ;  Professor  Arthur  Thomson  has  shown  that  the 
lower  end  of  the  shaft  of  the  femur  is  fitted  within  its  lower  epiphysis  by  a 
number  of  projections  not  well  marked  in  the  human  bone  but  pronounced 
in  those  animals  which  maintain  the  knee  in  a  flexed  position.  Epiphyses 
are  mammalian  characters  ;  their  rudiments  are  to  be  seen  in  reptilia. 

The  great  trochanter  is  the  traction  epiphysis  of  the  gluteus  medius 
and  minimus  ;  the  small  trochanter,  of  the  psoas  and  iliacus  ;  the  third 
trochanter,  in  which  a  centre  appears  in  the  20th  year  (Dixon),  that  of  the 
gluteus  maximus. 

As  examples  of  atavistic  epiphyses.  Parsons  cites  the  following  :  those 
of  the  ischium  and  pubis  (Fig.  479,  A)  from  the  median  pelvic  bar  (Figs. 
442,  479)  ;  the  coracoid  process  ;   the  epiphysis  on  the  os  calcis,  the  scale- 

^  The  account  given  by  Parsons  has  been  followed.  See  Journ.  Anat.  and  Physiol. 
1903,  vol.  37,  p.  315  ;  1904,  vol.  38,  p.  248  ;  1908,  vol.  42,  p.  388.  R.  L.  Moodie, 
Amer.  Journ.  Anat.  1907,  vol.  7,  p.  443  (Reptilian  Epiphyses).  A.  Kirchner,  Anat. 
Hefte,  1907,  vol.  33,  p.  513  (Epiphyses  of  Os  Calcis  and  5th  Metatarsal).  T.  Walmsley, 
Journ.  Anat.  1919,  vol.  53,  p.  326. 


MORPHOLOGY  OF  THE  LIMBS  461 

like  epiphysis  of  the  olecranon  (Fig.  479,  C).  The  internal  and  external 
condyles  of  the  humerus  may  be  derived  from  sesamoid  ossifications, 
such  as  are  now  seen  in  the  patellae,  in  the  tendons  of  the  popliteus,  outer 
head  of  gastrocnemius  (occasional),  peroneus  longus,  tibialis  posticus  and 
at  the  metacarpo-phalangeal  joints  of  the  thumb  and  great  toe.  The 
patella  is  usually  regarded  as  a  sesamoid,  but  Mile.  Bertha  Vriese  collected 
evidence  to  show  that  it  is  really  a  true  morphological  skeletal  element.^ 

Lines  of  Pressure  and  Tension  oJ  Bones.— The  trabeculae,  in  which 
the  bony  matter  is  deposited  by  the  osteoblasts,  are  arranged  so  as  to 
withstand  the  forces  to  which  the  body  is  subjected.  When  a  bone,  such 
as  th'e  astragalus,  rib  or  neck  of  the  femur,  is  laid  open  by  a  section,  the 
trabeculae  appear  to  form  straight  lines  or  septa  which  converge  and  meet 
at  various  angles  ;  when,  how^ever,  such  bones  are  examined  stereoscopic- 
ally  with  the  X-rays,  the  trabeculae  are  seen  to  be  arranged  in  a  double 
spiral — one  system  twisting  from  right  to  left,  the  other  from  left  to  right 
(Haughton  and  Dixon). ^  By  this  means,  the  greatest  strength  is  obtained 
with  the  least  expenditure  of  material. 

Split  Hand  and  Foot. — The  extremities  are  subject  to  a  remarkable 
series  of  malformations,  which  apparently  represent  arrests  of  their 
development.  The  digits  may  be  abnormally  short  (brachy-dactyly), 
owing  to  an  arrest  in  the  differentiation  of  the  blastema  of  the  phalanges, 
the  terminal  phalanx  being  unseparated  from  the  middle.^  Besides  errors 
in  the  separation  of  the  phalanges,  there  is  an  arrest  of  growth — usually  in 
th^  middle  phalanges,  while,  as  Dr.  Drinkwater  has  shown,  extra  phalanges 
may  be  intercalated.  This  is  of  frequent  occurrence  in  the  fifth  digit  of 
the  foot.  In  another  series  of  cases  the  hand  or  foot  appears  as  if  cleft 
— an  appearance  due  in  some  cases  to  the  fact  that  three  or  more  of  the 
digits  on  the  ulnar  side  of  the  hand  or  fibular  side  of  the  foot  have  remained 
joined  or  webbed,  as  in  the  embryo  of  the  2nd  month,  while  in  others  the 
condition  is  due  to  a  splitting  or  dichotomy  of  the  terminal  plate  of 
the  limb  bud.  The  condition  is  hereditary.*  In  more  extreme  cases  the 
digits  on  the  radial,  or  more  rarely,  those  on  the  ulnar  side  of  the  hand, 
may  be  absent ;  the  corresponding  bone  of  the  forearm  or  leg  is  also 
undeveloped.  Such  cases  lead  one  to  suppose  that  the  two  distal  segments 
of  the  limbs  are  developed  from  a  radial  and  ulnar  or  tibial  and  fibular 
buds,  and  in  such  cases  only  one  of  these  has  been  affected.  Both  may 
be  arrested,  the  extremities  terminating  at  the  proximal  segment.  In 
extreme  cases  the  limb  buds  are  undeveloped. 

1  Bertha  de  Vriese,  Bull,  de  VAcad.  Roy.  de  Sc.  Belgique,  1909,  March  27th. 

2  Dixon,  Journ.  Anat.  and  Physiol.  1910,  vol.  44,  p.  223. 

3  A.  Fischel,  Anat.  Hefte,  1909,  vol.  40,  p.  1  ;  J.  D.  Fiddes,  Anat.  Anz.  1912,  vol.  40, 
p.  544  (Supernumerary  Hallux)  ;  J.  Symington,  Journ.  Anat.  and  Physiol.  1906, 
vol.  40,  p.  100  (Hyper-phalangism  in  Cetacea)  ;  H.  Drinkwater,  Journ.  Anat.  1916, 
vol.  50,  p.  177. 

4  See  T.  Lewis,  Biometrika,  1908,  vol.  6,  p.  25. 


CHAPTER  XXVIII. 

SKIN  AND   ITS  APPENDAGES. 

Stages  in  the  Evolution  of  the  Skin. — We  have  already  seen  that  the 
structures  which  are  developed  in  the  human  embryo  can  be  best  explained 
by  supposing  that  at  one  stage  of  evolution  the  ancestry  of  mammals 
lived  and  breathed  in  water.  The  skin  of  the  human  embryo  until  the 
end  of  the  2nd  month  of  development  is  translucent,  and  has  many  points 
in  common  with  that  of  the  lowest  gill-bearing  vertebrates.  It  then 
consists  of  two  layers — a  deep  or  germinal,  consisting  of  cubical  epithelium 
and  a  superficial,  made  up  of  fattened  cells  (Fig.  481).  In  the  3rd  month 
this  superficial  layer,  known  as  the  epitrichium  or  periderm,  becomes 
horny  in  nature,  recalhng  a  stage  which  represents  the  evolution  into  a 


EPIDERMIC    SCALES 
HAIR     QROUP 

(5) 


HAIR    OROUP 

(3) 

Fig.  480. — Showing  the  Arrangement  of  Hair  Groups  in  the  Human  Foetus  and  their 
Relationship  to  Hypothetical  Dermal  Scales.     (Stohr.) 

terrestrial  form  of  life.  The  appendages  of  the  skin — its  hair  and  glands 
— appear  later  ;  they  seem  to  be  modifications  of  glandular  and  sensory 
structures  seen  in  the  soft  skin  of  amphibia.  The  hairs  are  developed  in 
groups  and  lines.^  Their  arrangement  can  be  best  explained,  according 
to  Dr.  Max  Weber,  by  supposing  that  the  skin  of  primitive  mammals  was 
covered  by  scales,  and  that  the  hairs  sprouted  out  in  groups  at  their 
tessellated  junctions,  as  in  certain  living  edentates  (see  Fig.  480).  The 
human  hairs  are  arranged  in  irregular  series,  but  in  most  instances  only 
the  chief  hair  of  a  group  is  developed.  In  later  period  of  foetal  life,  however, 
the  chief  hair  has  one  or  two  subsidiary  hairs  planted  on  either  side  of  it 
— making  one  of  a  group  of  three  or  five  hairs. 

The  skin  of  man,  compared  to  other  primates,  is  comparatively  hairless. 
We  must  regard  his  nudity  as  a  lately  acquired  character.     At  the  7th 

1  Stohr,  VerJiand.  Anat.  Gesellsch.  1907,  p.  153. 
462 


SKIN  AND  ITS  APPENDAGES  463 

month  of  foetal  life  the  chimpanzee  and  gorilla  have  hair  only  on  the 
scalp,  eyebrows  and  lips  ;  the  rest  of  the  body  is  nude,  except  for  fine  hairs 
or  lanugo.  This  is  also  the  condition  in  the  human  foetus  at  a  corresponding 
period  ;  in  man,  although  the  foetal  crop  of  lanugo  is  succeeded  by  a 
general  outgrowth  of  fully  developed  hair,  yet  we  may  regard  the  human 
condition  as  representing  an  arrest  of  hair  development  at  a  stage  seen  in 
foetal  apes.  The  human  skin  is  also  more  sensitive  and  more  richly 
supplied  with  sensory  nerves  than  is  the  case  in  other  primates.  In 
Professor  Elliot  Smith's  opinion  the  rich  sensory  supply  to  the  skin  must 
have  been  a  factor  in  bringing  about  the  large  size  of  the  human  brain. 
In  the  distribution  and  "  lie  "  of  the  hair  on  his  body  and  limbs  man  also 
resembles  the  hairy  anthropoids. 

There  are  on  record  a  number  of  cases  of  men  and  women,  in  whom  the 
whole  surface  of  the  body  was  covered  with  a  close  covering  of  hair.  The 
development  of  hair  on  the  face  is  certainly  regulated  by  a  secretion 
derived  from  the  sexual  organs,  for  in  eunuchs  the  beard  is  never  developed. 
It  is  also  well  known  that  the  thyroid  has  a  direct  influence  on  the  develop- 
ment and  growth  of  hair.  Desquamation  from  the  epidermis  begins  in 
the  3rd  month  of  foetal  life,  and  never  ceases  until  death.  In  a  certain 
disease  of  foetal  life,  named  Ichthyosis,  desquamation  does  not  take  place  ; 
the  unshed  epidermis  forms  cracked  cakes  on  the  surface  of  the  child  at 
birth. 

Development  of  the  Skin. — Considerable  assistance  in  the  under- 
standing of  the  diseases  to  which  the  skin  is  liable  and  of  the  nature  of  the 
growths  which  arise  from  the  epidermis,  such  as  corns,  bunions,  and 
cancer,  is  to  be  obtained  by  studying  the  manner  in  which  the  skin  is 
developed.  At  first  the  human  embryo  is  covered  by  a  single  layer  of 
epithelium  (epiblast  or  ectoderm),  as  is  the  case  in  the  adult  amphioxus. 
By  the  end  of  the  1st  month  there  are  two  layers,  the  lower  representing 
the  germinal  or  basal  layer  ;  the  upper  the  epitrichimn,  so  named  because 
it  was  supposed  that  hairs  are  developed  beneath  it,  and  when  they  grew 
out  in  the  sixth  month  this  surface  layer  of  flat  epithelium  was  shed.  This 
evanescent  foetal  layer  is  also  known  as  the  periderm. 

In  the  4th  month  we  find  developmental  processes  in  full  activity  in  the 
skin  ;  three  strata  are  recognizable  in  the  epidermis — all  derived  from 
the  single  germinal  layer.  These  are  (1)  a  basal  layer — a  single  stratum 
of  cubical  or  columnar  cells,  representing  the  primitive  germinal  epithelium 
(Fig.  481,  B) ;  (2)  an  intermediate  or  mucous  stratum,  several  cells  deep  ; 
(3)  a  heaped-up  superficial  or  corneous  stratum,  representing  the  protecting 
but  perishing  superficial  covering  of  the  skin.  At  the  same  time  the  opening 
phases  in  the  development  of  hair  follicles,  sebaceous  and  sweat  glands 
and  of  skin  ridges  and  papillae  are  to  be  detected.  In  the  5th  month 
the  stratum  lucidum  becomes  differentiated  between  the  mucous  and 
corneous  strata.^ 

The  epidermis  rests  at  first  on  undifferentiated  mesoderm,  consisting 
of  small  round  cells  closely  imbedded  m  a  mucoid  matrix.     This  is  the 

^Comparative  anatomy  of  epidermis:  see  F.  K.  Studnicka,  Anat.  Hefte,  1909, 
vol.  39,  p.  1. 


464 


HUMAN  EMBEYOLOGY  AND  MORPHOLOGY 


normal  structure  of  undiSerentiated  mesoderm.  The  superficial  meso- 
dermal cells  are  condensed  beneath  the  epidermis  to  form  a  corium  towards 
the  end  of  the  3rd  month  ;  an  areolar  or  subcutaneous  stratum  of  tissue 
is  differentiated  at  the  same  time.  Connective  tissue  fibrils  begin  to  de- 
velop in  connection  with  the  mesodermal  cells  and  by  the  fifth  month  the 
mucoid  substance  has  almost  disappeared ;  but  even  in  adult  life,  when  the 
thyroid  body  is  diseased  or  removed,  a  mucoid  substance  may  reappear, 
and  a  condition  resembling  the  foetal  state  be  thus  produced.  In  the 
mucous  membranes  of  the  lips,  anus  and  vulva,  the  superficial  layer  of 
epithelium  does  not  become  cornified. 

Formation  of  Dermal  Papillae.^ — Up  to  the  end  of  the  3rd  month  the 
epidermis  is  easily  detached  from  the  corium  as  a  flat  membrane,  but 
early  in  the  4:th  month  they  become  more  closely  united  by  ridges 
of    epidermis    becoming    folded    within    corresponding    furrows    on    the 


epitrich. 
germ.  I. 
dermis. 


_^      -_  corneous.  I. 

.^^'^s?^?^^-  mucous. I 


A. 


B. 


Fig.  481,  A. 


—Diagrammatic  Section  of  the  Skin  at  the  commencement  of  the  second 
month. 

B. — Diagrammatic  Section  of  the  Skin  at  the  commencement  of  the  fifth 
month,  a.  a.  a.  Infoldings  of  the  epidermis  between  the  primary 
ridges. 

corium.  About  the  ith  month,  the  dermal  papillae,  which  are  grouped 
in  lines  and  ridges  as  is  well  seen  in  the  palm,  are  formed  in  the  following 
manner  : 

Long,  linear  furrows  of  epidermis  grow  down  into  the  dermis  (corium) 
and  divide  its  surface  into  narrow  ridges  (Fig.  481,  B).  These  ridges 
are  subsequently  subdivided  into  papillae.  The  down-growing  nature  of 
the  ectodermal  (epidermal)  cells  which  is  here  exemplified,  is  of  the  greatest 
clinical  importance.  The  enamel  organs,  we  have  seen,  arose  by  a  species 
of  downgrowth  of  the  epidermis  ;  so  do  hairs,  sweat  glands  and  sebaceous 
follicles.  Prolonged  pressure  and  friction  welds  the  corneous  cells  into  a 
solid  plate,  such  as  the  callosities  seen  on  the  palms  of  manual  labourers. 
Normal  desquamation  is  arrested  ;  the  cells  produced  in  the  deeper  layers, 
unable  to  grow  to  the  surface,  grow  inwards  and  produce  corns.  In 
cancer,  the  epithelial  cells  of  the  skin  renew  their  youth  and  invade  the 
dermis  and  deeper  tissues. 

1  For  literature  see  E.  J.  Evatt,  Journ.  Anat.  and  Physiol.  1907,  vol.  41,  p.  66. 
Also  paper  by  Walter  Kidd,  same  volume,  p.  35.  M.  Heidenhain,  Anat.  Hefte,  1906, 
vol.  30,  p.  419.  0.  Schlaginhaufen,  Ergebnisse  der  Anat.  1905,  vol.  15,  p.  628.  H.  H. 
Wilder,  Amer.  Journ.  Anat.  1901,  vol.  1,  p.  423.  Walter  Kidd,  The  Sense  of  Touch  in 
Mammals  and  Birds,  London,  1907  ;  The  Initiative  in  Evolution,  1920. 


SKIN  AND  ITS  APPENDAGES 


465 


Sweat  glands  arise  as  buds  from  tlie  ectodermal  troughs  (Fig.  481,  B). 
Their  ducts  open  on  the  surface  of  the  skin  in  lines  or  rows  corresponding 
to  the  ]:)rimary  e})idermal  furrows.  In  the  5th  month  the  epidermis 
round  their  mouths  is  raised  up  into  ridges,  and  it  is  these  ridges  which 
give  rise  to  the  papillary  patterns  on  the  balls  of  the  fingers  and  elevations 
of  the  palm.     It  will  be  thus  seen  that  the  epidermal  ridges  correspond 


Fio.  482. — The  more  common  patterns  formed  by  the  Dermal  Papillae  on  the  Tips 

of  the  Fingers. 
A,  The  Loop  Pattern.     B,  The  Triangle  Pattern.     C,  The  Whorl  Pattern. 

not  to  the  lines  of  dermal  ]3apillae,  but  to  the  furrows  of  epidermis  lying 
between  the  papillae. 

The  papillary  lines  on  the  palms  and  fingers  give  security  of  grasp 
(Hepburn).  They  are  arranged  in  most  variable  patterns,  but  the  pre- 
vailing types  in  man  are  those  arranged  as  loops,  spirals  or  whorls  (Fig. 
482).  So  much  does  each  pattern  vary  and  so  variable  is  the  sequence 
of  the  patterns  on  the  pulps  of  the  digits,  that  no  two  people  show  exactly 
the  same  pattern  in  the  same  order  counting  from  thumb  to  little  finger 


HAND  FOOT 

Fig.  483. — The  "  Pad  "  Elevations  on  the  Palm  and  Sole  of  a  Human  Foetus  at  the 
end  of  the  2nd  month  of  development.     (After  Retzius.^) 

in  both  hands.  Hence  the  impress  of  the  ten  finger-tips  has  been  success- 
fully used  in  the  identification  of  criminals — a  practical  discovery  made  and 
put  into  use  by  Sir  Edward  Henry. 

These  epidermal  patterns  ^  are  formed  on  elevations  which  appear  on 
the  human  hand  and  foot  at  the  end  of  the  2nd  month,  and  which  certainly 
correspond  to  the  horny  pads  found  on  the  feet  of  quadrupeds.  Besides 
the  elevations  on  the  terminal  phalanges  there  are  five  situated  on  the 
palm  and  sole  at  the  base  of  the  digits.     Three  others  are  situated  on  the 


Biolog.  Untersuch.  1904,  vol.  11,  p.  33,  Jena, 

2g 


2  See  references,  p.  464. 


466 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


proximal  part  of  the  palm.  In  the  human  foot  the  elevation  corresponding 
to  the  hypothenar  elevation  of  the  palm  undergoes  a  remarkable  enlarge- 
ment to  cover  the  heel  (Fig.  483). 

The  Hairs.^^Hairs  begin  to  develop  in  the  4th  month,  although  in  some 
regions,  such  as  the  eyebrows  and  lips,  their  formation  begins  a  month 
earlier.  Morphologically,  a  hair  may  be  regarded  as  a  dermal  papilla 
which  has  sunk  in  the  subcutaneous  tissue,  and  become  capped  by  a 
process  of  epidermis.  Hairs  appear  to  have  been  primarily  touch  organs, 
and  are  modifications  of  the  touch  bodies  found  in  the  skin  of  reptilia 
(Gegenbaur).  These  touch  bodies  are  composed  of  epithelial  cells,  having 
the  same  shape  and  arrangement  as  those  which  form  the  taste  buds  in 
the  circumvallate  papillae  of  the  human  tongue.  The  cells  which  cap  the 
hair  papilla  evidently  represent  the  primary  sensory  cells  of  the  touch 


EPITRICHIUM 


MUC  STRAT. 

BASAL   STRAT. 


SEBAC.  GL. 


BASAL  STRAT. 

CENTRAL  STRAT. 

PAPILLA 


CENT  BAS.CE.LLb 


OUTER  SHEATH^ 
INNER  SHEATH 

PAPILLA 


PERIPHERAL 
CELLS 
BUlb 


(A) 


(B) 


(C) 


Fig.  484. — Three  Stages  in  the  development  of  a  Hair  Follicle.     (After  Stohr.) 

A,  Hair  Follicle,  commencing  to  form  in  a  Foetus  of  3  months. 

B,  The  downgrowth  of  the  follicle  and  mesodermal  thickening  to  form  papilla. 

C,  Invagination  of  Follicular  Bulb  by  Papilla  with  formation  of  Matrix  Cone. 

bodies  ;  they  are  situated  in  line,  and  continuous  with  the  basal  or  germinal 
layer  of  the  skin.  The  primary  function  of  the  hairs  as  touch  organs  is 
seen  in  the  vibrissae  round  the  mouths  of  carnivora.  Fnedenthal  has 
found  that  certain  of  the  hair-roots  in  the  lips  and  eyebrows  of  the  human 
foetus  develop  the  same  large  sensory  bulbs  as  are  found  in  the  roots  of  the 
vibrissae  of  lower  mammals. 

The  first  stage  in  the  development  of  a  hair  is  the  ingrowth  of  epidermis 
as  a  solid  bud,  which  pushes  in  front  of  it  the  dermis  to  form  the  papilla 
on  which  the  hair  grows  (Fig.  484).  Only  the  two  deeper  of  the  primary 
layers  of  the  epidermis  are  carried  inwards  to  form  the  hair  sheath  and 
hair  root.  Three  stages  in  the  development  of  a  hair  follicle  are  shown 
in  Fig.  484.     As  the  follicle  grows  downwards  the  fundus  of  its  shaft 

1  See  Friedenthal,  Zeitschrift  fur  Ethnol.  1911,  vol.  43,  p.  974;  K.  Backmund, 
Anat.  Hefte,  1904,  vol.  26,  p.  315  ;  P.  Stohr,  Aiiat.  Hefte,  1904,  vol.  23,  p.  1  ;  L.  Stieda, 
Anat.  Hefte,  1910,  vol.  40,  p.  285. 


SKIN  AND  ITS  APPENDAGES  467 

expands  to  form  a  bulb.  Outside  the  follicular  bulb  mesodermal  cells 
collect  to  become  a  papilla.  Presently  the  bullj  is  invaginated  by  the 
papilla,  which  thus  becomes  clothed  by  the  central  cells  of  the  basal  stratum 
while  the  surrounding  wall  of  the  bulb  is  lined  by  peripheral  basal  cells. 
On  the  papilla  and  within  the  shaft  of  the  follicle  is  produced  a  mass  of 
cells— the  hair  cone  (Fig.  484,  C) — the  first  rudiment  of  a  hair.  The 
central  cells  on  the  papilla  give  rise  to  the  cells  which  form  the  core  or 
pith  of  the  hair  shaft ;  from  the  peripheral  cells  arises  the  inner  root- 
sheath.  The  outer  root-sheath  is  formed  by  the  lining  cells  of  the  follicular 
shaft.  As  will  be  seen  from  Fig.  484,  C,  a  sebaceous  gland  is  produced 
from  the  shaft  of  the  hair  follicle,  while  the  erector  muscle  arises  from  the 
shaft  at  a  deeper  level. 

The  hairs  produced  at  the  4th  month  are  fine  in  texture  (lanugo),  and  by 
the  7th  month  the  whole  body  is  covered  by  them.  The  hair  roots  of  tbe 
eyebrows,  eyelids,  and  of  the  lips  and  scalp  are  the  first  to  appear.  The 
production  of  hair  buds  goes  on  until  birth,  the  later  buds  and  hairs  being 

termin.  of  corn,  layer 

.stratum  lucidum 

rete  mucosum  nail  fold 


.corneous  layer 
-stratum  lucidum 
nail  bed         ^'^^^^^         ^"^rete  mucosum 

Fig.  485. — Diagrammatic  Section  across  a  Nail. 

thicker  and  stronger.  After  birth,  new  hairs  are  constantly  reproduced 
within  the  sheaths  to  replace  the  old.  Probably  the  manner  in  which 
new  hairs  are  produced  resembles  that  of  teeth,  viz.,  from  processes  of  the 
original  bud.  Hairs  appear  first  on  the  head  and  then  on  other  parts  of 
the  body.  The  fat  in  the  subcutaneous  tissue  takes  the  place  of  hair  as  a 
heat  conserver.  Certain  sexual  hairgrowths  appear  at  puberty  on  the 
face,  pubes  and  axilla.  Morphologically,  the  pubic  region  represents  the 
separated  axillary  regions,  and  probably  th.e  explanation  of  sexual  hairs 
in  the  axilla  is  due  to  this  correspondence,  for  there  is  a  persistent  tendency 
towards  symmetry  of  development  in  the  upper  and  lower  extremities. 
The  primitive  mammary  ridges,  also  sexual  structures,  end  at  the  axilla 
and  groin. 

Nails. — The  nails  are  made  up  of  three  strata  representing  the  basal 
layer  of  cylindrical  cells,  the  stratum  mucosum  and  the  stratum  lucidum 
of  the  skin,  the  corneous  layer  being  lost  after  the  4th  month  of  foetal  life. 
They  appear  first  in  the  3rd  month  as  fields  of  thickened  epidermis  on  the 
tips  of  the  digits  (Fig.  473),  but  are  afterwards  shifted  dorsally,  carrying 
their  palmar  nerves  with  them,  so  that  the  terminal  phalanx  is  wholly 
supplied  from  the  palmar  digital  branches.  At  the  end  of  the  3rd  month 
the  germinal  layer  of  epithelium  at  the  proximal  margin  of  the  nail  field 
forms  a  lamina  which  grows  into  the  dermis  to  form  the  root  and  is  thus 
overhung  by  a  reflection  of  skin — the  nail  fold  (Fig.  486).     The  nail  of  the 


468 


HUMAN  EMBRYOLOGY  AND  MORPHOLOaY 


little  toe,  a  digit  in  a  retrograde  phase  of  development,  is  frequently 
shaped  like  a  claw,  probably  a  reversion  to  a  primitive  form.  The  nail 
is  produced  on  the  scattered  papillae  (the  matrix)  at  its  root.  The  area  of 
production  is  marked  by  the  lunule.  On  the  nail  bed,  in  front  of  the 
lunule,  the  papillae  are  arranged  in  longitudinal  rows.  If  the  nail  be 
pressed,  as  by  the  boot,  the  lateral  papillae,  under  the  nail  fold  (see  Fig. 
485)  are  directed  downwards,  and  their  epithelial  outgrowths  follow  the 
same  direction,  thus  causing  ingrowing  nail. 

About  the  end  of  the  7tli  month  the  matrix  of  the  nail  root  becomes 
differentiated,  active  growth  sets  in  and  the  terminal  margin  of  the  nail 
becomes  free  ;  it  grows  forwards  over  the  corneous  layer  which  covers  the 
terminal  row  of  papillae  of  the  nail  bed.     The  ridge  of  corneous  epithelium 


SUBUNGUAL  WELT 
(Frog-) 


FIBROUS   CAP 


TERM    PHAL: 


Fig.  486. — Diagrammatic  Section  of  the  Terminal  Joint  of  tiie  Digit  of  a  Human 
Foetus  to  show  the  Cap  of  the  terminal  Phalanx  and  the  Subungual  Welt. 

under  the  nail-tip  represents  the  central  part  of  the  hoof  ("  frog  ")  of 
ungulates  (Fig.  486). 

The  nail  is  carried  by  the  terminal  phalanges.  Professor  Leboucq 
observed  that  the  tip  of  the  terminal  phalanges  of  the  foetus  is  covered 
by  a  special  fibrous  cap  ^  (Fig.  486),  which  undergoes  ossification  directly 
from  membrane,  while  the  rest  of  the  phalanx  is  laid  down  and  ossified 
in  cartilage.  The  terminal  phalanges  have  thus  a  special  element  added 
to  them  for  the  support  of  the  nail  and  for  the  fixation  of  the  terminal  bulb 
of  the  digits. 

Sweat  Glands.^ — In  the  4th  month  solid  processes  of  epidermis  grow 
into  the  dermis  from  the  ectodermal  troughs  and  also  from  the  necks  of 
hair  follicles  and  produce  sweat  glands  (Fig.  481,  B).  They  arise  at  the 
same  time  and  in  the  same  manner  as,  and  often  in  common  with,  the  buds 
of  hair  roots  and  sebaceous  glands.  They  are  produced  within  the  epi- 
dermal ridges,  and  hence  the  ducts  of  sweat  glands,  as  may  be  seen  on  the 

^  F.  A.  Dikey,  Journ.  Anat.  and  Physiol.  1906,  vol.  40,  January. 
2  P.  Diem,  Anat.  Hefte,  1907,  vol.  34,  p.  187  ;    C.  Schoeppler,  Anat.  Hefle,  1907, 
vol.  34,  p.  429. 


SKIN  AND  ITS  APPENDAGES  469 

palms  and  fingers,  open  along  the  summits  of  these.  The  sweat  glands  in 
the  axilla  arc  peculiar.  In  section  they  resemble  the  acini  of  the  mammary 
gland,  also  believed  to  be  highly  modified  sweat  glands.  The  axillary 
glands  contain  much  epithelial  debris.  They  appear  to  be  sexual  in 
nature.  The  wax  glands  of  the  external  auditory  meatus  are  also  modified 
sweat  glands. 

Sebaceous  Glands. — The  sebaceous  glands  are  outgrowths  from  the 
more  superficial  part  of  hair  buds  (Fig.  484).  Their  epithelial  lining  is 
derived  from  the  germinal  layer.  In  hair  sheaths  which  have  become 
occluded  after  their  hairs  have  been  shed  or  lost,  or  when  the  mouth  of  a 
gland  is  blocked,  the  secretion  is  retained,  and  a  sebaceous  cyst  or  wen, 
so  frequently  seen  in  the  scalp,  is  produced.  Round  the  mouth,  on  the 
lips  and  nose,  the  sebaceous  glands,  especially  in  disorders  of  the  sexual 
organs,  are  apt  to  retain  their  secretions  and  become  inflamed,  smaU 
pustules  being  thus  produced.  The  Meibomian  glands  in  the  eyelids  are 
modified  sebaceous  glands.  At  birth  the  child  is  covered  by  the  vernix 
caseosa,  which  is  composed  of  desquamated  corneous  epithelium  and  the 
secretion  of  sebaceous  glands. 


MAMMARY  GLANDS. 

Evolutionary  History. — It  is  a  remarkable  fact  that  although  the  milk 
glands  do  not  come  into  use  until  adult  life  and  although  they  must  be 
regarded  as  among  the  later  evolved  structures  of  vertebrate  animals, 
yet  they  are  the  first  of  all  the  glands  arising  from  the  epidermis  to  appear 
during  development  of  the  embryo.  In  the  human  embryo  of  the  6th 
week  or  in  the  corresponding  stage  of  a  pig  (Fig.  487),  or  of  any  other 
mammal,  the  primary  mammary  ridge  or  milk  line — a  mere  surface  thicken- 
ing of  the  ectoderm — is  seen  extending  along  the  body  wall  on  either  side 
from  axilla  to  groin.  Breslau  ^  regards  these  primary  ridges  as  representa- 
tives of  the  brooding  organs  of  the  ancestors  of  mammals,  from  which 
structures  he  supposes  that  the  mammary  glands  were  evolved.  In  a 
large  number  of  human  beings  (15  %)  one  or  more  supernumerary  nipples 
are  to  be  found  between  the  axilla  and  groin,  indicating  the  wide  distribu- 
tion of  ancestral  glands.  There  is  no  longer  any  doubt  that  the  mammary 
acini  and  ducts  have  been  modified  from  sweat  glands  ;  a  mamma  represents 
a  group  of  sweat  glands  developed  from  a  circumscribed  area  of  skin 
lying  under  the  primitive  mammary  ridge.  Nor  are  there  two  opinions 
as  to  the  stages  in  the  evolution  of  the  human  nipple  ;  they  are  repeated 
in  its  development.  In  its  primitive  form  the  nipple  is  represented  by  a 
pocket — an  invaginated  area  of  mammary  skin — on  the  wall  of  which 
milk  ducts  open.  This  pocket — an  inverted  nipple — becomes  everted, 
chiefly  by  a  proliferation  of  the  tissues  round  the  terminal  parts  of  the 
duct,  which  raises  the  interior  of  the  pocket  first  to  the  level  of  the  sur- 
rounding skin  and  then  above  it  to  form  a  nipple — an  everted  mammary 

1  The  Mammary  Apparatus  of  the  Mammalia,  with  Introduction  by  Prof.  J.  P.  Hill, 
London,  1920, 


470 


HUMAN  EMBEYOLOGY  AND  MORPHOLOGY 


pocket.  Further,  the  mammary  ridge  appears  in  both  sexes  alike,  but 
this  may  not  mean  that  both  sexes  of  ancestral  mammals  were  concerned 
in  brooding  or  gave  milk.  The  male  is  the  father  of  girls  as  well  as  of  boys  ; 
it  is  therefore  necessary  to  provide  both  father  and  mother  with  a  complete 
sexual  outfit  if  each  sex  is  to  provide  equal  shares  to  the  making  of  their 
progeny.  In  females  the  breasts  undergo  a  great  development  at  puberty, 
while  in  males  they  retain  their  infantile  form. 

The  Female  Breast  is  composed  of  two  embryological  elements  :  (a) 
Glandular  tissue  derived  from  the  ectoderm  by  a  process  of  inbudding  ; 
(b)  An  intricate  arrangement  of  connective  tissue  derived  from  the  meso- 
dermal subcutaneous  tissue  over  the  pectoralis  major. 


MAMMARY 
RIDGE 


LEG  BUD 


Fig.  487. — Embryo  of  a  Pig,   showing  the  Mammary  Ridge  extending  from  Axilla 
to  Groin.     (After  Schultze.) 

Fig.  488. — Diagram  to  show  the  Position  in  which  Supernumerary  Nipples  are  usually 
found.     (After  Merkel.) 

Seven  stages  may  be  recognized  in  the  developmental  history  of  the 
glandular  mammary  tissue.     Four  of  these  take  place  before  birth  : 

(1)  The  stage  represented  by  the  ectodermal  ridge  passing  from  axilla 
to  groin — formed  during  the  6th  week  (Fig.  489,  A). 

(2)  The  production  of  a  bulb-like  downgrowth  of  ectoderm  from  the 
pectoral  part  of  the  mammary  ridge.  This  downgrowth  represents  the 
pocket  form  of  nipple  (Fig.  489,  B). 

(3)  From  the  deepest  stratum  of  the  ectodermal  bulb  arises  a  number 
of  solid  buds,  exactly  similar  to  those  of  sweat  glands  (5th  month).  The 
stalks  of  these  buds  form  the  epithelial  lining  of  the  lactiferous  ducts 
(Fig.  489,  C). 

(4)  The  lobular  buds,  for  each  bud  develops  into  a  lobe,  subdivide  at 
their  growing  extremities.  At  first  solid,  they  begin  to  canaliculize  (7th 
to  9th  months).  At  or  about  birth  the  pit  or  depression,  from  which  the 
lobular  buds  originated,  is  raised,  evaginated,  and  forms  the  surface  of 


SKIN  AND  ITS  APPENDAGES 


471 


the  nipple  (Fig.  489,  D).  Thus  the  ducts  come  to  open  on  the  apex  of  the 
nipple.  An  ampulla  is  developed  in  each  duct  within  the  base  of  the 
nipple.  It  is  normal  for  the  glandular  tissue  of  the  newly  born  child  to 
secrete  milk  during  the  two  weeks  following  birth  (Roger  Williams). 

Stages  after  Birth. — Stage  5  occurs  at  puberty  ;  the  latent  infantile 
lobular  buds  again  undergo  a  rapid  growth,  and  give  rise  to  the  minor 
lobules  and  acini.  Stage  6  occurs  towards  the  end  of  pregnancy,  and 
consists  of  a  renewed  production  of  glandular  tissue.  Stage  7  sets  in  with 
the  menopause,  and  is  characterized  by  an  atrophy  of  the  glandular  tissue 
formed  in  the  later  stages  of  development. 

In  the  process  of  subdivision,  minor  buds  of  adjacent  lobes  frequently 
unite  together.     Hence  it  is  found  difficult,  during  dissection,  to  separate 


A. 


B-^>-/V\£^ 


Fig.  489. — Showing  the  various  Stages  in  the  Development  of  the  Mamma. 
A,  during  the  2nd  month  ;  B,  at  the  commencement  of  the  3rd  month  :  C,  at  the 
5th  month  ;  D,  at  birth. 
A=Ectoderm;  B  =  Subcutaneous  tissue  (mesoderm) ;  c=Pectoralis  major. 

the  gland  into  its  primary  lobes.  In  any  of  the  three  later  stages  a  localized 
and  invading  hypertrophy  of  the  cells  of  the  glandular  tissue  may  take 
place.  In  this  manner  cancer  is  produced.  The  part  played  by  the 
lymphatics,  which  are  situated  in  the  mesodermal  tissue  of  the  gland, 
in  the  spread  of  this  disease,  makes  their  study  important. 

Origin   of    the    Capsular  or   Mesodermal   Part    of    the    Gland. — 

As  the  glandular  buds  grow  into  the  subcutaneous  mesodermal  tissue, 
which  reacts  and  hypertrophies  around  the  invading  processes,  they  divide 
it  (see  Fig.  490),  into  {a)  superficial,  and  (6)  deep  layers,  these  being  joined 
together  by  (c)  interstitial  septa.  The  superficial  and  deep  layers  are 
fused  in  {d)  the  circum-mammary  tissue  in  which  the  final  glandular  buds 
terminate.     The  processes  as  they  grow  outwards  also  take  on  (e)  perilobular 


472 


HUMAN  EMBRYOLOaY  AND  MORPHOLOGY 


and  periductal  sheaths.  The  deep  and  superficial  layers  are  also  connected 
with  the  anterior  sheath  of  the  pectoral  muscles  and  the  skin — for  they 
are  all  parts  of  the  same  subdermal  layer. 

Lymphatics. — We  have  already  seen  (p.  337)  that  during  the  3rd  month 
the  skin  and  subcutaneous  tissues  become  invaded  by  the  developing 
system  of  lymph  vessels,  the  pectoral  system  lying  chiefly  in  the  zone 
arising  in  connection  with  the  jugular  lymph  sac.  As  each  part  of  the 
capsule  carries  with  it  lymph  vessels  of  the  pectoral  subdermal  area  it 
will  be  seen  that  the  arrangement  of  the  parts  of  the  capsule  is  an  im- 
portant matter  in  both  the  physiology  and  surgery  of  the  gland.  The 
periductal  and  perilobular  lymphatics  communicate  through  the  septal 
or  interstitial  vessels  with  the  superficial  mammary  and  deep  (retro- 
mammary)   lymphatics    (Fig.    490).     The   superficial   communicate   with 

subcutaneous  /ympfi. 
superficial  mammary 

mam. 


sheath      pectoralis  major    ^"^^^''^^'^''^f 


retro-mammary 


Fig.  490. — Diagrammatic  Section  of  tlie  Breast  to  show  tiie  Arrangement  of  its 
Capsule  and  Lymphatics.  The  lymphatic  \'essels  are  represented  by  thin  wavy 
lines. 

the  subcutaneous  ;  the  deep  with  those  in  the  pectoral  siieath,  and  thus 
it  will  be  seen  that  mammary  cancer  may  spread  to  the  skin  or  pectoralis 
major.  The  deep  and  superficial  join  in  the  circum-mammary  lymphatics, 
and  from  these  pass  efferent  vessels  to  the  pectoral  and  central  glands  of 
the  axilla.  The  lymph  passes  from  these  to  the  deep  axillary  and  inferior 
deep  cervical  glands — all  of  which  are  involved  in  late  stages  of  cancer 
of  the  breast.  Other  efferent  vessels  pass  from  the  circum-mammary  to 
the  anterior  intercostal  glands  of  the  upper  four  spaces  ;  one  or  two  vessels 
may  go  to  the  cephalic  gland.  During  the  mammary  hypertrophy, 
which  takes  place  at  the  end  of  pregnancy,  there  is  a  further  formation 
of  lymphatic  glands  in  the  axilla  (Stiles). 

Peripheral  Remnants. — Besides  accessory  nipple  ingrowths,  which  are 
to  be  found  in  most  foetuses  of  the  3rd  month,  isolated  or  semi-isolated 
small  masses  of  glandular  substance  may  be  found  situated  in  the  circum- 
mammary  tissue,  beyond  the  body  of  the  gland.  Some  may  pierce  the 
sheath  of  the  pectoralis  major,  and  become  a  source  of  recurrent  cancer. 


SKIN  AND  ITS  APPENDAGES  473 

The  presence  of  glandular  remnants  is  explained  by  the  fact  that,  when  the 
primary  budding  takes  place,  the  subdermal  tissue  is  shallow  and  of  small 
extent  ;  in  the  subsequent  growth  of  the  thorax,  the  tissue  in  which  the 
mamma  is  developed  is  widely  spread  out. 

Fat  begins  to  be  deposited  in  the  subcutaneous  tissue  during  the  5th 
month  of  foetal  life.  It  forms  a  large  element  of  the  mammary  gland 
after  puberty.  The  subcutaneous  tissue,  out  of  which  the  capsule  of  the 
gland  is  formed,  normally  contains  much  fat.  After  lactation,  when  the 
glandular  tissue  atrophies  to  a  considerable  extent,  a  growth  of  fat  replaces 
it.  If  no  fat  is  deposited,  or  if  it  be  absorbed,  then  the  breast  loses  its 
plump  form  and  hangs  on  the  chest. 

The  mammary  nerves  (secretory)  come  from  the  3rd,  4th  and  5th  inter- 
costals  ;  the  nipple  is  supplied  from  the  same  nerves.  The  nipple  contains 
non-striated  muscle,  and  is  covered  with  touch  papillae,  and  surrounded 
by  modified  sweat  and  sebaceous  glands. 

Dermis  and  Subcutaneous  Tissue. — The  subectodermal  tissues,  out 
of  which  the  dermis  and  subcutaneous  stratum  are  differentiated,  is  at 
first  composed  of  cells  of  rounded  outline  embedded  in  a  homogeneous 
jelly-like  matrix — a  syncytium.  Mall  regarded  the  matrix  as  a  living 
substance  in  which,  quite  independently  of  the  cells,  connective  tissue 
fibres  are  differentiated,  both  white  and  yellow.  Processes  are  certainly 
developed  from  the  cells,  but  it  is  doubtful  if  these  ever  become  detached 
and  form  independent  fibres. 

Fat  Cells. — Certain  granular  cells  of  the  connective  tissue,  especially 
of  the  subcutaneous  layers,  have  the  property  of  secreting  fat,  which 
appears  first  as  diffuse  droplets.  These  ultimately  run  together  and 
produce  the  characteristic  outline  of  adipose  cells.  Fat  cells  appear 
first  in  the  subcutaneous  tissue  during  the  5th  month  of  foetal  life  ;  later 
it  appears  in  the  subserous  tissue  of  the  body  wall.  It  reaches  its  greatest 
normal  development  just  before  and  after  birth.  Two  theories  are  held 
regarding  the  origin  of  fat  cells  :  (1)  that  they  are  cells  of  the  connective 
tissue  differentiated  and  set  aside  permanently  to  form  and  store  fat ; 
(2)  they  are  ordinary  connective  tissue  cells  temporarily  laden  with  fat.^ 
There  is  present  at  birth  a  sharply  differentiated  mass  of  fat  and  lymphoid 
tissue  in  each  posterior  triangle  of  the  neck  and  extending  on  each  side 
beneath  the  trapezius  muscle.  Hatai  regards  this  mass  as  the  represen- 
tative of  the  interscapular  gland  of  hibernating  mammals  (see  p.  339). 

Touch  Bodies  and  Sense  Organs. — The  cells  of  the  ectoderm  in  the 
simpler  forms  of  invertebrate  animals  not  only  protect  the  body  but 
many  of  them  become  specially  sensitive  or  nervous  in  nature,  developing 
processes  which  link  them  with  neighbouring  or  even  distant  cells  and  thus 
are  able  to  afford  the  animal  knowledge  of  its  surroundings.  In  the 
development  of  the  olfactory  mucous  membrane,  of  the  auditory  cells  and 
of  the  taste  buds  of  the  human  embryo,  this  specialization  of  areas  of  the 
ectoderm  is  seen.  The  retina,  the  brain,  spinal  cord  and  nerves  are  also 
areas  of  the  ectoderm  which  have  been  highly  specialized  and  set  aside  for 

J  See  article  by  Batty  Shaw,  Journ.  Anat.  and  Physiol.  1902,  vol.  36,  p.  1. 


474      HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 

the  purpose  of  correlating  the  organism  with  its  surroundings.  Such  cells 
may  migrate  and  become  grouped  in  central  masses  of  the  nervous  system. 
Dart  and  Shellshear  have  shown  that  the  dermal  origin  of  neuroblasts 
is  a  factor  of  importance  in  the  origin  of  the  nerve  system.  Although 
the  various  forms  of  touch  bodies,  such  as  the  Pacinian  corpuscles  and  those 
of  Krause  and  Meissner,  have  not  been  traced  developmentally,  there  can 
be  little  doubt  that  they  arise  directly  from  the  epidermis  beneath  which 
they  are  situated. 


INDEX 


[The  numbers  refer  to  jmffcs.] 


Abbott,  Dr.  Maude,  327. 

Abdominal  musculature,  68,  404,  406,  423. 

Abdominal  ring,  399. 

Abnormalities      of      development,      44. 

(See  also  under  each  part.) 
Accessory  obturator  nerve,  437. 
Accessory  processes  of  vertebrae,  64. 
Accessory  supra  renals,  403. 
Accessory  thyroid  bodies,  264. 
Acetabulum,  443. 
Achondroplasia,  152. 
Achromatin,  8. 
Acoustic  ganglia,  237. 
Acroceplialy,  153. 
Acromegaly,  108,  152,  177. 
Acromion  process,  446. 
Adloff,  P.,  187. 
Adrenals,  402. 
Affenspalte,  124,  130. 
Agar,  W.  E.,  8,  155. 
Age  changes  in  foetus,  45. 
Age  of  foetus.  Estimation  of,  46,  48. 
Agger  nasi,  200. 
Aichel,  389. 
Air  sacs,  348,  353. 
Ala  temporalis,  137,  147. 
Alar  cartilages  of  nose,  163,  168. 
Alar  lamina  of  cord,  79. 
Alar  lamina  of  hind-brain,  85.- 
Albrecht,  165. 

Alimentary  tract,  17,  19,  267. 
Alisphenoid,  147. 
AUantois,  17,  24,  26. 
Allantois,  Evolution  of,  24. 
Allen,  B.  M.,  365. 
Alveolar  trough,  176,  185. 
Ameloblasts,  183. 
Ammoecetes,  97,  101. 
Amnion,  12,  14,  24. 
Amnion,  Evolution  of,  27. 
Amniotic  adhesions,  15. 
Amniotic  fluid,  14. 
Amphioxus,  Gastrulation  of,  38. 
Anal  fascia,  413. 
Anal  glands,  396. 


Anal  valves,  386. 

Andrews,  H.  R.,  15. 

Anencephaly,  83. 

Angioblastic  tissue,  302,  313,  334,  337. 

Angular  gyrus,  221. 

Anlage,  15. 

Ano-coccygeal  ligament,  410- 

Ano-coccygeal  tumours,  391. 

Anterior  commissure,  121. 

Anthropoid  apes,  52,  81,  128,  155,  174, 
199,  221,  278,  288,  347,  348,  355, 
357,  405,  418,  447,  453,  454,  463. 

Antrum  of  Highmore,  173,  197. 

Antrum  of  mastoid,  230. 

Anus,  Formation  of,  381. 

Anus,  Malformation  of,  385. 

Anus,  Musculature  of,  389. 

Aorta,  dorsal,  30,  70,  249,  251,  333. 

Aorta,  stenosis  of,  252. 

Aortic  arches,  243,  249,  251. 

Aortic  septa,  323. 

Aortic  stems,  323. 

Appendix,  295,  297. 

Aqueduct  of  Sylvius,  95,  219. 

Aqueductus  cochlea,  235. 

Aqueous  chamber,  212. 

Arachnoid,  84,  134. 

Archenteric  vesicle,  12,  17. 

Archenteron,  12,  38,  267. 

Archipallium,  118. 

Arch  of  aorta  on  right  side,  250. 

Arch  of  foot,  Development  of,  449. 

Arm.     See  Limbs. 

Arm,  Nerve  supply  of,  431. 

Arteries,  Morphology  of,  70. 

Arteries.     See  Blood  vessels. 

Arteries  of  body  segment,  69. 

Arytenoids,  352. 

Ask,  F.,  216. 

Assheton,  R.,  39,  42. 

Asterion,  143. 

Astragalus,  447. 

Atavism,  350. 

Atlas,  58,  62,  145. 

Atlas,  Joints  of,  62. 

Atria.     See  Auricles. 

Attic  of  tympanum,  229. 

475 


476 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


Attraction  sphere,  8. 

At  well,  W.  J.,  106. 

Auditory  capsule,  138,  145,  224. 

Auditory  centres,  132,  238. 

Auditory  meatus,  external,  179,  225. 

Auditory  meatus,  internal,  239. 

Auditory  nerve,  88,  98,  224,  237. 

Auditory  ossicles,  172,  222,  229. 

Auerbach's  plexus,  71,  292. 

Auricles  of  heart,  316,  320. 

Auricular  canal,  316. 

Auricular  septa,  322. 

Auriculo-ventricular  bundle,  318,  329. 

A uriculo -ventricular  valves,  327. 

Axillary  glands,  468,  472. 

Axillary  hair,  467. 

Axis,  62. 

Axis  cylinders,  82. 

Axon,  82. 

Ayrers,  H.,  200. 

Azygos  lobe  of  lung,  349. 

Azygos  uvulae,  170. 

Azygos  veins,  307. 


B 

Backmund,  K.,  466. 

Bailey,  P.,  134. 

Ballantyne,  J,  W.,  42. 

Bardeen,  C.  R.,  54,  59,  68,  135,  290,  425, 

467. 
Bardeleben,  54. 
Barniville,  H.  L.,  47. 
Barrv,  D.  T.,  396. 
Bartholin,  Glands  of,  396. 
Basal  ganglia,  116. 
Basal  lamina  of  cord,  79. 
Basilar  membrane,  236. 
Basilar  plate  of  skull,  144. 
Bashford  and  Murrav,  8. 
Basi-hyal  247. 
Basion,  153. 

Baumgartner,  E.  A.,  22,  106. 
Beard,  20,  42. 
Bell,  E.  T.,  262. 
Bell,  W.  Blair,  6. 
Berry,  James,  159. 
Berry,  R.  J.  A.,  142,  292,  294,  298. 
Biceps  of  arm,  453. 
Biceps  of  leg,  454. 
Bicipital  rib,  56. 
Bilaminar  blastocyst,  11. 
Bilaminar  blastoderm,  11. 
Bile  ducts,  273,  274. 
Binocular  vision  ,210. 
Birth,  Changes  at,  330,  351. 
Black,  Davidson,  89,  162. 
Bladder,  390. 
Bladder,  Fixation  of,  414. 
Bladder,  Malformations  of,  390. 
Bladder,  Musculature  of,  391. 
Blaisdell,  F.  E.,  50. 
Bland-Sutton,  63,  150,  173. 
Blastema  of  limbs,  432. 
Blastema  of  skeleton,  49. 


Blastocyst,  11. 

Blastopore,  38. 

Blastula,  11. 

Blood,  302,  335. 

Blood-islands,  302,  334. 

Blood  vessels,  Formation  of,  30,  302,  334. 

Body-stalk,  17,  30. 

Body,  Symmetry  of,  415. 

Bodv,  Ventral  line  of,  415. 

Body  wall,  404,  415,  423. 

Body  wall.  Stages  in  Evolution  of,  404. 
423. 

Bolk,  L.',  63,  161,  188,  431. 

Bolton,  J.  S.,  124. 

Bone,  cartilaginous,  140,  457. 

Bone,  membranous,   141,  458. 

Bone-marrow,  336,  339,  458. 

Bones,  Development  of,  143,  438. 

Bones,  Foramina  of,  150,  173. 

Bones,  Growth  of,  459. 

Bones,    Lines    of    pressure    and    tension 
in,  461. 

Bones,  Ossification  of,  143,  457. 

Bonnot,  339. 

Brachet,  A.,  353. 

Brachial  artery,  439. 

Brachial  plexus,  432. 

Brachycephalic  skull,  151. 

Brachvdactyly,  461. 

Bradley,  0.  Charnock,  271,  273. 

Brain,  101. 

Brain,  Arteries  of,  132. 

Brain  capsule,  140. 

Brain,  Commissures  of,  120. 

Brain,  Development  of,  101. 

Brain,  Evolution  of,  104. 

Brain,  Fissures  of,  124. 

Brain,  Growth  of,  47,  139,  144. 

Brain,  Membranes  of,  133. 

Brain,  Secondary  sulci  and  gyri  of,  132. 

Brain,    Significance    of    convolutions    of, 

123. 
Branchiae,  340. 
Branchial  arches,  20,  47,  228,  240,  244, 

352. 
Branchial  bodies,  265. 
Branchial  clefts,  228,  243. 
Branchial  nerves,  98. 
Branchial  segments,  41,  97,  156. 
Branchiomere,  97,  156. 
Brash,  J.  C,  54. 
Braus,  H.,  434. 
Breast  of  female,  470. 
Bregma,  143. 

Bregmatic  fontanelle,  140. 
Bremer,  J.  L.,  47,  302,  350,  359,  364. 
Breslau,  E.,  469. 
Broad  ligament,  4. 
Broman,  69,  70,  200,  205,  230,  269,  277, 

290,  334,  353. 
Bronchi,  Formation  of,  346. 
Bronchi,   Ramification  of,   348. 
Bruni,  A.,  57. 

Bryce,  T.  H.,  13,  84,  142,  204,  261,  263. 
Buchanan,  G.,  359, 


INDEX 


477 


Bulbar  canal,  316. 
Bulbar  cushions,  32(5. 
Bui  bus  cordis,  315,  324. 
Bulla  cthnioidalis,  199. 
Bulloch,  W.,  401. 
Burne,  R.  H.,  102. 
Bursa  omentalis,  276,  286. 
Butterfield,  260. 

C 

Caeco-colic  sphincter,  295. 

Caecum,  Development  of,  290. 

Caecum,  Morphology  of,  295. 

Calcaneo-scaphoid  ligament,  450. 

Calcar  avis,  220. 

Calcarine  fissure,  129,  220. 

Calloso- marginal  fissure,  129. 

Calvaria,  141. 

Cameron,  J.,  68,  84,  123,  207,  308,  409. 

Canalis  cranio-pharyngeus,  107,  149. 

Capsular  ligaments,  456. 

Cardiac  septa,  322. 

Cardiac  tube.  Flexures  of,  322. 

Cardiac  tubes.  Origin  of,  312. 

Cardinal  veins,  303,  306,  403. 

Carey,  E.  J.,  296. 

Carotid  arteries,  250,  252,  332. 

Carotid  body,  266. 

Carpale,  I.,  II.,  III.,  IV.,  V.,  447. 

Carpus,  446. 

Carsall,  0.,  200. 

Carter,  J.  T.,  187,  190. 

Carotid  sheath,  414. 

Cartilage  bone,  140,  455,  458. 

Cartilage,  Differentiation  of,  455. 

Cartilages  of  Jacobson,  163. 

Cartilages  of  visceral  arches,  246. 

Cartilaginous    part    of    skull,    DeveloiD- 

ment  of,  137,  140. 
Castration,  Effects  of,  396. 
Cauda  equina,  76. 
Caudal  part  of  spinal  cord,  77. 
Caudal  somites,  41,  65. 
Caudate  lobe  of  liver,  279. 
Caudate  nucleus,  112. 
Caudopore,  75. 
Cavernous  sinus,  133. 
Central  canal,  75. 
Central  fissure,  131. 
Centrosome,  8,  10. 
Centrum,  58. 
Cephalic  index,  151. 
Cephalic  vein,  440. 
Cerato-hyal,  247. 
Cerebellar  tracts,  92. 
Cerebellum,  85,  91,  239. 
Cerebellum,  Fissures  of,  92. 
Cerebellum,  Function  of,  92,  94. 
Cerebellum,  Peduncles  of,  92. 
Cerebral  hemispheres,  118. 
Cerebral  vesicle,  101,  111, 
Cerebral  vessels,  132,  306. 
Cerebro-spinal  fluid,  90,  134. 
Cerebrum,  Growth  of,  139. 


Cerebrum,  Origin  of,  101. 

Cervical  curvature,  53. 

Cervical  fascia,  414. 

Cervical  flexure,  49. 

Cervical  rib,  56,  442. 

Cervical  sinus.  Formation  of,  47,  244,  263. 

Cervico-occipital   region.     Variations    of, 

56. 
Cheatle,  A.,  235. 
Chemotactic  influence,  20,  88. 
Chevron  bones,  65. 
Choanae,  160,  169,  195. 
Choanoid  muscle,  214. 
Chondriosome,  10. 
Chondrocranium,  136. 
Chordal  base  of  skull,  136. 
Chorionic  vesicle,  16. 
Chorionic  villi,  29. 
Choroid  coat,  213. 
Choroid  fissure  of  eve,  210. 
Choroid  velum,  102",  105,  111. 
Choroidal  fissure  of  brain,  114. 
Choroidal  villi,  90,  102,  113. 
Chorion,  14,  16,  24,  27. 
Chorion,  Evolution  of,  24,  27. 
Chromaffin  cells,  403. 
Chromatin,  8. 
Chromogenic  tissue,  402. 
Chromosome,  8. 
Ciliary  ganglion,  218. 
Ciliary  muscle,  213,  217. 
Ciliary  processes,  208,  213. 
Cingulum,  188. 
Circulation  at  birth,  330. 
Circulation  established,  20,  30. 
Circulatory  system.  Development  of,  301. 
Clark,  E.  R.,  337. 
Clarke,  H.  R.,  372. 
Claustrum,  117. 
Clavicle,  445. 
Cleft-membrane,  243. 
Cleft  palate,  161,  165,  169. 
Cleido-cranial  dysostosis,   446. 
Clitoris,  387,  393. 
Cloaca,  26,  268,  361,  379. 
Cloaca,  Ectodermal,  380,  382. 
Cloaca,  Parts  formed  from,  381. 
Cloaca,  Sphincter  of,  389. 
Cloacal  membrane,  37,  381. 
Club-foot,  449. 
Coccygeal  body,  403. 
Coccygeus,  410,  454. 
Coccyx,  65,  405,  408. 
Cochlea,  Evolution  of,  223,  233. 
Cochlear  gandion,  224,  237. 
Cochlear  nucFei,  95,  238. 
Coeliac  axis,  268,  285,  334. 
Coelom,  Divisions  of.  19,  156,  208. 
Coelom,  Origin  of,  13,  39,  268. 
Coelom,  Pericardial,  40. 
Coelom,  Umbilical,  289. 
Coelomic  cavitj%  Origin  of,  13. 
Collateral  fissure,  130. 
Colliculi  of  mid-brain,  95. 
CoUinge,  W.  E.,  308. 


m 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


Coloboma  iridis,  210. 

Colon.     See  also  Hind-gut. 

Colon,  Development  of,  293. 

Columella,  225. 

Commissures,     Development     of.      111, 

121. 
Common  genital  mesentery,  3. 
Conception,  Period  of,  6. 
Concrescence  theory,  189. 
Congenital  pulmonary  stenosis,  327. 
Conjugal  ligament,  63. 
Connective  tissue,  412,  473. 
Contrahentes,  453. 
Conus  arteriosus,  316. 
Cope,  V.  L.,  149. 

Coraco-clavicular  ligaments,  446. 
Coraco -humeral  ligaments,  456. 
Coracoid,  444. 
Cornea,  206. 
Corner,  G.  W.,  283. 
Coronary  sinus,  317. 
Corpora  quadrigemina,  95,  218. 
Corpus  callosum,  123. 
Corpus  luteum,  6,  22. 
Corpus  striatum,  102,  104,  112,  115. 
Cortex,  Development  of,  117,  123,  124. 
Corti,  Organ  of,  236. 
Costal  cartilages,  418. 
Costal  processes,  56,  63,  417. 
Costo-colic  ligaments,  288. 
Costo-coracoid  ligament,  445. 
Cotyloid  ligament,  456. 
Cowdry,  E.  V.,  10. 
Cowper,  Glands  of,  10,  396. 
Crania,  Abnormal,  144,  152. 
Cranial  axis,  154. 
Cranial  base,  136,  154. 
Cranial  capsule,  134,  153. 
Cranial  nerves.  Nuclei  of,  85,  98. 
Cranial  nerves  in  visceral  arches,  248. 
Cranial  nerves.   Segmental  arrangement 

of,  98. 
Cranial  splanchnic  fibres,  73. 
Craniopharyngeal  canal,  107,  149. 
Cranium,  135. 
Cranium,  Base  of,  136. 
Cranium,  Growth  of,  139,  144. 
Cremaster,  398. 
Cribriform  plate,  167. 
Cricoid  cartilage,  252. 
Crista  gaUi,  164. 
Crown-rump  length,  48. 
Crucial  ligaments,  456. 
Crucial  sulcus,  131. 
Crura  cerebri,  96. 
Crural  ring,  407. 
Crusta  petrosa,  185. 
Crymble,  P.  T.,  286,  292. 
Cuboid,  448. 
Cuneate  nucleus,  88. 
Cunningham,  D.  J.,  54. 
Cushmg,  H.,  108. 
Cutaneous  structures,  462. 
Cuvier,  Duct  of,  303,  346. 


Cyclopia,  161,  201. 

Cysts  of  jaw,  187. 


D 


Dale,  284. 

Danchakoff,  Vera,  334. 

Dandy,  W.  E.,  18. 

Dareste,  41. 

Dart,  R.,  473. 

Darwin's  tubercle,  227. 

Decidua,  22. 

Decidua  reflexa,  23. 

Decidua,  Sensitization  of,  6. 

Decidua  serotina,  23. 

Decidua  vera,  23. 

Decidual  cells,  22. 

Dedekind,  F.,  217. 

Delbet,  413. 

Dendrites,  82. 

Dendy,  Prof.  A.,  97,  108. 

Dental  follicle,  184. 

Dental  lamina  or  shelf,  183. 

Dental  papilla,  183. 

Dental  sac,  184. 

Dentary  element,  176. 

Dentigerous     and     other    cysts     of     the 

jaw,  187. 
Dentine,  Origin  of,  184. 
Dentitions,  Number  of,  187. 
Dermal  bones.  Development  of,  141. 
Dermal  papillae.  Formation  of,  463. 
Dermatome,  58,  156,  426. 
Dermis,  463,  473. 
Derry,  D.,  63,  409. 
Diaphragm,  341,  353,  405,  421,  451. 
Dickie,  Dr.  Milne,  196. 
Diem,  F.,  468. 
Diencephalon,  101. 
Digby,  K.,  459. 
Digits,  426,  447,  468. 
Digits,  Muscles  of,  451,  453. 
Digits,  Supernumerary,  448. 
Dikey,  F.  A.,  468. 
Diploe,  144. 

Diverticula,  Congenital,  291. 
Dixon,  A.  F.,  61,  237,  366,  461. 
Dolichocephalic  skull,  151. 
Doran,  A.,  366,  390. 
Dorsal  aortae,  252,  333. 
Dorsal  commissure,  123. 
Dorso-cervical  region,  Variations  of,  55. 
Dorso-epitrochlearis,  452. 
Dorso-lumbar  region.   Variations  of,  55. 
Dorsum  sellae,  96,  148. 
Double,  A.  F.  I.e,  54. 
Douglas,  Pouch  of,  370. 
Drinkwater,  H.,  461. 
Drummond,  W.  B.,  339. 
Duckworth,  W.,  151,  153,  161,  353,  396. 
Ductless  glands.  Origin  of,  285. 
Duct  of  Gartner,  363. 
Ducts  of  Cuvier,  303,  346. 
Ductus  arteriosus,  250,  330,  351. 
Ductus  cndolyniphaticus,  232. 


INDEX 


479 


Ductus  vertosus,  273,  310. 

Duesberg,  J.,  10. 

Duodeno-jejunal  fossa,  298. 

Duodeno-jejunal  loop,  292. 

Duodenum,  284,  285. 

Duodenum,  Congenital  occlusion  of,  291. 

Duplication  of  parts,  43. 

Dura  mater,  84,  134,  143. 

D wight,  T.,  4.54. 

E 

Ear,  Development  of,  223. 

Ear,  Evolution  of,  223,  233. 

Ear,  External,  226. 

Ear,  Muscles  of,  227. 

Ear,  Nerve  centres  of,  238. 

Ear,  Ossicles  of,  172. 

Ectoderm,  12,  463. 

Ectoderm  of  hydra,  74. 

Ectopia  cordis,  333. 

Ectopia  vesicae,  390,  440. 

Ectorhinal  fissure,  129. 

Edgeworth,  F.  H.,  155,  190,  247,  253,  351. 

Elze,  C,  47. 

Embryo,  Differentiation  of,  1,  11. 

Embryo  and  membranes,  1,  14. 

Embryo  of  fourth  week,  18. 

Embryo,  Youngest  human,  13. 

Embryonic  cultures,  42. 

Embryonic  plate,  16,  37. 

Embryonic     structures.     Persistence     of, 

290. 
Emrys-Roberts,  E.,  24. 
Enamel  organ.  Remnants  of,  185, 
Enamel,  Origin  of,  183. 
Encephalocele,  83,  145. 
Endo-cardial  cushions,  322,  323. 
Endolymph  system,  235. 
EnsLform  process,  417. 
Entoderm,  12. 
Eparterial  bronchus,  349. 
Epaxial  muscles,  68. 
Ependyma,  78. 
Epiblast.     See  Ectoderm. 
Epibranchial  organs,  97. 
Epibranchial  placodes,  243,  248. 
Epicanthic  fold,  216. 
Epicoracoid,  422,  444. 
Epidermis,  463. 
Epididymis,  364,  398. 
Epiglottis,  258,  343,  352. 
Epi-hyal,  247. 
Epiotic,  235. 
Epiphyseal  lines,  459. 
Epiphyses,  Nature  of,  460. 
Epiphysis  (pineal),  108. 
Epipteric  bone,  150. 
Epipubis,  442. 
Epithalamus,  103. 
Epithelial  bodies,  265. 
Epitrochleo-anconeus,  453. 
Epitrichium,  462. 
Epoophoron,  364. 
Eruption  of  teeth,  189. 


Erythroblasts,  334. 

Erythrocytes,  26. 

Erythroplastids,  336. 

Esdaile,  P.  C,  13.5. 

Eternod,  A.  C.  F.,  29,  188,  311. 

Ethmoid,  Lateral  mass  of,  149,  196. 

Ethmoid,  Vertical  plate  of,  164 

Ethmoidal  cartilage,   137,   164,   196. 

Ethmoidal  infundibulum,  196. 

Ethmoidal  sinuses,  199. 

Eunuchs,  463. 

Eustachian  tube,  146,  224,  227. 

Eustachian  valve,  319. 

Evans,  H.  M.,  46,  132- 

Evatt,  E.  J.,  393,  464. 

Experimental    embryology,    41,    83,    89, 

206,  211,  223,  337. 
External  auditory  meatus,  245. 
Eye,  203. 
Eyeball,  203. 
Eyeball,  Growth  of,  214. 
Eyeball,  Muscles  of,  217. 
Eyebrows,  466, 
Eyelids,  215. 

F 

Face,  Development  of,  48, 153, 158. 

Face,  Evolution  of,  153,  158,  192. 

Face,  Malformations  of,  160. 

Face  and  scalp.  Muscles  of,  181,  253. 

Facial  angle,  153. 

Facial  nerve,  86,  237,  248,  253. 

Fairbank,  H.  A.  T.,  441. 

Falciform  ligament  of  liver,  278. 

Fallopian  tube,  6,  7,  363,  369,  372. 

Farmer,  J.  B.,  9. 

Farmer,  Sir  F.,  188. 

Fascia  dentata,  122. 

Fasciae,  Nature  of,  411,  428,  473. 

Fat  cells,  473. 

Fawcett,  E.,  61,  63,  135,  146,  163,  165, 

175,  322,  445. 
Federow,  V.,  350. 
Felix,  W.,  359. 
Female  pronucleus,  7. 
Femoral  hernia,  406. 
Femoral  vessels,  437. 
Femur,  429,  460. 
Fenestra  ovalis,  235. 
Ferguson,  J.  G.,  413. 
Fertilization,  Artificial,  41. 
Fertilization,  Phenomena  of,  7,  10. 
Fiddes,  J.  D.,  461. 
Fifth  cranial.     See  Trigeminal. 
Filum  terminale,  77. 
Fimbria,  Fallopian,  6. 
Fimbriae  of  fornix,  122. 
Fischel,  A.,  54,  461. 
Fischer,  G.,  189. 
Fish,  stage  of  development,  41. 
Fissure,  Longitudinal,  112. 
Fissure  of  Sylvius,  125. 
Fissures  of  brain,  124. 
Fissures  of  brain.  Formation  of,  123,  124. 


480 


HUMAN  EMBEYOLOGY  AND  MORPHOLOGY 


Fistulae  of  ear,  227. 

Fitzwilliams,  D.,  446. 

Flack,  Martin,  319. 

Flexor  accessorius,  450. 

Flexor  brevis  digitoruin,  450,  454. 

Flexure,  cervical,  49. 

Flexure,  Pontine,  89,  96. 

Flexures,  Neural,  96. 

Flint,  J.  M.,  349. 

Floccular  fossa,  236. 

Flocculus,  93. 

Foetal  circulation.  Remnants  of,  331. 

Foetus,  Definition  of,  48. 

Foetus,  Full-time,  50. 

Foetus,  Growth  of,  51. 

Foetus,  Measurements  of,  51. 

Follicular  cells,  5. 

Fontanelles,  143. 

Foot,  446. 

Foot,  Eversion  of,  449. 

Foot,  Muscles  of,  454. 

Foot,  Patterns  on  sole,  465. 

Foramen  caecum,  255,  264. 

Foramen,  Interventricular,  326. 

Foramen  magnum,  145. 

Foramen  ovale,  322,  331. 

Foramen  primum,  322,  328. 

Foramina  in  bone,  Formation  of,  150,  173. 

Fore-brain,  75. 

Fore-brain  in  human  embryo,  102. 

Fore-brain,  Roof  of,  102. 

Fore-brain,  Tracts  of,  104. 

Fore-gut,  17,  268. 

Fornix,  121,  195. 

Forssner,  H.,  291. 

Forster,  A.,  298. 

Forsyth,  265. 

Fossa  of  Rosenmiiller,  261. 

Fourchette,  388,  392. 

Fourth  ventricle,  87. 

Fovea  centralis,  209,  211,  214. 

Fox,  H.,  245. 

Fraser,  Eliz.  A.,  135,  217,  261,  359. 

Frassi,  L.,  47. 

Frazer,  J.  E.  S.,  106,  159,  170,  195,  225, 

227,  245,  271,  290,  326,  351. 
Friedenthal,  466. 
Frontal  bone,  141. 
Frontal  recess,  197. 
Frontal  sinus,  197. 
Fronto-nasal  process,  159. 
Fr  onto -pontine  tracts,  92. 
Froriep's  ganglion,  100. 
Fundus  of  stomach,  279. 
Funicular  process,  399. 
Funicular  tracts,  80. 
Furcula,  258,  343. 
Fiirst,  C,  209,  457. 
Futamura,  R.,  253. 


G 


Gadow,  H.,  61. 
Gage,  Sussana  P.,  18. 
Gall  bladder,  272,  274. 


Ganglia,  Posterior  root,  70. 

Ganglia,  Sympathetic,  71. 

Gartner,  Duct  of,  363. 

Gaskell,  W.  H.,  71,  75,  108,  155,  329; 

Gaskell's  theory,  157. 

Gastrocnemius,  448. 

Gastro -hepatic;  omentum,  275,  277,  287. 

Gastro-splenic  omentum,  273,  287. 

Gastrula,  38. 

Gaupp,  E.,  154,  165,  177. 

Geddes,  Sir  A.  C,  430,  431. 

Gemmill,  10,  43. 

Genial  tubercles,  177. 

Geniculate  bodies,  103,  238. 

Geniculate  ganglion,  86,  237. 

Genital  cord,  362,  370,  395. 

Genital  gland.  Differentiation  of,  21. 

Genital  organs  (external),  387,  392. 

Genital  ridge,  20,  362,  365,  396. 

Genital  tubercle,  387. 

Gennari,  Stria  of,  130,  220. 

Germ  cells,  5,  20. 

Germinal  epithelium,  5,  20. 

Germinal  neuroblasts,  78. 

Germinal  spot,  7. 

Germinal  vesicle,  7. 

Gestation,  Pelvic,  11. 

Gestation,  Period  of,  50. 

Gills.     See  Branchiae. 

Giraldes,  Organ  of,  365. 

Gladstone  R.  J.,  56,  144,  162,  291,  308, 

353. 
Glaesmer,  56. 
Glenoid  cavity,  445. 
Glosso-pharyngeal,  99,  248,  256. 
Goddard,  T.  R.,  397. 
Goeppert,  E.,  437. 
Goodrich,  E.  S.,  136,  155,  425. 
Gould,  E.  Pearce,  252. 
Graafian  follicle,  5. 
Gracile  nucleus,  88. 
Grail,  155. 

Great  omentum,  285. 
Great  sacro-sciatic  ligament,  454. 
Green,  Edridge,  214. 
Gregory,  W-  K.,  425. 
Greig,  D.  M.,  441. 
Greil,  324. 

Grosser,  0.,  244,  262,  267. 
Growth  of  foetus,  51. 
Gubernaculum  dentis,  186. 
Gubernaculum  testis,  3,  371,  397,  398. 
Gudernatsch,  J.  F.,  401. 
Gyri,  Formation  of,  123. 
Gyrus  dentatus,  122,  195. 
Gyrus  subcallosus,  122. 

H 

Habenula,  109. 
Haemal  arches,  55. 
Haemoblasts,  334. 
Haemolymph  glands,  339. 
Hair,  Eruption  of,  50,  462. 
Hairs,  462,  466. 


INDEX 


481 


HaUux,  447,  451. 

Hammar,  J.  A.,  261,  339. 

Hamulus  of  lachrymal,  215. 

Hand,  446. 

Hand,  Malformations  of,  461. 

Hand,  Muscles  of,  454. 

Hare  lijj,  161. 

Harrison,  R.  G.,  42,  69,  84,  89. 

Hart,  D.  Berry,  43,  373,  374,  388,  400. 

Hassall,  Corpuscles  of,  262. 

Head,  Shape  of,  47,  151. 

Healing  as  evolutionary  process,  162,  287. 

Hearing,  Structures  concerned  in,  223. 

Heart,  Abnormalities  of,  321,  325,  327. 

Heart,  as  placental  jiump,  312. 

Heart,  Changes  in  position  of,  254,  331. 

Heart,   Demarcation   of,   into   chambers, 

315,  322. 
Heart,  Development  of,  312. 
Heart,  Eminence  of,  48. 
Heart,  Evolution  of,  301. 
Heart,  Fixation  of,  303,  313,  332. 
Heart,  Septa  of,  322. 
Heart,  Valves  of,  327. 
Heidenhain,  M.,  464. 
Heiss,  R.,  346. 
Helicotrema,  235. 
Henry,  Sir  E.,  465. 
Hepatic  ducts,  273,  274. 
Hepatic  veins,  310. 
Hepburn,  D.,  142,  4G5. 
Hermaphrodites,  401. 
Hernia,  354,  406. 
Herring,  P.  T.,  106. 
Hertwig's  sheath,  186. 
Heschl's  gyrus,  238. 
Heterotypical  division,  8. 
Hett,  Seccombe,  260.' 
Hiatus  semilunaris,   196. 
Hibernating  gland,  339,  473. 
Hill,  E.  C,  396. 
Hill,  J.   P.,   11,  28,   39,    189,   261,   375, 

469. 
Hill,  Leonard,  351,  414. 
Hilton,  W.  A.,  292. 
Hind-brain,  75,  84,  97. 
Hind-gi-t,  17,  268,  293. 
Hip  joint,  429,  443,  456. 
Hip  joint.  Congenital  dislocation  of,  443. 
HiiDpocampal  commissure,  122,  195. 
Hippocampal  fissure,  129. 
Hippocamiml  formation,  117,  118,  122. 
Hippocampus  minor,  220. 
His,  Prof.  W.,  84,  321. 
Homotypical  division,  8. 
Horse-shoe  kidney,   368. 
Howes,  G.  B.,  156,  443. 
Huber,  G.  C,  366. 
Hubrecht,  A.  W.,  29. 
Hunter,  John,  400. 
Huntington,   G.  S.,  294,  306,  337,  349, 

350. 
Hutchison,  339. 
Huxley,  156. 
Hyaloid  artery,  211. 


Hyaloid  canal,  212. 

Hydatids  of  genital  glands,  364,  366,  376 

Hydrocephaly,  144. 

Hymen,  374,  377. 

Hvoid,  247. 

Hyoid  arch,  228,  244,  246,  253. 

Hyo-mandibular  cartilage,  172,  224. 

Hypaxial  muscles,  69. 

Hypoblast.     See  Entoderm. 

Hypochordal  bow,  55,  59. 

Hypogastric  arteries,  331,  437. 

Hypoglossal  nerve,  99,  136. 

Hyj5oischium,  443. 

HypopaUium,  116. 

Hypospadias,  389. 

Hypothalamus,  104,  HI,  116. 

Hypothenar  eminence,  466. 


Ichthyosis,  463. 

Ileo-caecal  bloodless  fold,  298. 

Heo-caecal  fossa,  298. 

Ileo-caecal  sjihincter.     See  Ileo-colic. 

Ileo-caecal  valves,  298. 

Ileo-colic  fold,  298. 

Ileo-colic  fossa,  298. 

Ileo-colic  part  of  bowel,  294. 

Ileo-colic    part    of    bowel.    Volvulus    of, 

300. 
Ileo-colic  sphincter,  294. 
Iliac  veins,  308. 
Ilio-coccygeus,  410. 
Ilio -femoral  ligament,  456. 
Ilium,  429. 

Imperforate  anus,  385. 
Incus,  172,  229. 
Inferior  medullary  velum,  89. 
Inferior  turbinate  bone,  196. 
Inferior  vena  cava,  307. 
Infra-cardiac  bursa,  277. 
Ingalls,  N.  W.,  17,  46,  360. 
Inguinal  canal,  398. 
Inguinal  fold,  371. 
Inguinal  hernia,  398,  406. 
Inguinal  ligament,  405. 
Inguinal  region,  405. 
Ingvar,  S.,~91. 
Inner  cell-mass,  12. 
Innominate  veins,  306. 
Inouye,  M.,  165. 

Inter-articular   cartilages,   455,   457. 
Interchondral  disc,   455. 
Interclavicle,  422,  445. 
Intercostal  artery,  69. 
Intercostal  muscles,  68,  423. 
Intercostal  space,  58. 
Intercostal  vein,  Left  superior,  305. 
Intermediate  cell-mass,  19,  40,  359,  362. 
Intermedium,  447. 
Internal  auditory  meatus,  239. 
Internal  bodies,  402. 
Internal  capsule.  111,  116. 
Internal  lateral  ligament  of  ankle,  450. 
Internal  pterygoid  plate,  171. 


2h 


482 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


Interossei,  453. 

Interparietal  bone,  150. 

Interscapular  gland,  339,  473. 

Intersegmental  septum,  58. 

Intersigmoid  fossa,  294. 

Interstitial  cells,  5,  397. 

Interventricular  foramen,  326. 

Interventricular  septum,  326. 

Intestinal  diverticula,  291. 

Intestinal  loop,  289. 

Intestine.     See  also  Gut. 

Intestine,  Great,  293. 

Intestine,  Great,  Fixation  of,  287. 

Intestine,  Small,  288. 

Intestines,  Musculature  of,  292. 

Intestines,    Retraction   of,    290. 

Intestines,  Rotation  of,  289,  293,  299. 

Intraparietal  fissure,  131. 

Iris,  211,  213. 

Ischium,  442. 

Island  of  ReU,  113,  116,  125. 

Isthmus  of  thyroid,  264. 

Iter  venosum,  304. 


Jacobson's  cartilage,  163. 

Jacobson's  organ,  163,  166,  193,  200. 

Jaekel,  0.,  62. 

Jansen,  Murk,  152. 

Jefferson,  J.,  131. 

Jenkins,  G.  T.,  443. 

Jenkinson,  J.  W.,  24,  29,  230. 

Johnson,  F.  P.,  271,  386. 

Johnston,  H.  M.,  447. 

Johnston,  T.  B.,  294,  308. 

Joints,  455. 

Jolly,  6. 

Jones,  F.  Wood,  15,  54,  56,  64,  258.  280, 

298,  373,  379,  388,  391,  404,  425. 
Jordan,  H.  E.,  290,  334. 
Jugular  vein,  133,  303,  306. 
Jugular  vein.  Primitive  external,  306. 
Junctional  cords,  365. 
Junctional  ring,  31. 


K 

Kallius,  E.,  204,  255. 
Kappers,  Dr.  Ariens,  89. 
Karyokinesis,  8. 
Kazzander,  423. 
Keibel,  F.,  204. 
Kelly,  H.  A.,  294. 
Kernan,  J.,  135. 
Kerr,  A.  T.,  431. 
Kerr,  Graham,  39,  84,  136. 
Kidd,  W.,  464. 
Kidney,  Cysts  of,  369. 
Kidney,  Horse-shoe,  368. 
Kidney,  Origin  of,  358,  366. 
Kidnev,  Vessels  of,  368. 
Kingsbury,  B.,  245,  261,  397. 
Kirchner,  A.,  460. 


Knee-joint,  457. 
Knowles,  189. 
Kohlbrugge,  J.  H.  F.,  10. 
KoUiker,  84. 
Korff,  413. 
Krabbe,  K.,  108. 
Kunitomo,  K.,  65. 
KupfEer,  84. 


Labia  minora,  393. 

Labio-alveolar  groove,  175. 

Labio-dental  groove,  184. 

Labyrinth,  223,  232. 

Lachrymal  bone,  168,  201,  215. 

Lachrymal  fistula,  217. 

Lachrymal  gland,  216. 

Lachrymal  sac,  167,  217. 

Lagena,  234. 

Lambda,  143. 

Lamina  termtnalis,  103,  105,  111. 

Lamprey,  101. 

Lander,  Miss  K.,  453. 

Langerhans  Islands,  284. 

Langhan's  cells,  15. 

Lanugo,  50,  463,  467. 

Laryngeal  nerves,  258,  352. 

Larynx,  343,  351. 

Larynx,  Muscles  of,  253,  352. 

Lateral  line  organ,  97,  223. 

Lateral  nasal  process,  167. 

Lateral  recess  of  pharynx,  261. 

Lateral  ventricle,  112. 

Latissimo-condyloideus,  452. 

Latissimus  dorsi,  451. 

Leboucq,  468. 

Left  innominate  vein,  306. 

Left  superior  intercostal  vein,  305. 

Leg,  425,  435. 

Leg,  Nerve  supply  of,  435. 

Legg,  T.  P.,  159. 

Lelievre,  336. 

Lemniscus,  lateral,  95. 

Lemniscus,  median,  96. 

Lenhossek,  M.  von,  208. 

Lens,  204. 

Lens,  Capsule  of,  206,  211. 

Lens,  Vascular  capsule  of,  211. 

Lesser  sac  of  peritoneum,  286. 

Leucoblasts,  334. 

Leucocytes,  Origin  of,  263,  336. 

Levator  ani,  409. 

Levator  claviculae,  452. 

Levator  glandulae  thyroideae,   264. 

Levator  palatae,  170. 

Levator  palpebrae,  218. 

Lewis,  F.  T.,  271,  273,  276,  283,  291,  327, 

339,  359,  366. 
Lewis,  T.,  320,  461. 
Lewis,  W.  H.,  66,  84,  135,  206,  257,  426, 

428. 
Lichtenberg,  A.,  393. 
Lieberkiihn,  Glands  of,  2"92. 
Lieno-phrenic  ligament,  283. 


INDEX 


483 


Lieno-renal  ligament,  283. 
Ligaments,  455. 
Ligaments,  Capsular,  455. 
Ligaments  of  ovary,  4. 
Ligaments  of  uterus,  4. 
Liganientum  teres,  456. 
Lillie,  F.  R.,  8,  401. 
Limb-buds,  47,  427, 
Limb-girdles,  428. 
Limbic  bands,  .319. 
Limbs,  425. 
Limbs,  Arrest  of,  461. 
Limbs,  Arteries  of,  428. 
Limbs,  Embryonic,  412,  425. 
Limbs,  Evolution  of,  425. 
Limbs,  Joints  of,  427,  455. 
Limbs,  Ligaments  of,  455. 
Limbs,  Morphology  of,  441. 
Limbs,  Nerve  supply  of,  82,  430. 
Limbs,  Segmental  nature  of,  430. 
Limbs,  Torsion  and  rotation  of,  429. 
Limbs,  Veins  of,  439. 
Limbs,  Vessels  of,  437. 
Limiting  membrane  of  cord,  77. 
Limiting  sulci  of  Island  of  Reil,  125. 
Linck,  A.,  57. 
Lindahl,  Dr.,  212. 
Linea  alba,  415,  423. 
Lineback,  P.  E.,  296. 
Lingual  papillae,  257. 
Lingual  tonsil,  261. 
Lingula,  94. 

Lingula  of  sphenoid,  148. 
Lips,  175. 
Lips,  Clefts  of,  161. 
Lisser,  H.,  351. 
Littre,  Glands  of,  396. 
Liver,  changes  after  birth,  279. 
Liver,  Development  of,  271. 
Liver,  Ligaments  of,  275. 
Liver,  Morphology  of,  278. 
Liver,  Veins  of,  273,  310. 
Lockwood,  305. 
Loeb,  41. 

Long  plantar  ligament,  450. 
Low,  A.,  18,  76,  175,  272,  292,  300. 
Lower    jaw.     Development     and     ossifi- 
cation of,  175. 
Lowsley,  0.  S.,  394. 
Lucas-Keen,  M.  F.,  53. 
Lumbar  curvature,  53. 
Lumbar  fascia,  69. 
Lumbar  plexus,  433. 
Lung,  Blood  supply  of,  350. 
Lung  buds,  345. 
Lungs,  changes  at  birth,  351. 
Lungs,  Development  of,  343,  417. 
Lungs,  Evolution  of,  341. 
Lungs,  Lobes  of,  346. 
Lutein  cells,  6. 
Lymphatic  glands,  339. 
Lymphatic  system,  337,  414. 
Lymphatic  vessels,  337. 
LymjDhocytes,  335. 
Lymphocytes,  Origin  of,  260,  262. 


Lymjjhoid  follicles,  292. 
Lymphoid  tissue.  Origin  of,  260. 

M 

MacBride,  E.  W.,  39. 

MacCormick,  Prof.  A.,  162. 

Mackenzie,  Ivjr,  320. 

MacKenzie,  W.  C,  282. 

Macklin,  C.  C,  135. 

MX'lure,  C.  F.  W.,  306,  337. 

M'Cotter,  R.  E.,  76,  200. 

M'Murrich,  J.  P.,  451. 

Macrostoma,  161. 

Magendie,  Foramen  of,  134. 

Maggi,  142. 

Malar,  173. 

Malar,  Orbital  plate  of,  215. 

Male  pronucleus,  7. 

Malformations,  Production  of,  41. 

Mall,  F.  P.,  7,  42,  45,  61,  132,  162,  271, 

327,  353,  413. 
MaUeus,  172,  175,  224,  230. 
Mamma,  Lymphatics  of,  472. 
Mamma,     Origin     of     glandular     tissue, 

470. 
Mamma,  PerijDheral  remnants  of,  472. 
Mammary  glands,  469. 
Mammarjr  ridge,  469. 
Mammillary  jjrocesses  of  vertebrae,  64. 
Mandible,  Evolution  of,  138,  176. 
Mandible,   growth  and  ossification,    175. 
Mandibular  processes  and  arch,  156,  159, 

171,  174,  244. 
Mandibular  segment,  98,  218,  253. 
Mantle.     See  Pallium. 
Marrow,  336,  458. 
Marshall  and  Jolly,  6. 
Marshall,  Francis  H.  A.,  4. 
Marshall,  Vein  of,  305. 
Mastication,   Muscles   of,    149,    155,    174, 

190,  253. 
Mastication,     Structures     concerned     in, 

182,  215. 
Mastoid,  155,  235. 
Masur,  A.,  183. 
Maturation  of  ovum,  9. 
Maxilla,  Ossification  of,  171,  215. 
Maxillae,  Evolution  of,  138,  168. 
Maxillary  processes,  158,  168,  172,  215. 
Maxillary  segment,   217. 
Maxillary  sinus,  173,  197. 
Maxillo-turbinal,  168,  196. 
Mayous,  216. 
Mazilier,  R.,  353. 
Meatus,  External  auditorj%  179. 
Meckel's  cartilage,  171,  175,  230. 
Meckel's  diverticulum,  288,  290. 
Meconium,  300. 
Medulla  oblongata,  86. 
Medullary  cavity,  458. 
Medullary  folds,  17,  75. 
Medullary  groove,  36. 
Medullary  plate,  17,  75,  82. 
Medullary  velum.  Inferior,  85,  89. 


484 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


Medullary  velum,  Superior,  94. 

MeduUation.     See  Myelinization. 

Meek,  A.,  155. 

Meibomian  glands,  469. 

Membrana  nicitans,  216. 

Membrana  pupillaris,  212. 

Membrana,  Tympani  of,    178,   224,   226, 

232,  246. 
Membranous  labyrinth,  Origin  of,  232. 
Meningocele,  83,  145. 
Mental  eminence,  177. 
Mesencephalon,  95. 
Mesenchyme,  40,  334. 
Mesenteric  recess,  275. 
Mesenteric  vessels,  288,  296,  334. 
Mesentery,  Common  genital,  3,  362,  396. 
Mesentery,  Fixation  of,  287,  297,  299. 
Mesentery  of  appendix,  298. 
Mesentery  of  gut,  268,  296,  299,  309. 
Mesentery,  Primitive,  268. 
Mesentery,  Ventral,  26,  39,  269. 
Mesial  nasal  processes,  163. 
Mesoblast.     See  Mesoderm. 
Mesocardia,  313,  332. 
Mesocephalic  skuU,  151. 
Mesoderm,  12,  19,  463. 
Mesoderm,  Origin  of,  39. 
Mesoduodenum,  290. 
Mesogastrium,  275,  280,  286,  297. 
Mesohepar,  275,  278. 
Mesonephros,  360. 
Mesorchium,  396,  400. 
Mesosalpinx,  3,  363. 
Mesothelium,  20,  334. 
Mesovarium,  3. 
Metacarpus,    Flexors    and    extensors    of. 

451. 
Metanephros,  359. 
Metathalamus,  103. 
Metopic  suture,  142. 
Meyer,  A.  W.,  330. 
Microcephaly,  144. 
Mid-brain,  75,  85,  95,  97. 
Mid-gut,  268,  288. 
MiUigan,  W.,  237. 
Mitochondria,  10. 
Mitosis,  7,  8. 
Moderator  band,  328. 
"  Mole  "  pregnancy,  43. 
MoUier,  336. 

Monro,  Foramen  of,  104,  112. 
Monsters,  Production  of,  41,  43. 
Moodie,  R.  L.,  460. 
Morgagni,  Columns  of,  386. 
Morula,  11. 
Mott,  Sir  F.  W.,  397. 
Miiller,  Charlotte,  421. 
Miiller,  E.,  425,  437. 
Miillerian  ducts,  362,  369,  379. 
Miiller's  muscle,  214. 
Mummery,  J.  H.,  183,  186. 
Murray,  H.  A.,  302. 
Muscle,  Development  of,  68. 
Muscle,  Growth  of,  69. 
Muscle  plate,  68,  426. 


Muscles,  Abnormalities  of,  452. 
Muscles,  Migration  of,  451. 
Muscles  of  body  segment,  68. 
Muscles  of  extremities,  451. 
Muscles,  Primary  groups  of,  68. 
Musculature  of  body  wall,  423. 
Musculature  of  trunk,  68,  404. 
Muthmann,  E.,  366. 
Myelinization,  81. 
Myelospongium,  77. 
Myoblast,  68,  69. 
Myocoel,  40. 
Myoplasm,  69. 
Myosjmcytium,  68. 
Myotomes,  41,  58,  68,  156. 

N 

Nails,  50,  467. 

Nares,  Anterior,  160,  202. 

Nares,  Atresia  of,  170,  202. 

Nares,  Posterior,  170,  195. 

Nasal  air  sinuses.  Development  of,  196. 

Nasal  bones,  168. 

Nasal  capsule.     See  OHactory. 

Nasal  cavities,  160,  192,  195. 

Nasal  duct,  166,  201,  216. 

Nasal  passage.  Evolution  of,  158,  192. 

Nasal  processes,  159,  163. 

Nasal    processes.    Arteries    and    nerves 

of,  168. 
Nasal  spine,  164. 
Nasmyth's  membrane,  183. 
Naso-palatine  foramen,  166. 
Nasoturbinal,  200. 
Neanderthal  teeth,  189. 
Neck,   Development    of,   240,   254,   332, 

414. 
Neocerebellum,  94. 
Neocranium,  157. 
Neopallium,  118. 
Nephric  ducts,  359. 
Nephric  tubules,  358,  366. 
Nephrocoel,  40,  359. 
Nephrostome,  369. 
Nephrotome,  156,  358. 
Nerve  cells.  Nature  of,  74. 
Nerve  cells  of  spinal  cord,  79,  82. 
Nerve  plexuses.  Nature  of,  432,  434. 
Nerve-roots,  71. 
Nerve  tracts,  81,  119,  123. 
Nerves,  Development  of,  84,  89. 
Nerves  of  body  segment,  70. 
Nerves  of  limbs,  431. 
Nerves,  Somatic,  70. 
Nerves,  Splanchnic,  70. 
Nervous    system.    Evolution    of    central, 

74. 
Nervus  bigeminus,  435. 
Nervus  furcalis,  435. 
Nervus  terminalis,  200. 
Neural  arch,  59. 
Neural  canal,  17,  19,  75. 
Neural  canal.  Divisions  of,  75. 
Neural  canal.  Malformations  of,  83. 


INDEX 


485 


Neural  canal,  Membranes  and  vessels  of, 

83. 
Neural  crest,  70. 
Neural  flexures,  96. 
Neural  plate.     See  Medullary  plate. 
Neurenteric  canal,  16,  35,  391. 
Neurobiotaxis,  20,  88. 
Neuroblasts,  78. 
Neuroblasts,  Grouping  of,  79. 
Neuroblasts  of  cord,  79. 
Neuro-central  suture,  60. 
Neurocranium  135,  153. 
Neuroglia,  78. 
Neuromeres,  82,  85,  98. 
Neuron,  80,  82. 
Neuropore,  75,  103. 
Nicholls,  97. 
Nipple,  469. 

Nij^ples,  Supernumerary,  469. 
Norris,  E.  E.,  264. 
Nose,  Air  sinuses  of,  196. 
Nose,  Development  of,  160,  193. 
Nose,  Malformations  of,  201. 
Nose,  Septum  of,  160,  163,  195. 
Notochord,  19,  36,  40,  56. 
Notochord,  Cranial,  136. 
Notochord,  Fate  of,  57. 
Nuchal  flexure,  96. 
Nutrient  canals,  460. 

0 

Oblique  sinus,  333. 

Oblique  vein  of  Marshall,  305. 

Obturator  fascia,  413. 

Obturator  nerve,  437. 

Occipital  bone,  57,  136,  144,  150. 

Occipital  condyle  (median),  63,  146. 

Occipital  fontanelle,  145. 

Occipital  joint,  62. 

Occipital  lobe,  112,  220. 

Occipital  ridges,  154. 

Occipital  somites,  41. 

Occipital  vertebrae,  56,  156. 

Occipito-atlanto-axial  articulations,  62. 

Oculomotor  nerve,  95,  99,  218. 

Odonto-blasts,  184. 

Odontomes,  184. 

Oesophageal  sphincter,  352. 

Oesophagus,  270,  343. 

Olecranon  process,  461. 

Olfactory  capsule,   149,   159,   167. 

Olfactory  lobe,  194. 

Olfactorj'  nerves,  193. 

Olfactory  peduncle,  194. 

Olfactory  pits,  160,  193. 

OKactory  plates,  82,  160,  192,  193. 

Olfactorj''    sense    epithelium,    Origin    of, 

102,  193. 
Olfactory  structures,  192. 
Olfactory  tract.  Termination  of,  195. 
OKactory  vesicle.  111,  194. 
Olivary  body.  Inferior,  88. 
Omentum,  285,  297. 
Omo-hyoid,  451. 


Omo-sternum,  419. 

Omo-trachelian,  452. 

Omo- vertebral  bone,  441. 

Oocyte,  2. 

Opercula  of  Island  of  Reil,  127. 

Opisthion,  153. 

Oppel,  A.,  267. 

Optic  chiasma,  105,  121,  207,  211,  218. 

Optic  cup,  206. 

Ojitic  foramen,  147. 

Optic  lobes,  95,  219. 

Optic  nerve,  207. 

Optic  radiations,  219. 

Optic  thalami,  102,  218. 

Optic  tracts,  218. 

Optic  vesicle,  75,  103,  203,  206. 

Ora  serrata,  208. 

Oral  plate,  106,  180. 

Orbit,  Formation  of,  201,  214. 

Orbit,  Primitive,  135. 

Orbital  fissure,  131. 

Orbital  muscles,  214,  271,  218. 

Orbital  nerves,  217. 

Orbito-sphenoid,  137,  147. 

Orthograde  posture,  52,  288,  297,  350, 
405,  417. 

Osborn,  425. 

Os  calcis,  447,  458. 

Os  centrale,  447. 

Os  epactal,  150. 

Os  Japonicum,  150,  173. 

Os  styloideum,  448. 

Os  trigonum,  447. 

Ossification  of  bones,  145,  168,  176,  457. 

Osteoblasts,  458. 

Ostium  abdominale,  6,  369,  373. 

Otic  capsule.     See  Auditory. 

Otis,  W.  J.,  390. 

Otocyst,  82,  86,  223,  232. 

Ova,  Discharge  of,  5. 

Ova,  Origin  of,  5,  20. 

Ova,  Primordial,  5. 

Ovarian  triangle,  4. 

Ovario-pelvic  ligament,  3,  397. 

Ovario-testis,  401. 

Ovarj;-,  Descent  of,  2. 

Ovary,  Ligament  of,  371. 

Ovary,  Mesentery  of,  3. 

Ovary,  Position  of.  4. 

Oviducts,  369. 

Ovum,  1,  4,  9. 

Ovum,  Discharge  of,  5. 

Ovum,  Division  of,  2. 

Ovum,  History  of,  within  the  Fallo- 
pian tube,  7. 

Ovum,  Implantation  of,  12,  22. 

Ovum,  Maturation  of,  9. 

Ovum,  Nourishment  of,  31. 

Owen,  Sir  R.,  140. 


Pacinian  corpuscles,  473. 
Palaeostriate  body,  116. 
Palatal  folds,  170,  246. 


486 


HUMAN  EMBEYOLOGY  AND  MORPHOLOGY 


Palatal  rugae,  173. 

Palate  bone,  171. 

Palate,  Cleft,  161,  165. 

Palate,  Contracted,  190. 

Palate,  Development  of,  168. 

Palate,  Evolution  of,  135,  158. 

Palate,  Malformations  of,  169. 

Palate,  Primitive,  158. 

Palate,  Soft,  170. 

Palato-glossus,  170. 

Palato-pharyngeus,  170. 

Palato-quadrate  bar,  138,  148,  156,  171. 

Paleocranium,  157. 

Pallial  area,  104. 

Pallio-spinal  tracts,  81. 

Pallium,  111,  117. 

Palmar  fascia,  414. 

Palmar  pads,  465. 

Palmaris  longus,  453. 

Pancreas,  283. 

Pancreas,    Relationship    to    peritoneum. 

285. 
Papillae  foUiatae,  257. 
Papillary  patterns,  465. 
Parachordal  cartilages  or  plate,  57,  144, 

146,  156. 
Paradidymis,  365. 
Paraflocculus,  93. 
Paraganglia,  403. 
Parallel  fissure,  132. 
Paramastoid  process,  154. 
Paramore,  R.  H.,  404,  409. 
Para-occipital  process,  154. 
Paraseptal  cartilages,  163,  167. 
Parasitic  foetus,  43. 
Para  terminal  bodjr,  118,  122. 
Para-thyroids,  262,  265. 
Paravertebral  ganglia,  71. 
Paraxial  mesoderm,  19,  39. 
Parietal  bone,  141. 
Parietal  eye,  109. 
Parietal  plate,  137. 
Parieto-occij)ital  fissure,  130,  220. 
Parker,  Miss  K.  M.,  106. 
Paroophoron,  364. 
Parotid  gland,  259. 
Parovarium,  364,  366. 
Pars  ciliaris  retinae,  207. 
Pars  membranacea  septi,  327. 
Pars  triangularis,  127. 
Parsons,  F.  G.,  154,  178,  247,  294,  298, 

332,  414,  430,  443,  445,  460. 
Patella,  430,  461. 
Paterson,  A.  M.,  55,  390,  409. 
Patten,  142. 
Paulet,  J.  L.,  163,  255. 
Pectoral  muscles,  452. 
Pelvic  colon,  293. 
Pelvic  fascia,  411,  413. 
Pelvic  floor,  408. 

Pelvic  floor,  Development  of,  408. 
Pelvic  girdle,  406,  419,  428,  442. 
Pelvic  musculature,  409. 
Penis,  379,  387,  392. 
Penis,  Dichotomy  of,  43. 


Pensa,  A.,  274. 

Pericardial  coelom,  40. 

Pericardiopleural  passage,  304. 

Pericardium,  269,  304,  313. 

Perichondrium,  455,  458. 

Pericranium,  143. 

Periderm,  462. 

Peridontal  membrane,  185. 

Perilymph  system,  234. 

Perineal  body,  385,  388. 

Perineal  folds,  387. 

Perineal  nerves,  436. 

Perineal  septa,  384. 

Perineum,  387. 

Perineum,  Muscles  of,  389. 

Periosteum,  458. 

Periotic  capsule,  224,  234. 

Peritoneal  adhesions,  287,  294,  297. 

Peritoneal  cavity,  269. 

Peritoneal  coelom,  40,  269. 

Peritoneal  fixation.  Process  of,  286,  294, 

405. 
Peritoneal  funnels,  366, 
Peritoneum,  269,  277. 
Peritoneum,  Lesser  sac,  277,  287. 
Peroneus  brevis,  449. 
Peroneus  longus,  449. 
Peroneus  quinti  digiti,  451. 
Peroneus  tertius,  449. 
Peter,  K.,  159. 
Peters,  14. 

Petersen,  Otto,  63,  364. 
Petro-mastoid,  Origin  of,  142,  146,  235. 
Petro-squamous  sinus,  231. 
Peyer's  patches,  292. 
Pfitzner,  448. 
Phalanges,  Terminal,  468. 
Pharyngeal  diverticulum,  273. 
PharATigeal  pouches,  243,  262. 
Pharyngeal  recess,  243,  246,  259,  261. 
Pharyngeal  tonsil,  259,  261. 
Pharynx,  Evolution  of,  240. 
Pharynx,  Muscles  of,  253. 
Pharynx,     Structures     developed    from, 

255. 
Phrenic  nerve,  353,  354. 
Pia  mater,  84,  134. 
Pineal  body,  103,  105,  108, 
Pinna,  226. 
Pisiform,  448. 
Pittard,  E.,  396. 
Pituitary  body.  Origin  of,  103,  104,  105, 

106,  149,  180. 
Pituitary  in  nasal  septum,  161. 
Placenta,  Formation  of,  31. 
Placenta,    Formation    of    blood    spaces 

in,  16,  30. 
Placental  circulation,  30. 
Placodes,  98,  243. 
Plantar  arches,  450. 
Plantar  fascia,  414,  450. 
Plantar  ligaments,  449. 
Plantaris,  450,  453. 
Plantigrade  posture,  405. 
Plasma,  335, 


INDEX 


487 


Platysnia  sheet,  2.53. 

Pleural  cavities,  342,  34.3,  353,  40.5. 

Pleuro-peritoneal  ojiening,  287,  3.54,  356. 

Pleuro-i^eritoneal  passages,  270,  343. 

Plexus,  Formation  of,  432,  434. 

Plicae  fimbriatae,  258. 

Plica  gubernatiix,  397. 

Plica  semilunaris,  215,  260. 

Plica  triangularis,  260. 

Plica  vascularis,  3.  396. 

Pohlmann,  A.  G.,  380. 

Polar  body,  7. 

Poles  of  brain,  112. 

Poles  of  ovum,  12. 

PoUex,  447,  451. 

Polocytes,  7. 

Pons  Varolii.  86,  92. 

Pontine  bend,  89,  96. 

Popliteal  artery,  437. 

Popliteus,  457. 

Portal  vein,  308. 

Post -anal  gut,  381,  391. 

Post-anal  pit,  408. 

Posterior  cardinal  vein,  304. 

Posterior  commissure,  97,  109. 

Posterior  root  ganglion,  70. 

Post -frontal,  150. 

Post-orbital  process,  139. 

Posture,  Change  in,  405. 

Poupart's  ligament,  405. 

Prechordal  base  of  skuU,  136,  163. 

Pre-chorion,  28. 

Pre-coracoid,  422,  444, 

Pregnancy,  Tubular,  7. 

Premaxillary  bones,  158,  163,  164. 

Prepuce,  388. 

Preputial  glands,  396. 

Presphenoid,  147. 

Pre-sternum,  417,  422. 

Prevertebral  ganglia,  71. 

Prevomer,  164. 

Primitive  groove,  35. 

Primitive  mesentery,  268. 

Primitive  segments,  58. 

Primitive  streak,  16,  35. 

Primitive  utricle,  234. 

Primitive  vein  of  head,  133,  306. 

Primordial  ova,  5,  20,  397. 

Processus  globularis,  160. 

Processus  hyjDochiasmata,  147. 

Processus  vaginalis,  399. 

Proctodaeum,  381. 

Prognathism,  165. 

Projection  tracts,  119. 

Pronephros,  359,  363. 

Pronograde  posture,  52. 

Pronuclei,  7,  10. 

Prosencephalon,  101. 

Prostate.  393. 

Prostate,  Nature  of,  10,  395. 

Prostate,  Sheath  of,  413. 

Protovertebrae,  20,  58. 

Psoas  parvus,  454. 

Pterion,  143. 

Pterotic,  235. 


Pterygo-jialatine  canal,  173. 

Pterygoid  muscles,  190. 

Pterygoid  plate.  Internal,   148,  171. 

Pubes,  442. 

Pubo-coccvgeus,  409. 

Pulmonary  artery,  249,  323,  3-50. 

Pulmonary  artery.  Stenosis  of,  325. 

Pulmonary  diverticulum,  343. 

Pulmonary     system.     Development     of, 

302,  330,  340. 
Pulmonary  veins,  321,  333. 
Pulp  of  te"eth,  184. 
Purkinje,  Cells  of,  92. 
Purkinje,  System  of,  329. 
Pylorus,  281. 
Pyramidal  tracts,  81. 
Pjrramidalis,  4.54. 
Pyramid  of  thyroid,  264. 
PjTamids  of  spine,  53. 
Pyriforni  fascia,  414. 
Pyriform  fossa,  246,  265. 
Pyriform  lobe,  118,  195. 

Q 

Quadrate,  172. 
Quadrigeminal  plate,  95,  218. 

R 

Rachischisis,  Total,  83. 

RacUale,  447. 

Rami  communicantes,  72. 

Ramm,  M.,  161. 

Rathke's  pocket,  107,  180,  195. 

Rauber's  layer,  12. 

Reagan,  F.,  249. 

Recapitulation,  Law  of,  35. 

Receptaculum  chyli,  338. 

Recto-vesical  fascia,  413. 

Rectum,  293. 

Rectum,  Malformations  of,  385» 

Rectum,  ilusculature  of,  391. 

Rectum,  Separation  of,  383,  386. 

Rectum,  Sheath  of,  414. 

Rectus  abdominis,  69,  355,  423. 

Referred  pains,  72. 

Reid,  D.  G.,  288,  297. 

Reid,  R.  W.,  77. 

Reil,  Island  of,  125. 

Renal  arteries.  70,  368. 

Renal  ganglia,  71. 

Renal  organs,  Succession  of,  359. 

Renal-portal   circulation.    307. 

Rennie,  284. 

Respiratory  centre,  85. 

Respii'atory  passages,  343. 

Respiratorjr  svstem.   Evolution   of,   340, 

404. 
Respiratory  system.  Morphological  parts 

of,  343,  419. 
Restiform  body,  92. 
Rete  testis.  365.  398. 
Retina,  82.  203,  207. 
Retina,  Pigmentary  layer  of.  207. 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


Retro -caecal  fossa,  298. 

Retro-calcarine-fissure,  130. 

Retro -f)haryngeal   diverticulum,    273. 

Retterer,  336. 

Rhinal  fissure,  119. 

Rhinencephalon,  119,  123,  130,  194. 

Rhombic  lip,  89. 

Ribs,  63,  404,  417. 

Ribs,  Cervical,  56,  442. 

Ribs,  Formation  of,  58,  417. 

Ribs,  Reduction  of  12th,  56,  63. 

Ribs,  Vestigial,  63,  417. 

Riedel's  lobe,  279. 

Rischbieth,  H.,  308. 

Robinson,  A.,  4,  38,  58,  144,  207,  302. 

Rolando,  Fissure  of,  131. 

Rosenberg,  E.,  54,  55. 

Rosenmiiller,  Fossa  of,  24G. 

Rosenmiiller,  Organ  of,  364. 

Rubashkin,  259. 

Rudel,  E.,  106. 

Rutherford,  N.  C,  445. 

Rynberk,  G.  van,  66. 


Sabin,  Florence,  81,  302,  306,  334,  337. 

Saccule  of  ear,  232. 

Saccule  of  larynx,  353. 

Sacral  curvature,  53. 

Sacral  plexus,  435. 

Sacral  vertebrae,  55,  63,  442. 

Sacro-coccygeal    region,     Variations    of, 

55. 
Sacro -lumbar     regions.     Variations     of, 

54. 
Sacro-sciatic  ligaments,  454. 
Sacrum,  55,  442. 
Sagittal  fontanelle,  143. 
Salivary  glands.  Origin  of,  256,  258. 
Sandstrom,  265. 
Santorini,  Duct  of,  284. 
Saphenous  artery,  439. 
Saphenous  vein,  440. 
Scammon,  R.  E.,  310. 
Scansorius,  454. 
Scaphocephaly,  153. 
Scaphoid,  447. 
Scapula,  429,  444. 
Scapula,    Congenital    elevation    of,    73, 

441. 
Schaeffer,  J.  P.,  174. 
Schaffer,  J.,  258. 
Schlaginhaxifen,  0.,  464. 
Schoeppler,  C,  468. 
Schorr,  G.,  165. 
Schulte,  H.  W.,  302. 
Schumacher,  56,  155. 
Schwalbe,  E.,  42. 
Sciatic  plexus,  433. 
Sclerotic,  213. 
Sclerotome,  57,  58,  156. 
Scrotum,  393. 
Sebaceous  glands,  463,  469. 
Seessel's  pocket,  107,  259. 


Segment,  Constitution  of,  67. 

Segmental  vessels,  69. 

Segmentation,  18,  41,  66. 

Segmentation,  Abnormal,  73. 

Segmentation  of  body,  41,  58,  61,  416 

Segmentation  of  limbs,  426. 

Selenka,  E.,  30. 

Semicircular  canals,  232. 

Semilunar  cartilages,  457. 

Semilunar  ganglion,  71,  402. 

Seminal  guides,  383. 

Seminiferous  tubules,  9,  397. 

Senior,  H.  D.,  437. 

Sense  organs,  82. 

Sensori-motor  areas  of  brain,  119,  125. 

Sensory  cells,  82. 

Sensory  tracts  of  fore-brain,  104. 

Septal  cartilage,  163. 

Septum  lucidum,  122. 

Septum,  Nasal,  160,  163,  195. 

Septum  primum  of  auricle,  322. 

Septum  secundum  of  auricle,  322. 

Septum  transversum,  269,  272,  304,  332, 

342,  354. 
Serratus  magnus,  440,  451. 
Sesamoid  ossification,  461. 
Sewell,  R.  B.  S.,  447. 
Sex,  Differentiation  of,  370,  387,  401. 
Sex.     See  Genital. 
Shaw,  Batty,  473. 
Shaw,  D.  M.,  188. 
Shellshear,  J.,  117,  473. 
Sherrington,  433. 

Shoulder,  Congenital  elevation  of,  441. 
Shoulder  girdle,  419,  428,  444. 
Shoulder  joint,  456. 
Sight,  nerve  centres,  218. 
Sight,  Structures  of,  203. 
Sigmoid  flexure,  294. 
Simian  fissure,  124,  130. 
Sino-auricular  node,  319. 
Sinus  pocularis,  346,  376. 
Sinus  subpericardiacus,  350. 
Sinus  venosus,  304,  308,  316. 
Sinus  venosus.  Fate  of,  317. 
Sinus  venosus.  Valves  of,  318. 
Skeleton  of  body  segment,  67. 
Skene's  tubules,  395. 
Skin,  Development  of,  462. 
Skin-plate.     See  Dermatome. 
Skin,  Sense  organs  of,  466,  473. 
Skull,  Base  of,  136,  154. 
Skull,  Development  of,  140. 
Skull,  Evolution  of,  135. 
Skull,  Growth  of,  139. 
Skull,  Primitive  membranous,  140. 
Skull,  Segmentation  theory  of,  140,  155, 

217. 
Skull,  Shape  of,  151, 
Smith,  E.  Barclav,  54,  278. 
Smith,  G.  Elliot,  56,  70,  91,  102,  116,  144, 

219,  409,  463. 
Smith,  G.  M.,  298. 
Smith,  Priestley,  205. 
Smith,  T.  Manners,  54,  437,  447. 


INDEX 


489 


Smith,  W.  Ramsay,  189. 
Solitary  tract,  88. 
Somatic  centres  of  hind-brain,  87. 
Somatic  nerves,  70. 
Somatopleure,  14,  19,  416. 
Somites,  18,  20,  41,  58,  66. 
Spermatocyte,  9. 
Spermatogone,  9.  ^ 

Spermatozoa,  Formation  of,  9,  20. 
Sphenoid,  Development  of,  137,  147. 
Sphenoid,  Foramina  of,  173. 
Sphenoidal  sinus,  148,  149,  199. 
Sphenoidal  turbinate,  148,  196,  199. 
Spicer,  J.  E.,  393. 
Spigelian  lobe  of  liver,  278. 
Spigelian  recess,  276,  287. 
SiDina-bifida,  83. 
Spinal  accessory,  100. 
Spinal  column,  52. 
Spinal  column.  Curves  of,  54. 
Spinal  column.  Evolution  and  Develop- 
ment of,  52. 
Spinal  column.  Unstable  regions  of,  54. 
Spinal  cord,  caudal  part,  77. 
Spinal  cord,  Cysts  of,  80. 
Spinal  cord.  Development  of,  75,  77. 
Spinal  cord.  Malformations  of,  83. 
Spinal  cord.  Median  septum,  80. 
Spinal  cord,  Membranes  of,  83. 
Spinal  cord.  Segmentation  in,  72,  82. 
Spinal  tracts,  80. 
Spinal  vessels,  83. 
Spindle,  Achromatin,  8. 
Spine,  Curves  of,  53. 
Spine,  Pyramids  of,  53. 
Spinous  processes  of  vertebrae,  64. 
Splanchnic  arteries,  70,  288,  296,  334. 
Splanchnic  centres  of  hind-brain,  87. 
Splanchnic  nerves,  71,  72. 
Splanchnocranium,  135,  153. 
vSplanchnopleure,  13,  19. 
Spleen,  282. 

Sprengel's  shoulder,  441. 
Squamosal,  142,  226. 
Stapes,  224,  230. 
Sternabrae,  417. 
Sternalis,  453,  455. 
Sterno-clavicular  joint,   422,   456. 
Stemo-manubrial   joint,    422. 
Sterno -mastoid,  453. 
Sternum,  404,  417. 
Stevens,  T.  G.,  4. 
Stieda,  L.,  466. 
StUes,  Sir  H.  S.,  291. 
Stockard,  C.  R.,  42,  334. 
Stohr,  P.,  261,  462,  466. 
Stomach,  279. 
Stomach,  Coats  of,  281. 
Stomodaeum,  107,  159,  179. 
Streeter,  G.  L.,  13,  17,  42,  65,  76,  79,  91, 

100,  133,  223,  233,  432. 
Striae  pinealis,  109. 
Striate  area,  104,  111,  221. 
Striate  arterj%  117. 
Stricht,  O.  van  der,  236. 


Struthers,  Ligament  of,  452. 

Studni^ka,  463. 

Stylo-hyal,  246. 

Styloid  process,  246. 

Stylomandibular  ligament,  259. 

Subarachnoid  spaces,  90,  134. 

Subarcuate  fossa,  236. 

Subcallosal  gyrus,  122. 

Subcardinal  vein,  306. 

Subclavian  arteries,  250,  251,  437. 

Subclavian  veins,  306. 

Subclavius,  451. 

Subcommissural  organ,  97. 

Subcutaneous  tissue,  464,  473. 

Sublingual  gland,  258. 

Submaxillary  ganglion,  259. 

Submaxillary  gland,  258. 

Sulcus  lunatus,  130. 

Sulcus  rectus,  131. 

Sulcus  terminalis  of  heart,  317. 

Sulcus  terminalis  of  tongue,  257. 

Sunier,  A.  L.  J.,  66. 

Superior  medullary  velum,  94. 

Superior  vena  cava,  303. 

Supra-condylar  process,  454. 

Supraorbital  ridges,  155,  198. 

Suprarenal  bodies,  Development  of,  402, 

Suprarenals,  Accessory,  403. 

Supra-scapula,  444. 

Supra-sternal  bones,  419,  422,  444. 

Swale-Vincent,  403. 

Sweat  glands,  463,  465,  468. 

Swim -bladder,  340. 

Sylvian  fissure,  125. 

Sylvius,  Aqueduct  of,  95,  219. 

Symington,  63,  225,  -446,  461. 

Symmetry  of  body,  415. 

Sympathetic    nerves    of    bodv    segment, 

71. 
Symphysis  pubis,  442. 
Syncytium,  15,  23,  30. 
Syndactyly,  427. 
Synovial  membranes,  455. 
Szily,  A.  von,  208,  413. 


Taenia  semicii'cularis,  115. 

Taenia  terminalis,  319. 

Tail,  Muscles  of,  405,  408,  454. 

Tail,  Vestiges  of,  41,  65,  77,  408. 

Tandler,  J.,  249,  291,  313. 

Tannreuther,  G.  W.,  43. 

Tapetum  lucidum,  213. 

Tarsale,  L,  II.,  III.,  IV.,  V.,  447. 

Tarsus,  446. 

Tarsus,  Extra  bones  of,  448. 

Taste,  Sense  of,  257,  473. 

Taussig,  F.  S.,  374. 

Tawara,  328. 

Teacher,  J.  H.,  13. 

Tectal  plate,  137,  145. 

Teeth,     Development     and    morj)hology 

of,  182. 
Teeth,  Effect  of  civilization  on,  190. 


490 


HUMAN  EMBRYOLOGY  AND  MORPHOLOGY 


Teeth,  Eruption  of,  174,  176,  189,  190. 

Teeth,  Gemmination  of,  189. 

Teeth,  Irregularities  of,  189. 

Teeth,  Morphology  of,  186,  188. 

Teeth,  Neanderthal,  189. 

Teeth,  Nerves  of,  191. 

Teeth,  Roots  of,  189. 

Tegmen  tympani,  228,  235. 

Telencephalon,  101. 

Temporal  bone,  138,  142. 

Temporal  fissures,  132. 

Temporal  lobe,  125,  238. 

Temporal  muscles,  190. 

Temporal  region,  138. 

Temporal  ridges,  154. 

Temporo-mandibular    articulation,    139, 

172,  176,  177,  456. 
Tenon,  Capsule  of,  214. 
Tensor  palati,  175,  228. 
Tensor  tympani,  175,  228. 
Tentorium  cerebelli,  96,  115. 
Teratology,  44. 
Teratomata,  Origin  of,  21. 
Terry,  R.  J.,  135. 
Testes,  396. 

Testicle,  Descent  of,  396,  400. 
Testicle,  Mesentery  of,  396,  400. 
Testicle,  Torsion  of,  401. 
Thalamencephalon,  101. 
Thallus,  42. 
Thaysen,  T.,  296. 
Thebesian  valve,  319. 
Thompson,  F.  T>.,  265. 
Thompson,  Peter,  18,  269,  271,  276,  279, 

324,  409. 
Thompson,  R.,  389. 
Thomson,  Arthur,  4,  7,  152,  177,  278. 
Thoracic  duct,  338. 
Thorax,  349,  405,  417. 
Thymus,  261. 

Thyng,  F.  W.,  47,  283,  291. 
Thyro-glossal  duct,  247,  264. 
Thyroid,  264. 
Thyroid  cartilage,  352. 
Thyroids,  Accessory,  264. 
Tibialis  anticus,  450. 
Tibialis  posticus,  450. 
Tilney,  Dr.  F.,  106. 
Tims,  Marett,  187. 
Tissues,  Differentiation  of,  427. 
Todd,  T.  W.,  54,  56,  188,  442,  443. 
Tomes,  Sir  C.  S.,  188. 
Tongue,  Development  of,  175,  255. 
Tongue,  Muscles  of,  257. 
Tongue,  Papillae  of,  257. 
Tonsil,  246,  260. 
Tonsillar  recess,  245. 
Tooth,  Structure  of,  1 82. 
Touch  bodies,  473. 
Tourneux,  F.  and  J.  P.,  404. 
'Trabeculae  cranii,  146. 
Trachea,  270,  344. 
Trachea-oesophageal  septum,  270. 
Traction  bands,  290. 
Transversalis,  423. 


Transverse  processes  of  vertebrae,  64. 

Transverse  sinus,  332. 

Trapezius,  69,  441,  451. 

Treitz,  Band  named  by,  290,  292. 

Triangle,  Ovarian,  4. 

Triangular  fibro -cartilage,  447,  456.  ■ 

Triangular  ligament,  413. 

Tribe,  Margaret,  283. 

Triepel,  H.,  17. 

Trigeminal  nerve,  95,  99,  149,  172,  191, 

218,  248. 
Trigonum  olfactorium,  186. 
Tritubercular  theory,  188. 
Trochanters,  429,  460. 
Trochlear  nerve,  95,  99,  218. 
Trophoblast,  11,  15. 
Truncus  arteriosus,  316,  323. 
Tschassownikow,  S.,  8. 
Tuberculum  impar,  255. 
Tubular  pregnancy,  7. 
Tunica  vaginalis,  398. 
Turbinate  processes,  196,  199. 
Twins,  42. 

Tympanic  bulla,  229. 
Tympanic  plate  and  articular  eminence, 

Development  of,  178,  225,  228. 
Tymj)anic  ring,  178. 
Tympano-hval,  246. 
Tympanum^  138,  224,  229. 
Tyson's  glands,  396. 

U 

Ulnare,  447. 

intimate  branchial  bodies,  265. 
Umbilical  arteries,  27,  30,  334. 
Umbilical  cord.  Formation  of,  30. 
Umbilical  faecal  fistula,  31,  290. 
Umbilical  hernia,  31,  291,  423. 
Umbilical  urinarv  fistula,  31. 
Umbilical  veins,  30,  273,  310. 
Umbilical  veins.  Nature  of,  27. 
Umbilicus,  Formation  of,  20,  24,  31,  289, 

416,  423. 
Unciform,  448. 
Uncus,  195. 
Urachal  cysts,  390. 
Urachus,  390. 
Ureter,  366. 
Ureter,  Double,  368. 
Urethra,  376,  383,  388,  393. 
Urethra,  Musculature  of,  389,  391. 
Urogenital  cleft,  387. 
Urogenital  mesentery,  362,  396. 
Urogenital  sinus  or  canal,  376,  381,  394. 
Urogenital  system,  358. 
Urorectal  septum,  383. 
Utero-sacral  ligaments,  371. 
Uterus,  Formation  of,  371. 
Uterus,  Malformations  of,  372. 
Uterus  masculinus,  364,  376. 
Uterus,  Round  ligament  of,  370. 
Utricle,  232,  234. 
Uvea,  207,  213. 
Uvula,  170. 


INDEX 


491 


V 

Vagal  nuclei,  87. 

Vagina,  Atresia  of,  373. 

Vagina,  Formation  of,  371. 

"N-'agina,  Metamorjihosis  of,  373. 

Vagina,  Sphincter  of,  390. 

Vaginal  cords,  373. 

Vagus,  99,  228,  248. 

Valvulae  conniventes,  292. 

Vasa  aberrantia,  365. 

Vasa  efferentia,  364. 

Vas  aberrans,  439. 

Vas  deferens,  364. 

Vasoformative  cells,  335. 

Veins  of  trunk.  Development  of,  303. 

Velum,  Inferior  medullary,  89. 

Velum  interpositum,  102,  112,  113. 

Velum,  Superior  medullary,  94. 

Vena  azygos,  303. 

Venae  cavae,  303,  306. 

Venous  sinuses,  133. 

Venous  valves,  316. 

Ventral  aortic  stems,  249. 

Ventral  mesentery,  269. 

Ventral  mesogastrium,  275. 

Ventricle,  Terminal,  77. 

Ventricles  of  brain.  Third,  102. 

Ventricles  of  brain.  Fourth,  86. 

Ventricles  of  brain.  Lateral,  112. 

Ventricles  of  heart,  315,  325,  326. 

Ventre -lateral  muscles,  68. 

Vernix  caseosa,  50,  469. 

Vertebra,  Development  of,  58. 

Vertebra,  Morphological  parts  of,  58. 

Vertebrae  of  mammalia,  61. 

Vertebrae,  Ossification  of,  61. 

Vertebral  arteries,  132. 

Vertebral  bow,  59. 

Vertebral  column.     See  Spine. 

Vesiculae  seminales,  364. 

Vestibular  folds,  384. 

Vestibular  ganglion,  86,  224,  238. 

Vestibular  mechanism,  86. 

Vestibule  of  ear,  235. 

Vestibule  of  left  auricle,  321. 

Vestigial  fold  of  Marshall,  305. 

Vestigial  muscles,  452. 

Vestigial  turbinates,  199. 

Vidian  canal,  173. 

Villi  of  intestine,  292,  294. 

Vincent,  Swale,  403. 

Visceral  arch.     See  also  Branchial  arch. 

Visceral  arches,  156,  241. 

Visceral  arches,  Arteries  of,  243,  248. 

Visceral  arches.  Cartilages  of,  246,  352. 

Visceral  arches.  Muscles  of,  253. 

Visceral  arches.  Nerves  of,  248. 

Visceral  arteries,  288,  296,  334. 

Visceral  clefts,  243. 

Visual  cortex,  221. 


Vitelline  veins,  273,  308. 
Vitelline  vessels,  288,  291. 
Vitello-intestina]  duct,  20,  25,  268,  288, 

290. 
Vitreous  humour,  211. 
Vocal  cords,  343,  352. 
Vomer,  163. 

Vriese,  Bertha  de,  132.  461. 
Vulval  cleft,  376. 


W 

WaddeU,  419. 
Wakeley,  C.  P.,  291. 
Walker,  C.  E.,  8. 
Walker,  T.,  394,  403. 
Wallace,  C.  S.,  396. 
Walmsley,  T.,  68,  457,  460. 
Wang,  C.  C,  302. 
Warren,  J.,  108. 
Waterston,  D.,  18,  271,  324. 
Watson,  D.  M.  S.,  135,  425,  444. 
Watson,  E.  M.,  364. 
Watson,  J.  H.,  376. 
Watt,  J.  C,  18,  42,  82. 
Weber,  Max,  462. 
Weed,  L.  H.,  90,  134. 
Weidenreich,  F.,  336. 
Weigner,  K.,  56. 
Wharton's  jelly,  31. 
Whitehead,  R.  H.,  396. 
WhitnaU,  S.  E.,  214. 
Wichmann,  S.  E.,  369. 
Wijhe,  J.  W.  van,  155. 
Wilder,  H.  H.,  464. 
Williams,  L.  W.,  57. 
Willis,  Circle  of,  132. 
Wilson,  J.  T.,  39,  90,  189. 
Winslow,  Foramen  of,  276. 
Wirsung,  Duet  of,  284. 
Wolffian  body,  360. 
Wolffian  body.  Fate  of,  363. 
Wolffian  ducts,  26,  360,  361,  376. 
Wolffian  tubules,  361,  398. 
Woodland,  400. 
Wormian  bones,  143,  144,  150. 
Wrai,  H.,  149. 
Wright,  W.,  155. 


Yolk  sac,  15,  17,  24,  288. 
Yolk  sac.  Nature  of,  17,  24. 
Yolk  sac.  Vessels  of,  288,  291. 


Zona  radiata,  7. 
Zuckerkandl,  E.,  200.  403. 
Zygosis,  287,  356,  399. 


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